Tag Archives: Antennas

Guest Post: Supercharging the XHDATA D-808 with a 7.5″ loopstick

Many thanks to SWLing Post contributor, Gary DeBock, for sharing the following guest post:


Supercharging the XHDATA D-808

Installation of High Performance AM and LW Loopsticks

By Gary DeBock, Puyallup, WA, USA, September 2018

Introduction

As a stock receiver the Chinese-made D-808 AM-LW-FM-SW-AIR portable is a very capable performer, with AM reception superior to that of any current Ultralight model, and impressive FM reception as well. The radio was certainly “inspired” (to use a generous term) by the C.Crane Skywave SSB model, which coincidentally was manufactured in the same part of China by C.Crane’s Redsun partner—with the first units going out the door a few months before the D-808 came into existence.

Because foreign intellectual property is routinely copied in China with no punishment from the government, XHDATA essentially had the chance to copy all the good points in the Skywave SSB design and improve upon its weak points as well. The only precaution that XHDATA took after this wholesale design appropriation was to forbid direct shipments of the D-808 from China to North America—presumably to avoid a copyright lawsuit by C.Crane. As such, the first D-808 models were sold to the rest of the world around January of 2018 at a price about half that of the Skywave SSB, while North American DXers were told that since the model couldn’t be shipped to the USA or Canada, they were out of luck.

Of course some D-808 models did make it into North America, where it was found to be a very capable portable with astonishing value for the price. Finally around March, an enterprising Chinese eBay seller came up with a plan to ship the model to North America through Israel, thereby skirting around XHDATA’s direct shipment prohibition. As of late August this eBay seller (harelan ecommerce) has already sold 62 of the D-808 models this way, even though he charges a premium for shipment to North America. Whether this single supply source will continue to serve North American customers is currently unknown, but out of the 7 models that I have purchased from him there hasn’t been a single D-808 model with any issues– despite the apparent lack of any manufacturer’s warranty offered on the radio.

Despite the D-808’s rather dubious design pedigree there is no doubt that the Chinese engineers (or reverse engineers?) did a superb job in creating an awesome radio for the money. Besides directly copying the Skywave’s SSB design and controls, XHDATA also made significant improvements, including a longer loopstick (providing clearly superior AM sensitivity), a much more powerful audio amplifier (correcting a serious shortcoming in the Skywave SSB) and a much lower price (about half that of the $169.99 Skywave SSB, for models shipped outside North America). Another great advantage for someone wishing to perform this loopstick upgrade are the perfectly located, highly accessible Litz wire connections on the RF circuit board—apparently used by the Chinese engineers to conveniently test out various loopsticks, and retained in the final product.  The radio’s high quality construction and survivability in adverse conditions were proven repeatedly over the summer here, with the model surviving accidental exposure to a 104 degree (43 degrees C) car trunk temperature, exposure to moderate rain, repeated travel bumps, and use as the main receiver during a 9-day DXpedition to a plunging ocean side cliff in Oregon state. The 3.7v lithium-ion rechargeable battery provides superior run time for extended DXing sessions, and is included in the D-808 shipping package, along with a USB cord to charge the battery, a plug-in wire antenna (for FM,SW and AIR), a vinyl carrying case, and a pretty basic English instruction manual.

One thing you will NOT find supplied with the D-808 is a warranty card– either in the shipping box, or online. This is pretty standard practice in China, incidentally, where concepts like refunds and warranties aren’t generally part of customers’ expectations. This doesn’t necessarily mean that XHDATA won’t repair obvious problems in a new D-808, but it does mean that they aren’t assuming the obligation to do so. I have heard from one North American purchaser who received a new D-808 with a defective speaker, and he is still waiting for the model to be repaired (after paying the shipping charge to send it back to China). Each individual purchaser must decide whether or not this lack of any warranty is a deal breaker. But if you are looking for a final reason to perform this loopstick transplant, why not consider the fact that you will not be violating any manufacturer’s warranty by doing so??

Realistic Expectations

Although this 7.5” loopstick upgrade will certainly make your D-808 far more sensitive than the stock model on Medium Wave or Longwave, it is not designed to compete with large (2’ sided or larger) inductively coupled box loops, or any of the new FSL antennas. The sensitivity upgrade will boost the D-808’s MW band weak-signal performance up to the level of classic portables like the ICF-2010 and RF-2200; however, and since the D-808’s DSP-enhanced selectivity will generally exceed that offered by these classic portables, the overall DXing capability in the AM mode could be considered slightly greater. The D-808 does have SSB capability, although it lacks the SSB tuning convenience offered by the ICF-2010 and RF-2200. It also lacks the ICF-2010’s superb Synch detector, a big advantage in weak signal DXing. But in portability, versatility and DXing value for the price, the “Supercharged” D-808 is a real winner.

Project Overview

This construction article will provide the builder with step-by-step instructions to upgrade the XHDATA D-808’s loopstick to a much more sensitive, externally-mounted 7.5” Medium Wave or Longwave loopstick replacement. Both the Medium Wave and Longwave 7.5” loopstick designs have been thoroughly tested and proven effective in actual DXing by hobbyists other than the author, and as long as the instructions are followed carefully, this relatively inexpensive modification will provide a major improvement in the D-808’s weak-signal reception capability.

This modification project involves close-order soldering on the D-808’s circuit board, and should only be attempted by builders with reasonably good eyesight, good hand coordination and soldering experience. The project also calls for the use of a precut plastic loopstick frame to attach the antenna to the top of the D-808’s top back cabinet surface, and the construction of this precut plastic frame requires either the use of a 12” (or larger) power miter saw, or some rather lengthy cutting with a hacksaw. Use of a power miter saw SHOULD NOT be attempted by those without serious power tool experience! The author assumes that only qualified power tool operators will attempt to use a 12” miter saw to cut these frames quickly, and that other builders who wish to construct them will use a hacksaw. As such, only basic cutting instructions are provided for the 12” power miter saw users, while detailed instructions are provided for the hacksaw users. To assist builders who are not qualified to use power tools, the author has prepared a LIMITED number of these precut plastic loopstick frames on a power miter saw, which will be offered at cost to these builders on a first come, first served basis.

