Category Archives: How To

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

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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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Guest Post: Backpack Shack 3.0 – Part 2

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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

Firestik K-11 Magnetic Mount
Palomar Engineers Magnetic Longwire Balun
Sistema 3 liter food container
12 foot LMR-240 UltraFlex coax assembled by sagemaiden
Daiwa 2-position coax switch
Kiwa Electronics Broadcast Band Rejection filter
RF Systems Magnetic Longwire Balun (discontinued)
Hard Case for Tecsun S-8800 – (This perfectly-fit case for the Tecsun S-8800 will NOT fit into the backpack, so I carry it separately)
Vic Firth Marching Snare Bag Pair (to safely hold the Trucker Antenna Shafts)
TDK 13mm Clamp-style ferrite for RF choke
2 right-angle UHF coax adapters

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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Video: N1SPY builds and describes the components of a simple receiver

Guest Post: Tom’s Backpack Shack 3.0

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Many thanks to SWLing Post contributor, TomL, who shares the following guest post:

Backpack Shack 3 – Amplified Whip Antenna

by TomL

So, having enjoyed using the Ferrite Sleeve Loop I created last year, I have wanted something a little more sensitive and less bulky. I will eventually create a much BIGGER FSL antenna on the order of 2 feet long and perhaps 18 or 24 inches in diameter for indoor/attic use. But that is not a priority at the moment.

Since I already have the DX Engineering Pre-Amplifier and the very nice Cross Country Preselector from the loop project, I thought it might be useful to create an active whip antenna for it. And the cool looking Solar Red backpack needed something to do!

Power

Now that the bulky loop was not taking up the main compartment of the backpack, I could think about what else to put in there, like a larger power pack. I scoured FleaBay for ideas and stumbled upon this contraption for backup power to network systems, the CyberPower CyberShield for Verizon.

This has 12 spaces for D-cell batteries and was mounted inside the demarcation terminal to provide backup power for things like cable systems and Copper-to-Ethernet networks. It is not waterproof, so would be inside the premises of the customer getting the internet/cable service. But my Pre-Amp needs 12-18Volts and would love to have nearly unlimited power. So, I bought a used one, cut the end off of the power lead and put on my own 2.1×5.5mm plug (carefully glued down and tie-wrapped). Then I filled it with 1.2Volt Tenergy D-cells.

Everything was just fine until I forgot to double check the polarity of the plug that I had wired onto the end. Plugged it into the DX Engineering Pre-Amp, flipped the power switch and fitzzz…. The Pre-Amp light went on, then off (permanently!).

So, my expensive mistake is that I start using the FREE multimeter I got from Harbor Freight and check the polarity before I connect homemade battery packs to anything!!

DX Engineering charged me $60 to fix my mistake and it is working fine now after I swapped the wires on the plug. Yes, their Pre-Amp is NOT reverse-polarity protected! Disappointing, since the price tag for that device is $148!!! The CyberShield now sits comfortably inside the bottom of the backpack.

Antenna

Now that the drama was over regarding the Power pack, I could think about the whip. I did not want a wimpy whip! (No one should rightly aspire to this, in my opinion). More FleaBay searches found me looking at Trucker parts. Loaded whips, magnetic mounts, 10 foot tall MFJ telescoping whips, etc was looking a bit expensive.

Besides that, I cannot fit a 10 foot tall telescoping whip into the backpack, I am limited to at most 18 inches (and that is at an angle to fit it in there). But I found an old-fashioned mirror mount that looked promising since it had a nice SO-239 connector at the bottom and standard CB antenna fitting on top of 3/8”-24.

Then I found the 44 inch SuperAntenna with the same threads; then found the replacement Stainless Steel Shafts for a Wilson antenna in different lengths (I ordered the 10 inch version to test). With a couple of rod coupling nuts and I was ready for testing!

Test Locations

I had already scheduled a short vacation to Sleeping Bear Dunes on the thumb of Northwestern Michigan, so I took this test setup with my Sony ICF-2010. This area is a very nice remote National Lakeshore with minimal noise. I tried a beach setting and a couple of hilltop picnic areas (including meeting a local Porcupine) and had very nice reception at all locations. The hilltop locations are approximately 400 – 600 feet above the Lake (yes, the Dunes are THAT big there!).

Meeting a local Porcupine

Later on, I went to Grand Haven, MI on the way home and stopped at their very lovely beach.

