Category Archives: How To

Build an SDR station and balcony antenna farm for less than 150 Euros

UPDATE 11 May 2020: We recently learned that the MSI.SDR software defined radio dongle in the following post and tutorial is a clone of the SDRplay RSP1 SDR. We did not realize this when the post was published. Grayhat had done research prior to purchase and believed it not to be a clone, but only using the same chipset as the RSP1 (hence the compatibility with SDRuno). We have confirmed that it is indeed a clone now via SDRplay (clear here to read more via the excellent RTL-SDR blog). What follows isn’t an SDR review. Indeed, Grahat’s post has little to do with the receiver and much, much more to do with building proper antennas! We’ve removed links to the MSI.SDR and would encourage you to invest in the excellent SDRplay RSP1a instead (click here to read our RSP1a review). Thank you for understanding!

Many thanks to SWLing Post contributor, Grayhat, who shares the following guest post. He lives in Italy and has been in lock-down since the beginning of the pandemic. He pitched the idea of building an entire SDR setup from scratch–receiver and antennas–for less than 150 Euros (roughly $163 USD). I thought it was a brilliant idea and I believe he thoroughly enjoyed the challenge of sourcing the components and building a mini antenna farm on his balcony while in quarantine:

From Zero to SDR

by Grayhat

What follows doesn’t pretend to be some kind of “definitive guide” or “last word”, on the contrary, it’s aimed at people who have little or no experience with SDR but want to try putting together a decent station without paying an arm and a leg.

The idea of writing this came to me after reading a number of messages and discussions on various online groups/forums, in a lot of cases, someone bought an SDR (usually the ones coming with a telescopic whip antenna), and after connecting it was expecting it “just to work” or, better said, pretending that the SDR connected to that whip (usually placed on a table right near the computer) could receive ANY POSSIBLE signal, including transmissions coming from the “dark side of the moon.” Those folks got scared by the fact that the SDR “didn’t work” and decided to give up; now, this short “guide” should allow anyone to setup what’s needed to have a working SDR

My self-imposed limitations for this project/experiment were the following:

The whole setup shouldn’t cost more than 150 Euros so that, if after trying the SDR one doesn’t like it, (s)he won’t have paid $$$, otherwise, if (s)he decides to keep it, the resulting station will allow for further expansion/improvement
The available space was considered to be that of an apartment, that is, no large field to put up huge wire antennas or to raise towers, the limit was the one of a balcony (in my home) that is 8 meters (max antenna length) by 3 meters (available height) by 2 meters (balcony width)
The whole setup should be simple and straightforward, no need to solder components or to build special types of antennas
Given the current Covid-19 sheltering, most components should be available online, while for others one may arrange with whatever is locally available (e.g. duct tape)

With the above limitations in mind, I took pencil, paper and rubber eraser (high-tech instruments, indeed) and started writing down a list of the needed stuff, after some writing, wiping and second thought, I came out with the following list, available on Amazon:

Bill of materials

Insulated wire (100mt x 2.5mm)
Paracord (100mt x 3mm)
Nylon cable tiers (100 pcs x 2.5mm x 100 mm)
Waterproof electrical junction box
NooElec Balun 1:9 V2
iChoose Ltd RG58+BNC coax, 15mts
BNC female to SMA male adapter (2 pcs)
Snap-on ferrite chokes for 9mm cables (10 pcs)
MSI.SDR 12 bit ADC was used for this experiment before realizing it is a clone of the SDRplay RSP1. We recommend the RSP1a as an upgrade.

The above includes all the needed stuff to put together a number of wire antennas (random wire, random dipole, loop…) the coax to connect the SDR, a balun to match the coax to the antenna and the accessory parts needed to put up the antenna. The selected SDR isn’t the common “RTL SDR” type, not that they don’t work, but their 8 bit ADC is far from being a good performer, so I decided to pick a different SDR which offers a 12 bits ADC and which also “presents itself” to the system as an SDRplay RSP1.

[Please note: we’ve since learned from SDRplay that the MSI.SDR is indeed a clone of the SDRplay RSP1. Here’s a post from the RTL-SDR blog confirming this. We recommend purchasing the RSP1a as a better alternative.]