A final warning is in order concerning the step of gluing the precut plastic loopstick frame to the D-808’s top back cabinet surface. Although this step is not dangerous, it is pretty tricky. Since the superglue “grips” very rapidly, you will only get one chance to ensure that the frame is straight, and centered on the D-808’s top cabinet surface. Do yourself a favor, and make multiple “dry runs” to practice this important step before applying the glue! Failure to take this step seriously will probably result in a crooked loopstick frame—which will hold the antenna just fine for DXing purposes, but which will be an eternal reminder to the DXer (and everyone else) of the hazards of haste.

Construction Parts Required

This 7.5” loopstick D-808 construction article will guide you through the assembly of either a 7.5” Medium Wave loopstick D-808 or a 7.5” Longwave loopstick D-808, so make sure that you order the parts necessary for construction of your chosen model. The picture above shows the parts that will be necessary for construction of either model, but the Litz wire and 7.5” ferrite rod components differ according to whether you are building the Medium Wave or Longwave model.

A)  XHDATA D-808 Receiver, currently available to North American purchasers (for $112.87 + $10. Click here to search eBay.

B)   Scotch brand “Extreme” strapping tape (any size roll)

C)   15 feet (4.6 meters) of 250/46 Litz wire (Medium Wave model). Click here to view on eBay.

OR 25 feet (7.7 meters) of 100/44 Litz wire (Longwave model). Click here to view on eBay.

D)  Two 120 lb. test plastic tie wraps (any length over 6”)

E)  Johnson Level & Tool Mfg. Co., Inc. 48” orange plastic carpenter’s level, part # 7748-O (provides enough plastic for two loopstick frames)

F)  Two 3/4” lengths of 1/2” I.D. clear vinyl hose

G)  Two 1” lengths of 5/8” I.D. rubber hose

H)  Roll of 2” Johnson & Johnson waterproof (medical) tape OR roll of 1” Rite-Aid waterproof tape

I)  Amidon 7.5” x .5” ferrite rod, part no.  R61-050-750 (MW model) OR part no. R33-050-750 (LW Model), available at http://www.amidoncorp.com/rods-and-tiles/

J)  6” of 1/16” shrink tubing

Miscellaneous:  One packet of Duro Super Glue (.07 ounce size), solder, 25w (low heat) soldering iron, hacksaw (or power miter saw), screwdriver set, sandpaper, needle nose pliers, diagonal cutters

D-808 Radio Preparation

Before starting the modification give the radio a thorough test on all bands, ensuring that all the stock model functions work properly, and that there are no issues with the display, speaker, headphone jack, battery or charging system. It’s also a good idea to run a daytime DX band scan on the AM or Longwave band (for whichever band you plan to construct an upgrade loopstick) and document the results—to use as a benchmark for the upgrade loopstick’s performance.

Step-By-Step Construction

Antenna Frame and 7.5 inch Loopstick Preparation

1)   Refer to the photo below. Using the “Supercharging the Tecsun PL-380” article (posted at  http://www.mediafire.com/file/du3sr5cd9thqvau/7.5inch-LS-PL380.doc/file or available directly from the author) carefully prepare the orange loopstick antenna frame according to construction steps 1-9, EXCEPT note that the lower (glue surface) edge of the antenna frame should be cut to a length of 5 3/4” (147mm), NOT 5” (127mm) as described in the PL-380 transplant article. Pay close attention to the safety precautions concerning power tool usage, and DO NOT attempt to use a power miter saw unless you have SERIOUS power tool experience!

2)   If you are constructing an AM (Medium Wave) loopstick, follow construction steps 10-16 in the PL-380 transplant article to construct the antenna. If you are constructing a Longwave loopstick, follow construction steps 10a-16a in the PL-380 transplant article to construct the antenna. If you are constructing both loopsticks, MAKE SURE that the ferrite rod and Litz wire are only used in the antennas for which they were designed. Mixing up these items is very easy, and such a mistake will make both loopsticks perform like clunkers.

3)   After construction of either the AM or Longwave loopstick, follow the instructions in steps 29 and 30 of the PL-380 transplant article to install a piece of 3 1/8” (79mm) shrink tubing, EXCEPT note that this length is slightly longer than the 3” (76mm) length called for in the PL-380 article.

4)   Refer to the photo below for the following three steps. [NOTE: Although this photo shows the AM (Medium Wave) loopstick, the procedures in this step are the same for the Longwave loopstick, although the position of the rubber hose lengths and clear vinyl inserts will be closer to the ends of the ferrite rod]. Carefully slide the length of 3 1/8” shrink tubing into the position shown, ensuring that there are no Litz wire kinks or bends inside the shrink tubing.

5)   Take the two 3/4” (19mm) clear vinyl inserts and slide them onto the ferrite rod ends, twisting them up against the border of the Scotch “Extreme” tape ends to lock the tape in place under the vinyl inserts. Ensure that the clear vinyl inserts do not touch any Litz wire leads or coil turns.

6)    Slide the 1” (25mm) lengths of rubber heater hose over the clear vinyl inserts until the appearance of the loopstick resembles the above photo. Ensure that the rubber hose sections also do not touch either the Litz wire leads or any coil turns. Finally, place the completed loopstick in a safe place until it is called for in Step  .

Radio Disassembly

7) Refer to the photo above for this step. Remove the battery from the radio, and using a Jeweler’s Phillips screwdriver of the correct size, remove the six identical screws in the positions shown (NOTE: These screws have a tendency to stick inside their slots, even when the slots are turned upside down. If you cannot remove all six screws it’s not a major problem, but at least ensure that the screws are completely loose in their slots, and that you don’t lose any of them during the remaining steps). Grasp the tuning knob, and pull it out horizontally in a completely straight manner to remove it from the radio. Ensure that the battery, tuning knob and all removed screws are placed in a safe place until the radio is reassembled.