Reception was just as good as the hilltop locations at Sleeping Bear! In both areas, I was next to a large body of water (in this case, Lake Michigan) and makes for an advantageous place for DXing! I had also stopped at a Rest Area off the highway and that was a terrible place even though it was electrically quiet but nowhere near the big Lake. I guess the rumors are true about being near a large body of water somehow enhances reception of weak signals–?

I will submit recordings later since I lost the mini-B cable for the Sony digital recorder and had to order a replacement. However, this was a nice project that freed up some space inside the backpack. I will add an 18 inch extension to the whip that will give me a total length of 72 inches. Plus, it is mounted 12 inches up on the poly cutting board and I place the backpack on a small hunters folding chair that is about 24 inches tall. So, the tip will be about 9 feet off the ground.

Not pictured but I was also able to easily fit inside a used CCrane Twin Coil Ferrite antenna for mediumwave use that also performed very well. I noticed that the picnic benches at some locations are made of metal, so that gives me a future idea of trying to leverage that to use as a ground plane somehow. The battery pack is heavy but also gives great ballast to the backpack and will not tip over. Cannot wait for the Tecsun S-8800 to arrive so I can try leaving the radio inside the bag and just use the remote control to tune!

Happy Listening,

TomL

Parts List

As always, I’m so impressed with your spirit of radio adventure, Tom! I love the fact that your goal is to make a field-deployable DX kit that isn’t cumbersome or time-consuming to set up on site. I imagine you only need a couple of minutes to open the pack and have it on the air.

Those DXing spots are stunning! I had no idea one could find 400-600′ dunes in NW Michigan–! With that said, I’ve heard that part of the state is one of exceptional natural beauty. If you could somehow turn the lake into a body of salt water–thus increasing ground conductivity–you’d really enhance that already impressive reception! I’m guessing that sort of project would be a bit outside your budget! Ha ha! That and the freshwater fish might protest!

To me, there is no better way to enjoy radio than finding a nice RF quiet spot in the great outdoors…no matter where you live in the world. On top of that, Tom, you’re constantly building, experimenting, documenting and sharing your findings–you’re a true radio zealot! Huzzah!

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

Click here to read Backpack Shack 3 – Part 2.

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How to build SM0VPO’s 20 meter magnetic loop antenna

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The completed 20 meter loop antenna.

A few days ago, SWLing Post contributor, Robert Gulley (AK3Q), pointed me to an excellent website by Harry Lythall (SM0VPO) which is chock-full of various homebrew radio projects. In particular, we both were impressed with Harry’s 20 Meter Loop Antenna–it’s such a simple project and requires no special order components. In fact, all of the components (save, perhaps, the antenna connector) can be purchased at a DIY store.

I reached out to Harry and he has kindly allowed me to republish this project as a guest post:

20m Loop Antenna

by Harry Lythall – SM0VPO

Introduction

I recently saw that my 80m (3.5MHz) loop (or frame) antenna has been really popular, and that there are loads of other radio amateurs who have taken my design and “ran with it” to produce variations that all have some great improvement. There have been many in-depth tests and simulations, all with exceedingly good results and reports. This is exactly what I am aiming for with my homepages – free information for all and my designs being improved upon. That way we all win 🙂

One small point all variations have in common is the need for an expensive tuning capacitor and a very restricted RF power level. Of course, you can throw money at the problem, but for me this hurts. I got to thinking that there must be a way of adjusting the design a little and finding another technique to tune the antenna, and to make the best use of the little radio transceiver I have in Sweden, given the limited space.

My limited space apartment.

As you can see there is not much opportunity for grand antennas. And to add to this, the equipment I have in Sweden is also limited to a single 5-Watt unit.

My limited equipment – only 5-Watts.

The Design Thoughts

Today I have no area of land to use for antennas. I have a glassed-in balcony on the 4th floor of an apartment block. I really like the 20m (14MHz) band so I will concentrate on that. I am not really interested in the CW end of the band, except perhaps 14.070MHz for the digimodes. So my requirements are:

As efficient as possible (useable)
Small size, also portable so I can use it for field use
No expensive components, everything available locally
No TVI, QRM or interference to stereos or computer sound
Total price less than $2

The antenna I have created is based on my original 3.5MHz loop (or frame) antenna. This time I built it out of scrap components. I cadged (tiggade) some plastic conduit tubes from an electrical contractor at work. The same guy also gave me the remnants of a roll of 2.5mm C.S.A. multi-strand mains cable. That was all I needed.In my junkbox I found no tuning capacitors, but WAIT!! Why do I need to tune the antenna? Once it is tuned I should not need to tune it again, just set the centre-frequency to 14.175MHz. If I can get the Q-factor to around 100 then my useable 3dB bandwidth should be more than 150kHz. That will give me 14.10MHz to 14.25MHz.