Anyhow; all I can say is that after some tests, the MSI.SDR is a quite good unit and offers quite a lot of bangs for the buck, so I believe it may be a good unit for people willing to get their feet wet with SDRs

The above being said, here’s a pic of the MSI.SDR unit with the included stuff:

The unit is very small and the box has two connectors, an SMA for the antenna and a micro-USB (like the ones used in cellphones) for the USB cable which is used to both power and control it; the other bits are the telescopic whip antenna (around 98cm fully extended) with a magnetic base and a short run of coax, and the USB cable.

Once I got the SDR I decided to give the included whip antenna a try… well, to be clear, while it will allow you to pick up some strong local FM stations and maybe a bit else, it will only be useful to test if the SDR unit is working (before putting together our antenna), so don’t expect to receive much with that whip, yet… don’t throw it away, it may become useful (more later).

The other important piece is the BalUn. I picked a NooElec “Nine to One” v2, since I’ve used their v1 model and I’ve found it to work well, I decided to pick the newest model which has a better antenna wire connector. The BalUn, which is in effect a so-called “transformer balun” is really small and the junction box I bought is much bigger, but it isn’t a problem. All in all, the box may host a preamplifier in the future, but for the moment it’s fine for the balun. The following pic shows the balun “installed” inside the junction box:

The scissors are there to give you an idea of the sizes; to put together the whole thing, I started by preparing two pieces of wire (the 2.5mm one), made a turn with each wire and locked them with a nylon cable tier. Those turns will prevent the wire from sliding out and putting a strain on the balun connector. I did that since I didn’t have plastic washers at hand, otherwise you may just slide two plastic washers (or proper diameter) over the wires and use two nylon tiers to lock them. In either case, the idea is that the “loops” or the washers won’t slide through the box holes and will support that (little bit of) strain caused by the wire connection.

Next, I stripped some of the insulation from the ends and connected the wires to one of the balun connectors (I chose the one in the pic since I believe it’s the most suitable for this setup), at that point I continued cutting the smaller “ring” of the box insulation caps (the two at top and the bottom one). Then I placed a piece of carboard roll (from a kitchen-paper roll) at the bottom to serve as a support (you can see it below the balun). At that point, I slid the balun SMA connector through the bottom hole and used the SMA to BNC adapter to hold it, done so I slid the two wires (connected to the green wire connector) through the side hole and then inserted the connector into the balun. I then placed the other piece of paper roll above the balun and closed the box with its cap. As a note, to properly close it, start by inserting the screw into the cap holes till end, so that they’ll extrude from the bottom, then place the cap over the box and tighten the screws–you may need to use some force to properly tighten it.

Notice that the wire shown in the pic are SHORT, later on I replaced them with longer wires (outside the box) to be able to better connect the balun box to the antenna, but the remainder of the build is the same.

Now that I had my “balun box” ready, I measured the antenna wire and, using the paracord and some nylon tiers, I installed it. I also installed the “counterpoise” wire. For the latter, at first I tried just connecting the remainder of wire to the “gnd” of the balun, leaving the spool laying on the floor, but later on I decided to hang up the counterpoise and the final result was the following:

Click to enlarge

Not a work of art, but then since I was experimenting, I decided not to add PTFE and tape to allow me to quickly rearrange the antenna to run other configurations, yet, the whole setup worked quite well and stood fine to some wind and rain, the picture below shows the balun box with the antenna/counterpoise wires and the coax with the snap-on ferrite chokes.

Click to enlarge

Notice that to avoid putting strain on the balun wires, I used a wire clamp I had in my junkbox–the clamp is then tied to the paracord using a nylon tier and the paracord holds the assembly and keeps the antenna wire in position. The ferrite chokes aren’t properly seated, and I’m planning to remove and re-place them, but for the moment they’re okay. The balcony faces to south/south-west so the antenna has a free horizon of about 270 degrees ranging from the Adriatic coast to the Appennines (Mt. S.Vicino can be seen behind the paracord)–not bad. Here’s another pic showing the horizon to West, just to give you an idea:

Getting back to the antenna installation, the other end of the antenna wire is tied to the opposite side of the balcony as shown below (let aside the tent/awning, I raise them when using the SDR, also, the bowline knot isn’t correct, I’ll need to tie that again):

The counterpoise instead is supported by a lamp I’ve on the terrace, here’s it’s setup:

The “paracord” goes down to a plastic bottle filled with a water/chlorine mixture which serves to keep it in place. The remainder of the wire is just hanging down for about 1.5 meters (the counterpoise is shorter than the antenna wire, it’s about 2/3 of its length).