8)   Carefully separate the front and back cabinet sections and place them down in the position shown in the photo below. Note that the front and back sections of the radio are connected by a ribbon wire plug-in system– ensure that this plug remains securely inside its slot at all times, and that no great stress is placed on the speaker wires.

9)   Refer to the close up photo below, and note the position of the two Litz wire soldering points on the circuit board (in the lower right corner of the photo). Using diagonal cutters, cut the two Litz wire leads at the position shown, UNLESS you wish to salvage this stock loopstick for other projects—in which case you should desolder the entire lengths of the Litz wire leads from the circuit board at the positions shown in the lower right corner (NOTE: The stock loopstick is of a fairly good design, and has an inductance that would be compatible with any DSP-chip Ultralight radio, providing an AM sensitivity boost in the process).

10)   Refer to the photo below. Using a flat Jeweler’s screwdriver with a 1/16” blade, carefully probe around all four sides of the stock loopstick to break all of the glue bonds. Work slowly and carefully around the perimeter of the ferrite rod, including the plastic covers on each end. Once most of the glue bonds have been broken the ferrite rod will begin to shift around as you break up the few remaining bonds, but until this point work slowly and patiently to break up the glue.

11)   Refer to the photo below. Using the flat Jeweler’s screwdriver, once all of the glue bonds have been broken and the ferrite rod is loose in its slot, lift the ferrite rod out of its slot on one side by prying up under the plastic cover on the end of the ferrite rod. Ensure that the Litz wire leads have either been cut or desoldered from the circuit board, then grasp the ferrite rod with your fingers and pull it completely out of the slot with a slight twisting motion.

 

12)   Remove the wrist strap, and refer to the photo below. Carefully pick up the two sides of the radio and place the back section in a vertical position as shown, with a heavy flat weight (barbell, or other heavy flat item) pressing up against the back cabinet section to keep it in a vertical position. Ensure that there is adequate, even lighting on the top cabinet section for the gluing process in the next step, and that the back cabinet surface will not shift around as you make the gluing “dry runs,” and perform the actual gluing of the loopstick frame to the top of the cabinet.

 

13) Take the previously prepared orange plastic loopstick frame, and ensure that its bottom glue surface is completely smooth and flat, with no uneven ridges on the edges of the glue surface (remove these with fine sandpaper, but ONLY on the ridges, and not on the rest of the flat glue surface). Using a damp paper towel, wipe the top cabinet glue surface and the loopstick frame glue surface to remove any dust or debris, then wipe them again with a dry, clean paper towel to ensure that they are both completely dry.

Take the loopstick frame and gently slide the frame over the top cabinet surface to ensure that both surfaces are smooth and flat. Refer to the photo at the top of the next page. Ensure that there is even, bright lighting on the top cabinet surface, and make several “dry runs” to place the loopstick frame in the exact center of the top cabinet surface (with 1/16”, or 1.5mm of space between the frame ends to the cabinet ends), and also 1/16” (1.5mm) of overhang above the front edge of the cabinet’s glue surface (NOTE: if you wish to simplify the process by lining up the front edge of the loopstick frame with the front edge of the cabinet’s glue surface it will still provide an acceptable result, but you will need to do some minor sanding of the whip antenna’s plastic slot post, as shown in the photo below. In either case, make repeated “dry runs” with the loopstick frame to practice placing it in the exact center of the top cabinet’s glue surface, since you will only get one chance to place it in the proper center position once the superglue is applied.

NOTE: The back of the loopstick frame has a beveled surface to permit full operation of the radio’s whip antenna after the frame is glued on the top of the cabinet surface. If the loopstick frame is glued with a 1/16” (1.5mm) overhang in front of the front edge of the cabinet surface then the whip antenna should have enough space for free operation. The alternative is to glue the two front edges lined up with each other to simplify the gluing process, in which case minor sanding may be required on the whip antenna slot post, as shown in the photo below.

14)   After making multiple “dry runs” and becoming familiar with accurate placement of the loopstick frame on top of the cabinet, refer to the photo at the top of the next page. After once again ensuring that the back cabinet section will not shift around during the gluing process, take the Duro superglue packet and apply a thin (1/8”, or 3mm) bead of glue along the center of the cabinet’s glue surface, extending it 5 1/4” (133mm)long, with equal spaces on both ends (as shown). While sighting the two sides place the loopstick frame carefully down in the correct center position as practiced previously, with the 1/16” overhang if desired. If satisfied with the position, press down on the frame to lock the two surfaces together securely. Usually the frame may be shifted around slightly within 1 or 2 seconds of placing it on the superglue, so use this brief time to promptly shift the frame to a straight position, if necessary. After a couple of seconds, though, you will need to be satisfied with whatever position the frame has ended up with (regardless, it will still hold the loopstick just fine, for DXing purposes).

15)   After the loopstick frame is securely placed and locked on top of the D-808’s cabinet surface, place downward pressure on the loopstick frame along its length in order to ensure a tight glue bond throughout the entire top cabinet surface. Continue this process for about one minute, and sight both ends of the loopstick frame to ensure that they are both completely flat against the D-808 cabinet.

16)   Inspect the front and back edges of the loopstick frame’s border with the D-808 cabinet for any glue seepage, and if any is found,  remove it promptly with the 1/16” flat Jeweler’s screwdriver blade. Glue should not be allowed to run past the frame edges. This completes the process of gluing the frame to the D-808 cabinet.

7.5” Loopstick Installation

17)   [NOTE:  The installation procedures of the Medium Wave (AM) and Longwave loopsticks are identical, except that the plastic tie wraps and rubber hose sections are closer to the ends of the ferrite rod in the Longwave version. The following photos are for the Medium Wave (AM) version,  but Longwave loopstick builders should follow the same steps, while referring to the Longwave model photo in the “Operation” section as a guide]

Refer to the photo below. Carefully take the previously prepared 7.5” loopstick and hold it in the position shown—in its slot, centered in the middle of the orange antenna frame, with the shrink tubing and Litz wire leads running down to the left. Take the two plastic tie wraps and install them in the position shown, centered over the rubber hose sections on the loopstick, while ensuring that no Litz wires or shrink tubing is bound under the plastic tie wraps.