Ok, I need a 1-off, preset tuning capacitor. Why not use a Gimmik Capacitor? Just twist two bits of wire together and cut it short to get the resonant frequency I want. So I need to get the coil wound so that there is sufficient cable length and self capacitance to give a resonance of about 14.5MHz without any extra capacitance what-so-ever. That means I need just a few pf. That sounds like a good plan.

Construction

The 15mm Diameter plastic tube I “aquired” were 80cm long. After much trial and error I found that exactly 3 turns, with 2.5cm spacing, gives about 14.9MHz self resonance. The wire support holes are exactly 4cm spaced, beginning 1cm from the end of each tube. The two tubes are fixed into a and X using zip-straps (tie-wraps, buntband). The feed loop is 1/2 turn.

Note the size and position of the feed loop. Also the Gimmik capacitor.

One problem I had with the original loop antenna was that of RF coming back down the cable braid. Using on old FT-101ZD it was possible to feel the RF on the microphone with your lips. The cure for this is to use a balanced feed and at least 5m of RF cable.

I robbed the ferrite ring for the balun from an old ATX computer PSU and made a triflar wound torroidal transformer. That is to say, twist together three lengths of 1mm x 7-strand insulated hookup wire together. Use this to make a 7-turn coil and connect the three coils in series, with four connections. Feed connections (numbered in the picture below) 1 and 3 are connected to to the antenna feed loop. Connect the coaxial cable braid to connection 2, and the coax centre to connection 4. My balun is self-supported on the connection leads.

The 1:1 Balun I used.

The coaxial feed cable was found to affect the resonance slightly, so I fed that through an extra bit of tube to make it stay in one place. It works fine.

Feeder cable secured in the support tube.

Testing

Testing is very easy. I used my GDO-2 to check the middle-turn of the loop for a dip. Twist the two tails together to form the Gimmik capacitor and adjust the length of the twist until the centre-frequency is 14.175MHz. With the GDO you can get it within about 100kHz to 200kHz, but then you can check the VSWR using your HF radio. You can also sweep the band for maximum noise and get a very close approximation.

The Gimmick capacitor.

The centre frequency of my 20m Loop antenna is 14.175MHz, and the VSWR is better than 1.05:1 (I can hardly see any movement on my meter). The Q-factor is somewhere approaching 100. The useable bandwidth is just a little narrower than I would have wished, but the antenna certainly works well and meets all the other criteria. But the slightly less useable bandwidth criterion is at the expense of better performance, and it still allows me to use 14.070MHz, although it is a little quieter down there.

The completed antenna.

Conclusion

No-matter how you play with the figures, the best indoor antenna cannot replace a full-size dipole antenna. But the indoor antenna can give some extra features, such as just reaching out your arm and trimming a little, which you cannot do with a long-wire antenna up a tree out in the garden, especially when it is raining.

This antenna gets me on the air on 14MHz, and it has a useable frequency range. The VSWR is almost perfect at the centre-frequency, and this time I don’t burn my lips on the microphone (not that I am likely to do so with just 5-Watts of power). The design uses no expensive components, in fact the only item I bought was the block-connector for the balun. That cost me US$1.50 for a pair of 12-contact screw-terminals. The construction is ridiculously simple and easy to build.

On the air I can hear traffic on 14.070 digimodes, and from 14.130 to 14.220MHz I have a near-perfect VSWR aqnd good clear reception of SSB. I can also rotate the antenna to cut out rubbish, and most of all, using the Gimmik capacitor I don’t need to re-tune it: it seems temperatore-stable. The weight is less than 500g and when I poke it out of the balcony window the reception improves, the VSWR does not change, and I can make myself heard among the big boys.

I hope that you have some fun building and using this antenna. If you have any ideas for further improving it then please use my forum.

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.

Many thanks, Harry, for sharing this excellent project on the SWLing Post!

Side note: I’m impressed with the fact that the main SM0VPO website is actually hosted on a bedside Raspberry Pi computer (running the Linux-based Lighttp server). Very cool!

Check out other homebrew mag loop antenna projects on the SWLing Post by clicking here.

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