Ok, time to put the antenna and SDR to test, so I brought the coax inside home, connected the other SMA to BNC adapter to the SDR and connected the coax going to the antenna. Note that 15 meters of coax is enough for me, but if one wants a length of up to 25 mt, it won’t be a problem.

I already installed the SDR software, in my case since the unit identifies itself as an “SDR1” I downloaded the SDRPlay “SDRuno” software https://sdrplay.com/windl2.php and since I was at it I also downloaded the PDF manual https://www.sdrplay.com/downloads/ and the “CookBook” http://www.nn4f.com/SDRuno-cookbook.pdf and I heartly recommend reading and digesting them before starting the whole thing (while you wait for all the stuff to be delivered). An important note is that you MUST install the SDRuno software BEFORE connecting the SDR since that way, the SDRuno setup will install the proper drivers and you won’t have issues.

Anyhow, I connected the coax to the SDR and then it was time to fire up the whole thing and give it a spin; so I powered up the laptop (technically, a “transformable” laptop/tablet), started SDRuno, opened the “RX control” and “Main Spectrum” windows and then clicked the “play” button, clicked the “broadcast” band, and the “MW” one and got this:

Not exceptional maybe, but not bad, either; in particular if one considers that it’s from a quite short piece of wire which isn’t exactly placed in an ideal position.

Deutsche Welle

So I went on and explored the bands a bit. On ham bands the SDR picked up signals from the whole mediterranean basin (Cyprus, Lebanon, Spain and then some) and from north too (Russia, Germany, Denmark); then depending on time, I was able to clearly receive broadcasts from China, South America, Africa and more ham QSOs from a lot of places.

BBC Ascension Island 5/9+ and just a bit of QSB

I must admit I didn’ record the callsigns or stations identifiers (“guilty” your honor–!) but I was more focused on testing the SDR and antenna than running a “DX session” at any rate. On the BCB bands I picked up WWV, BBC, VoA, China Radio International, Radio Free Asia, Radio Romania and a bunch of others from Middle East, Asia, Africa and South America. While on the ham bands, I was able to pick up some quite interesting QSOs and then… well, I went hunting for higher frequencies signals!

I got Police, Ambulances, Air control…so even if that “piece of wire” isn’t optimized for VHF/UHF it seems to be working decently there too. By the way, when changing bands you may (and probably will) need to adjust the gain control, but that will be almost the only thing needed to pull in signals

At the end of the day, I can say that I’m quite pleased with the performance offered by this simple and cheap setup. For less than 150 euros you have everything you need, not just the SDR.

Sure, the setup may be improved, but then again you’ll have all of the basic parts, so you won’t need too much. For example, if you live in a really noisy environment, it would be a good idea to use a loop antenna. You would only need a “cross shaped” support (PVC pipes or wood will do). You could quickly put together the SRL (Small Receiving Loop) designed by Matt Roberts (KK5JY) http://www.kk5jy.net/rx-loop/ the balun will be the SAME (yes, no need to wind whatsoever!) so building it will just be a matter of assembling a cross shaped support for the wire (which we already have because it’s the same used for the wire antenna) and you’ll have it. While I already tried the SRL, I didn’t build one to test with this SDR, but I’ll probably do that as soon as SWMBO will start complaining about those “wires on the balcony.”