18)   Refer to the photo below. Lay the two cabinet sections down flat as shown, ensuring that the Litz wire shrink tubing is in the exact position shown (if it isn’t, carefully slide it along both Litz wires until it is in this exact position). Carefully thread one Litz wire end through the empty wrist strap hole, then thread the other Litz wire end through the hole, as shown. Finally pull on the two Litz wires together from the right while guiding the end of the shrink tubing into the empty wrist strap hole, and pull a short section of the shrink tubing through the hole (as shown) to protect the Litz wire insulation from friction damage.

19) Refer to the photo below. Using the previous procedure to install shrink tubing (which is described in the PL-380 transplant article) install a 2.5” (63mm) length of shrink tubing over the two Litz wire ends, and shift the shrink tubing into the position shown in the photo. After this is done cut the two Litz wire leads to the lengths shown in the photo (NOTE: make sure that the ends of both Litz wires are cleanly cut, not frayed and at the minimum diameter before attempting to insert them into the shrink tubing. The process is much easier when the Litz wires pass smoothly through the shrink tubing).

20) Refer to the close up photo below. Using a low heat (25w) pencil-type soldering iron, remove the two stock Litz wire leads at the positions shown, taking care not to use excessive heat, or touch the adjacent components. Ensure that the new Litz wire leads are at the length shown when the leads are in a horizontal position throughout the cabinet, and cut them to this length if they are not.

21) NOTE: When tinning the 250/46 Litz wire it is essential that all of the individual Litz wire strands be completely soldered together for a length of at least 1/4” (6mm), with bright, shiny solder around the circumference of the Litz wire ends for this minimum (1/4”) length. The Litz wire must be heated with a clean, hot soldering iron around its circumference in order to melt the solder properly for this step]

Refer to the photo above. Pull the Litz wires up out of the previous position, and place a clean rag underneath them (on top of the circuit board) to completely protect the circuit board from any solder which might accidentally drop down during the tinning process. Using your hot 25w soldering iron melt a generous amount of solder on its tip, and work the soldering iron tip slowly and patiently around the circumference of each Litz wire end until there is a bright, shiny solder length of at least 1/4” (6mm) in a cylindrical pattern at the end of each Litz wire. When doing this, take great care not to allow any solder to drip down onto the circuit board below (i.e., make sure that your rag completely covers the circuit board). The final appearance of your Litz wire lead ends should resemble those in the photo.

22) When your Litz wire lead ends resemble the photo above, cut the soldered portion down to a length of 3/16” (5mm) and observe the appearance of the end of the Litz wire. It should have a bright, solid circular shape, with no gaps or individual Litz wires showing. If not, reheat the end of the Litz wire while adding some solder, and repeat this step.

23) NOTE: The Litz wire connection points on the circuit board are surrounded by other important components. It is important to avoid solder drips on these components, or solder bridges to their leads. Solder the Litz wire leads down at an angle to avoid these surrounding components, and use the minimum amount of heat and solder to ensure good electrical connections)

Refer to the close up photo above. Following the precautions described, solder the two Litz wire leads down onto the circuit board at an angle, as shown in the photo. After soldering, make a close visual inspection to ensure that there are no solder bridges across the Litz wire connections, or nearby components. The remaining length of the Litz wire leads should be routed in a horizontal manner to the wrist strap hole.

24) Carefully pick up the front and back cabinet sections, and hold the back cabinet section fairly close to the front section (as the radio would normally be oriented, when assembled). Refer to the photo below, and carefully insert the “Fine Tuning” control thumbwheel from the front cabinet section into its slot on the back cabinet section in a sideway movement. This will allow you to fully close the front and back cabinet sections in the next step.

25) Refer to the photo below. Pick up the two cabinet halves and carefully snap them together (this action should not require any great force). Place the radio face down in the position shown (with a soft surface underneath, for protection), and using the Jewelers Phillips screwdriver of the correct size, carefully screw in the six screws that were loosened previously, starting with the screw near the whip antenna post (you should pick up the radio temporarily and hold the two cabinet sections together tightly at this corner, as you do this).

After all six screws have been retightened take the Tuning control knob and press it back onto its shaft in a straight horizontal motion. Finally, reinstall the battery and battery compartment cover to finish up the reassembly.

TESTING AND OPERATION– MEDIUM WAVE MODEL

This 7.5” transplant loopstick is designed to provide a major boost in sensitivity from 530-1700 kHz, and if the antenna is working properly both the weak signal reception and the radio’s nulling capability should be greatly enhanced. It is normal for the antenna to receive more background noise on the low band frequencies, although the sensitivity boost should be substantial across the band.

The construction design of the orange antenna frame allows full usage of the whip antenna for checking SW parallels of MW-DX stations, although if you chose to glue the antenna frame flush with the front of the back cabinet surface to simplify the gluing process, you may need to sand the whip antenna slot post slightly to allow free movement of the whip antenna (see step #13).

In the photo above, some of the important controls for Medium DXing are highlighted. The AM Bandwidth control allows you to choose multiple DSP filtering selections to enhance selectivity as desired, with the narrowest filtering (1 kHz) providing both the sharpest selectivity and the best weak-signal sensitivity. However this 1 kHz setting also has the poorest audio fidelity, with the higher audio frequencies typically cut off by the DSP filtering. As such, for regular DXing far away from strong local pests, the other AM Bandwidth settings may be more suitable. The Direct Frequency Entry key allows you to manual enter in any MW frequency, to which the radio will shift once the numbers are pressed on the keypad. The Tuning knob has three different modes, which can be toggled by pressing the knob horizontally. The first mode is tuning in either 9 kHz or 10 kHz steps (depending on which of these step you have selected), while the second mode is tuning in 1 kHz steps. The third mode is to lock the frequency in place. Pressing the knob again will return the tuning to 9 or 10 kHz steps.

The XHDATA D-808 has multiple display functions, which can be toggled by the indicated key. The first option is the temperature in either Centigrade or Fahrenheit (depending on your pre-set preference), while the second option is the alarm time. The third option is the current time (which you need to set according whether you prefer UTC or local time), while the fourth option is the received signal strength in both dBu and dB.

The supplied 3.7v lithium ion battery has superior run time, and may be easily charged using the supplied USB cable to either a computer or AC outlet (with the appropriate adapter). As reported in various posts throughout this year, the D-808 model has rugged construction with an excellent record of survival under tough conditions, including hot summer days, moderate rain exposure and extended usage as the main receiver during a 9-day ocean cliff DXpedition in Oregon—performing flawlessly at all times.

Conclusion

It is the author’s sincere hope that this “Supercharged” D-808 model will bring you a lot of DXing fun during travel, as well as at other times. When conditions are good you should never underestimate this enhanced model’s potential of receiving awesome DX beyond your expectations—as an example, here is the stand-alone performance of a 7.5” loopstick D-808 in receiving 1017-A3Z in Nuku’alofa, Tonga (10 kW at 5,632 miles/ 9,063 km) on the ocean cliff near Manzanita, Oregon at 1301 UTC on August 8th of this year:

Not only Tonga is received, but even the Australian horse racing station 1017-2KY in Sydney (5 kW at 7,630 miles/ 12,280 km) is received as a weak co-channel in the middle of the recording. My hope is that you all will be so lucky with your new Supercharged D-808!

73 and Good DX,

Gary DeBock (in Puyallup, WA, USA)


Absolutely amazing!  Thank you for taking the time to put this procedure together and describing the process in such fine detail, Gary! Hats off to you! 

Click here to read all of Gary DeBock’s posts on the SWLing Post.

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Guest Post: SM0VPO’s 3D Printed 10KV Tuning Capacitor

The completed antenna.

Many thanks to SWLing Post contributor,Harry Lythall (SM0VPO), who shares the following update to his excellent homebrew 20 meter magnetic loop antenna post:


3D Printed 10KV Tuning Capacitor

by Harry Lythall (SM0VPO)

Introduction

You may have seen my 20m (14MHz) loop, or frame, antenna, and the ease of construction with just a bit of wire and a bit of plastic tube. The tuning arrangement is a little primitive, using just a “gimmick capacitor”, comprising two bits of wire twisted together.

The original “Gimmick” capacitor that burns.

This arrangement works very well for QRP, where the average RF power is about 5 Watts or less. If you exceed this power level, then the twisted-wire capacitor tends to warm up and the tuning drifts a little. But if you use more than about 10 Watts of continuous RF power, then things start to burn. Cheap insulated wire also smokes. This is because the impedance at the ends of the coil is so high that you can get many 100s of volts and the insulation, normally intended for house wiring, breaks down.

In this page I will show you how to build a super-cheap tuning capacitor that will tolerate up to 10,000 Volts of RF and allow you to use up to about 100 Watts of RF into my 14MHz (20m) antenna. The capacitor is also tunable so that you can adjust it by hand (when the RF is removed, of course :-). The tuning range is about 8pf to well over 30pf when really compressed. The normal range for the antenna is about 12pf to 15pf.

Construction

My prototype does not look very pretty, and it is not supported on anything other than the connection wires from the antenna. Very few components are used:

  • two plastic foam pan scrubbers or one thick plastic bath sponge
  • two pieces of metal 4cm x 6cm. Copper-clad board works fine
  • one plastic nut and bolt – see text
  • one heavy-duty 3-pole block connector with centre-pole removed

Components for the 10KV tuning capacitor

The plastic nut and bolt need to be about 6cm long and totally non-conductive. If you want to make my bolt, then do NOT use black plastic because some black plastics use carbon as a colouring agent. You can use a nylon bolt, as used to secure IKEA toilet seats, but you will also need a washer to spread the stress, otherwise the copper-clad board tends to bend under the stress with time. I chose copper-clad board because it is easy to solder – no need for drilling or connection bolts. I made my plastic compression tuning bolt using my 3D printer. I have included the project files for you to download.

Ikea toilet-seat bolt

Note that the connector for the wires needs to be well spaced between the metal inserts, in order to tolerate up to 10KV, so I used a 3-pole connector and took out the centre pole. At 100-Watts continuous there is a very slight warming after a few minutes, but no sign of smoke, sparks or corona. 🙂

The connector with the centre-pole removed.

I tried a few different types of plastic dielectric and all worked well, providing they are 100% dry. The best ones those I stole from the kitchen cupboard (when Maj-Lis was not watching). I think it costs about $1.50 for a packet of 10 pieces. Perhaps I should have used a nicer colour? A pretty pink? Heart-shaped? No! maybe that would be going a bit too to far ;-). Here is my finished capacitor using my 3D printed tuning screw.

The assembled 10KV capacitor.

3D Files

Once more, this project is ridiculously easy to make with a 3D printer. The hardest part was to get the pitch of the threads right, then clone/connect the pieces to get a longer thread. They were printed with the bolt vertical, so the slope under each thread is a steeper angle than the upper slope. This makes the printout a lot easier to print. If the angle is too steep, then it may extrude PLA into mid-air. My nut and bolt heads are about 3cm Diameter, and the 10mm thread for the nut was cut using boolean subtraction. I then enlarged the nut by about 3% so it still fits but there is a little slack so that it does not bind. I may have overdone it a little, but not much.

Project rendered in 3D Studio Max

Here are the files:
3D studio MAX file – 3d-cap-01.max
ASCII STL file – 3d-cap-01.stl
ASCII OBJ file – 3d-cap-01.obj
the GCODE file for my Wanhao (Prusa) Duplicator i3 – 3d-cap-01.gcode

Just right-click on the file and select “save as”. Some web browsers try to open ASCII files instead of saving them. I usually export STL files, but on this occasion, I tried comparing OBJ files since CURA slicer will accept both. Although I use a WANHAO replica of the PRUSA Duplicator i3 printer, the GCODE files are rather generic and will probably work on most printers.

My printer settings in this GCODE are:

  • Nozzle temperature = 200°C
  • Bed temperature = 60°C
  • Support structure = brim
  • Layer height = 0.1mm
  • Print speed = 60mm/s
  • Fill density = 40%
  • Shell thickness = 1.2mm

The printer temperature is 200°C, which is 5°C hotter than recommended for PLA filament. I find that I get a better print at 200°C as it sticks to the bed a lot easier. When printing this screw thread, it may be advisable to start at 200°C and then turn down the nozzle temperature to 195°C after a few layers have been printed.


I hope that you find this project interesting. It is a bit small, but it gets me active on 14MHz from my car with this portable antenna. I have a new car and I don’t want to start throwing long aluminium tubes in it that scratch the interior to bits.

Don’t forget to visit my messageboard if you have any questions about this or any other project. I always look forward to receiving feedback, positive or negative ?

Very best regards from Harry Lythall
SM0VPO (QRA = JO89WO), Märsta, Sweden.
EA/SM0VPO (QRA = IM86BS), Nerja, Spain.


Thank you so much for sharing this, Harry! I love both the frugality and ingenuity in this unique capacitor design!

Post Readers: be sure to check out Harry’s website which is loaded with radio projects of all stripes. You’ll easily spend a few hours digging through his tutorials and downloads. Harry also maintains an alternate mirror server located here.

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Guest Post: Backpack Shack 3.0 – Part 3

Many thanks to SWLing Post contributor, TomL, who shares the following guest post as a his Backpack Shack 3 continues to evolve:


Backpack Shack 3.0 – Part 3

by TomL

I have now gone overboard since I think bigger must be better.  The temptation was just too great and now there is an MFJ-1979 17-foot telescoping whip antenna in my car (with consequences).

MFJ Angst

I have a love/hate relationship with MFJ products because of what I think are useful ideas that are made somewhat poorly.  But I went ahead and bought the large whip since I figured they could not possibly screw up something so simple, right?

Wrong.  As I excitedly tried to screw the supposed 3/8”-24 threaded end into the nice standard Firestik K-11 magnet mount, I realized I was turning and turning it but it was not going in!!!  I even had a small steel sliver of metal sticking into my flesh to prove I was not dreaming. The previous day, it had screwed in very tightly, but it did screw in. So, there I was after a long day of work, ready to listen to some SWL-Nirvana and I could not get the blasted antenna into the mount–?  That Firestik mount is a VERY standard 3/8”-24 female thread and the other third-party antenna shafts fit perfectly and easily EVERY time I use them. I hate $60 of poor workmanship and MFJ seems to be the poster child of overpromising and underdelivering.

I was determined to make this work, by force if needed.  One of the Trucker antenna shafts by necessity had an extra coupling nut on it to allow the extra 18 inch shaft to connect, so I took it off there and tried to thread it onto the MFJ-1979.  It barely moved. Not to be thwarted, I dug out an adjustable wrench and 3/8” socket wrench with ½” socket and grunted and twisted and tightened until the coupling nut was threaded all the way “up its shaft”.  That is what I feel like telling MFJ! That coupling nut is never coming off and now that I truly have bought it and cannot return it, I might as well use it.

The stainless steel telescoping rod is extremely thin and feels like it can bend and dent with any kind of mishandling.  So it resides collapsed in a 27 inch PVC pipe with plumbing pipe foam inside to baby it when it is not being used. It remains to be seen if I can remember to “Handle With Care” when extending/collapsing it.  We’ll see.

Ready-to-go

OK, so using the 18 inch antenna shaft attached to the magnet mount, then the coupling nut on the MFJ antenna, I extended it to a total of about 13 feet.  With the DX Engineering Pre-amp turned on, and using the SDR Play RSP2, I was getting many signals booming in. All the usual names we are familiar with – RMI, CRI, Turkey, Cuba, etc.  But also the noise level was very high. I know it is summer but I may have been overloading the Pre-amp a little bit. Here is an example, Radio Progresso from Cuba with some very nice acapella music but also a noisy background (plus, a noisy laptop computer pulse!):

Click here to download MP3 audio.

So I decided to come back in the morning before my workday started and see if the static crashes would have died down.

Preamp Angst

The next morning I had everything hooked up again in the same spot at the Forest Preserve (located in a suburb of Northern Illinois).  I moved the Cross Country Preselector to be directly connected from the roof, then to the antenna switch on the “Breadboard” (see part 2) to better prevent overloading.  I turned on the Verizon battery pack and nothing. No Pre-amp light. Switched it on, off, on, off – nothing. So, I thought I must have burnt it out the previous session?

Later on, I found it was some sort of short in the switch and I will have to move the D-cell batteries to a backup battery pack. In the meantime, I had to do without the Pre-amp and was forced to extend the MFJ antenna all the way.  With the 18 inch extension attached to the magnet mount, that was a total of 18.5 feet from antenna tip to the top of my car roof.

This was actually fortuitous since I was already concerned about overloading the Pre-amp or perhaps amplifying background noise.  This forced me to test it in a more “barefoot” manner, hearing what it would natively hear without any Pre-amp. It was also lucky there was no wind to blow it over!  It seems that if one is in an RFI-quiet area with decent view of horizons, the 20+dB Pre-amp may not be needed, depending on frequency band involved.

I have read that “Norton” style 10 dB Pre-amps and custom handmade transformer baluns are used by Dr. Dallas Lankford in his Low Noise Vertical antennas.  I don’t want to get into winding baluns so I am using one Palomar Longwire Balun to simulate the “magnetic” transfer. His design uses two, one 10:1 at the antenna and a 1:1 balun at the feedline into the house.  For more reading on LNV antennas, see these references:

UNAMPed Results

I purposely monitored Voice of Korea for their news statement on the De-Nuke talks on the 25 meter band and found it came in great, just as many others have heard it.  This was encouraging. Examining carefully the Data file from the SDR, here is what I pulled from it. I am pleasantly surprised and happy with the results; some stations I had never heard before and the language and music are very exotic.  All of it was a little more than one half hour of recording time (14 June 2018, 1300 UTC). You may have to crank up the volume on the weaker recordings to hear those properly.

Recordings

(Station, Frequency, Language(s), Transmitter site from www.short-wave.info):

Voice of Vietnam, 12020 kHz, English, Hanoi Vietnam (with local UFO noises near me)

Click here to download MP3 audio.

HCJB Beyond Australia-India, 11750 kHz, Nepali, Kununurra OZ

Click here to download MP3 audio.

Veritas, 11850 kHz, Vietnamese, Quezon City Philippines

Click here to download MP3 audio.

VOA, 11695 kHz, Cambodian, Tinang Philippines

Click here to download MP3 audio.

KCBS, 11680 kHz, Korean, Kanggye North Korea

Click here to download MP3 audio.

CRI, 11650 kHz, Esperanto (they get PAID to speak Esperanto!), Beijing China

Click here to download MP3 audio.

Taiwan International, 11640 kHz, Chinese, Kouhu Taiwan (blasting in strongly plus strong echo of broadcast at top of the hour – is a second transmitter signal going around the earth the other way and getting to me??)

Click here to download MP3 audio.

FEBA India, 11580 kHz, Malayalam scheduled but announcer says “Kannada”, Trincomalee Sri Lanka

Click here to download MP3 audio.

Radio Free Asia, 11540 kHz, Tibetan, Tinian Island signoff and transition to Kuwait (very faint)

Click here to download MP3 audio.

BBC, 12065 kHz, English, Kranji Singapore (ETWN not on air to mask this)

Click here to download MP3 audio.

Japan NHK, 11740 kHz, Thai, Kranji Singapore

Click here to download MP3 audio.

CRI, 11910 kHz, Amoy signoff transition directly to English, Beijing China

Click here to download MP3 audio.

FEBC, 12095 kHz, Hmong signoff transition directly to Khmu, Bocaue Philippines

Click here to download MP3 audio.

Radio Free Asia, 12140 kHz, Cambodian signoff transition directly to Burmese, Saipan

Click here to download MP3 audio.

Extreme Loading

Eighteen feet of whippy rod can sway in the gentlest breeze (consequences of “bigger must be better”).  The described setup has fallen over in as little as a 12 mph sustained wind when fully extended because I had the base in a plastic box.  I want plastic under the magnet(s) in order to get it off easily and put away out of sight!  Now installed is a larger QUAD magnet mount for better stability:

ProComm PCTM54 Quadruple Mag Mount

I am using the flat plastic lid from a 20 gallon tote container under the quad mount and a mover’s tie down strap to the main bar of the quad (I have room for multiple straps if needed). Ten foot fits just fine:

Erickson 34415 Black Retractable Ratchet Straps 2 Pack

A spring is attached to the base as well (strongest one I could find):

Hustler SSM-3 Super Heavy Duty Spring

Finally, the connecting stainless steel shaft at the base is a 5 inch Wilson 305-5 stainless steel shaft.

Because the backpack and quad mount can fit inside the 20 gallon tote container, this setup can be attached to a picnic table in a state park or campsite if I choose.  The Firestik single magnet mount will be recycled as a VHF antenna mount. I can go virtually anywhere now.

Instead of the 20+dB DX Engineering Pre-amp, perhaps one of those “Norton” 10 dB Pre-amps might be optimal (Kiwaelectronics.com broadband-preamp).  And I need to figure out why my Verizon battery pack failed as each Tenergy D cell measured fine.  Oh yeah, I have to buy an extra coupling nut, too……

Happy Listening,

TomL


Thanks so much for sharing this latest iteration of the BackPack Shack 3.0, Tom! It seems to me, as you imply, your current setup could be installed pretty much anywhere. 

I’m sorry to hear about your troubles with MFJ. I’ve only had good experiences with them in the past, but I suspect the specs on the 3/8”-24 thread were simply incorrect or perhaps metric and mislabeled.

Post readers: Read Tom’s past contributions and articles by clicking here

Click here to read Backpack Shack 3.0 – Part 1 and Part 2.

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Tom’s field portable HF antenna snags VOK’s summit broadcast

Tom’s field portable car roof HF antenna.

Many thanks to SWLing Post contributor, TomL, who shares a recording he made of the Voice of Korea on June 14, 2018. This English broadcast focuses on the Singapore summit and is, no doubt, historic in its content. [Note that we’ve posted other recordings on the Shortwave Radio Audio Archive.]

TomL notes:

Recorded on my noisy Lenovo laptop, SDRPlay RSP2, and an unamplified 18.5 foot antenna on the roof of my SUV.

I’m most impressed with the quality of his recording–VOK is not the easiest station to snag in the US midwest:

Click here to download the audio recording.

Thank you for sharing, Tom! I love your field portable vertical–obviously, it’s doing a fine job and your car must make for a decent ground plane!

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Guest Post: Backpack Shack 3.0 – Part 2

Many thanks to SWLing Post contributor, TomL, who shares the following guest post as a his Backpack Shack 3 evolves:


Backpack Shack 3 – Part 2

by TomL

Wanting MOAR options for my recent amplified whip antenna experiment, I decided to add a second antenna input to the kitchen cutting board (can I call it a “Breadboard”? – Ha, that’s an electronics joke!).  The idea behind it came from realizing that I might not want to spend all my time outside at a picnic table or on the beach, especially if it is drizzly and windy. And I still wanted a better ground for the antenna.  So, I thought I could use more Trucker Parts and put an antenna on top of the roof of my vehicle so I could listen in the relative comfort and safety of my small SUV (or even a friend’s car).

Breadboarding

Here is the crowded “Breadboard” with some extra items added.

I thought of the vertical antenna as a short longwire and had an old, original RF Systems Magnetic Longwire Balun.  That device allows for an improvement in signal/noise ratio (in theory) if used on a longwire. Perhaps it works on this, too??  You can see the gray cylinder connected right beneath the trucker mirror mount on the left (this will not be tested at this time, see External Antenna below).  The output goes to a greenish Daiwa switch on the right.

A large amplified antenna has the real possibility of overloading the amplifier.  With the Magnetic Balun, I am hoping the VHF band is attenuated enough to preclude any problems because its response naturally tapers off past 40MHz.  But Mediumwave is well within its bandpass. I remembered an old Kiwa Electronics Broadcast Band Rejection filter not being used for a long, long time, so I connected that right after the Daiwa switch (the metal box with red plate).

This output then goes to an RF choke just before entering the pre-amp.  I figure I will be using my SDRPlay RSP2 and noisy laptop and wanted to try to reduce any interference traveling on the outside coax braid before it gets amplified.

External Antenna

OK, now for the other Daiwa switch selection.  The external antenna will be connected and disconnected as often as I use it.  I attached two right-angle coax adapters to be the connection point for the antenna.   This is so that the physical switch threads do not have to handle that wear-and-tear. You can see it as the fuzzy out of focus thing sticking up out of the left side switch position.

The wire going out the top of the Breadboard goes to a Firestik K-11 magnetic mount placed on top of the roof of the SUV.  I also wanted this to be connected to a magnetic balun. I just happened to have a nearly unused Palomar Engineers Magnetic Longwire Balun.  It has its own ground lug for use with a counterpoise. Temporarily, I left the 18 feet of wire that came attached to the K-11 Mount and attached an adapter and BNC test lead; on the other end is connected the spade lugs to the Balun (red wire to the lanyard nut, black wire to the ground lug).  It all fits neatly inside a Sistema 3 liter container.

The magnet and box self-clamp easily onto the roof of the vehicle.  I added a new 18 inch section to the Trucker Antenna Shafts creating a full 72 inch antenna, complete with mag mount, ground plane (car body), and magnetic balun.  It is very easy to put up and take down and the box helps keep everything contained.

I am pushing things a bit here.  Magnetic Baluns are not really meant to be used on vertical antennas.  It probably breaks some sort of Cosmic Electrical Law somewhere that causes electromagnetic waves to get very confused and die a horrible, twisted, circular death.  But I figure that it is an unbalanced “line” similar to a longwire antenna; it’s just a little short and goes straight up instead of horizontal! I like the idea, so I am going to run with it.

Warnings!

It goes without saying that the Antenna Shafts, magnet mount, and magnetic balun are weatherproof (but NOT lightning proof!).  Take proper lightning precautions and take it down.  Even so, I might add a small drainage hole to the box since it did rain a tiny bit during testing.

Secondly, this setup is ONLY FOR STATIONARY VEHICLES!!!!  DO NOT TRY TO DRIVE DOWN THE ROAD OR HIGHWAY!!!! The magnetic mount will NOT stay on the car and will damage your vehicle and maybe a vehicle travelling next to you!

Performance

As you can see from the picture, my new Tecsun S-8800 is getting a workout while connected to the Cross Country preselector (not shown behind it) and to the backpack next to the back seat window.  The Tecsun S-8800 is a nice radio. My copy has a couple of quirks that I might have to send it back (the AM band tunes incorrectly 2 kHz lower than it should and the SW SSB tunes 140 Hz higher than indicated and I have to compensate using the fine-tuning dial for these modes–FM seems correctly tuned).

Other than this, the actual performance is really quite good!  DSP does have sharp cutoffs to the IF bandwidth (especially resolving strong station interference when selecting 3 kHz vs. 4 kHz).  With all my filters/balun/choke, I did not notice any MW or FM breakthroughs and signals on those bands were nicely contained and “normal”.  Interference from my cell phone while looking up internet frequency listings was minimal – seems like the cable shielding, choke, and car roof are doing a good job.

The audio output jacks have very thin clearance between the jacks and the housing of the radio. So for the second time, I will not have recordings since the cable I wanted to use has home theater style construction with very thick plug outer connectors and will not fit!

From an RF-quiet “Forest Preserve” (County park), there were a variety of stations received from the 25 through 19 meter bands (Local time 11am-1:30pm). A few stations I have never heard before until now:

  • Radio Free Asia in Korean on 11985 KHz (Tinian Island)
  • Radio China International in Esperanto on 11650 KHz (Xian China)
  • Radio Farda in Persian on 12005 KHz (Wooferton England) – broadcast opposite my direction
  • Radio Bible BCI in Somali on 15310 KHz (Nauen Germany) – Strange sounding but interesting  Christian Somali music
  • Radio Free Asia in Chinese on 13675 KHz (Dushanbe Tajikistan)
  • Voice of Hope Africa in English on 13680 KHz (Lusaka Zambia) – had to use ECSS USB to get away from strong interference from RFA on 13675, fairly good intelligibility (including music)!  I wish there was a 6 KHz option for SSB mode since the audio was slightly muffled and could not compensate much with the tone controls. That kind of feature usually comes with radios costing 3X more, however.
  • Voice of Korea in French (Kujang North Korea) being squashed by Radio China International (Kashi China) in English on 13760 KHz

It was so nice not to be on a beach and have people walk by STARING at me with my weird radio/antenna setup.  And I was dry and comfy sipping a cool drink while there was a drizzle of rain pelting the windshield. Downside might be that the car setup cannot always be located optimally if I want to be next to that Very Large Body of Water (Lake Michigan) to help enhance reception but this is not a bad alternative.   The next test will have to be during early evening when signals are booming into my location and see if performance holds up under those conditions!

Happy Listening,

TomL

Modified Parts List

Parts Repeated from Part 1 article


As always, Tom, a most impressive setup powered by home-grown ingenuity! Thanks so much for sharing the evolution of your field kits with all of us here at the SWLing Post!

By the way, you still have me chuckling about your use of the term, “breadboard!” 🙂

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