Also, at the beginning I wrote “more later” when writing about the telescopic whip included with the SDR. Here’s the idea–it requires soldering, so if you don’t want that, skip this: remove the adhesive sheet on the bottom of the antenna base to expose the bottom cap and then remove (extract) the bottom cap. You’ll see a magnetic ring and a “bell shaped” piece of metal (the “ground” for the whip). In the middle of the “bell” there will be the antenna connector which is soldered to the coax wire with a nut holding the connector (and the “bell”) in place. De-solder the coax, unscrew the antenna connector and extract it, at that point you’ll have the telescopic whip and its connector, now you may use them to build the active “whip” antenna described here:

http://www.techlib.com/electronics/antennas.html#Improved%20Active%20Antenna

Notice that it is NOT the “usual” active whip–the circuitry and idea behind it is totally different–yet it works fine and will serve you from VLF (not kidding) up to around 100MHz. It might be a good companion for the SDR. It won’t be as quiet as the loop, yet it may be a valid “all rounder.”

To conclude, I believe that the setup described above is something anyone can afford. You don’t need to be an engineer or to have special knowledge or abilities–it’s just a matter of putting together some bits and pieces.

Obviously, this setup doesn’t require a large space and offers good performance across the bands. Plus it’s so easy to improve since the 12bit SDR is a good starting point

All the best everyone and STAY HOME, STAY SAFE !

Thank you so much, Grayhat!

I love the fact that you invested (however modestly) in a proper antenna setup to better serve you rather than relying on the basic whip antenna that comes with the SDR. You’re right: too often, we invest a receiver, yet invest no money or time into building an appropriate antenna. The antenna is the most important component in a proper radio setup and those included telescoping whip antennas simply don’t perform well on the HF bands.

Based on our correspondence, I know you had fun piecing together this little system using a simple bill of materials and items you had on hand during the Covid-19 quarantine. Thank you for sharing it here with your SWLing Post community!

Patrizio’s simple homebrew NCPL antenna

Finding local Emergency Alert Stations in the US

4 Replies

Many thanks to SWLing Post contributor, Mario Filippi (N2HUN), who shares the following guest post:

Emergency Alert Stations: A great source of local information

by Mario Filippi

During the pandemic a source of local information for residents in certain areas of the country can be found on Emergency Advisory Radio stations that dot the country and provide 24/7 information pertinent to a community. Not all communities have these stations, which can be found from 1610 – 1710 kHz and operate at varying power outputs.

Author’s Yaesu FRG-100 tuned to EAS station

For example, a station I regularly hear is WRBX655 about 12 miles away in Franklin Township, NJ operating on 1630 kHz : https://www.franklintwpnj.org/Home/Components/News/News/6384/1130?cftype=News

At the moment it is broadcasting information on COVID-19 from the Center for Disease Control. Every EAS station has a call sign and wattage generally is from about 10 – 50 watts. However some stations do not necessarily announce their call signs so you can check theradiosource at: http://www.theradiosource.com/resources/stations-alert.htm

Now some of these stations are part of the HAR (Highway Advisory System) that broadcast on major roadways and usually have prominent road signs announcing where to tune your car’s AM radio for latest traffic conditions. These stations were also termed TIS (Traveler’s Information Stations) at one time and were the precursors of HAR. However, over the years the FCC allowed more leeway on what information could be broadcast and as a result these EAS stations appeared in communities and even state parks.

You can look up the locations of these stations to ascertain if one serves your community but the best way is to tune regularly from 1610 – 1710 kHz. The optimal time to listen is during daylight hours as propagation changes greatly after dark and you’ll hear commercial AM radio stations coming in and overpowering most EAS. As for range, I’ve heard HAR stations as far away as 40 miles depending on ground wave conditions which can vary greatly. QSB is common. Many of these stations will rebroadcast NWS weather information when no pertinent emergencies exist and that is another way to spot them. Some highway stations I’ve heard will begin each broadcast loop with a tone, they’re all different in their approach.

Attached [at the top of the page] is a picture of the author’s Yaesu FRG-100 tuned to WRBX655 from Franklin Township, New Jersey. For an antenna I’ve used a 31 foot vertical and a loop and success will depend on using an outdoor antenna but when away from the home QTH, I’ve heard many of these stations while traveling on the roadways of America, They’re a good break casual AM radio listening. Give it a try.

Thank you, Mario! I must admit that when I travel, I often hunt down EAS transmitters via my car’s AM radio. Besides being a good source of local information, I do know some DXers who’ve identified and logged an impressive number of distant stations when conditions were ideal.

If you live outside the US, do you have similar networks for local information? Please comment!

Bruce’s passion for SWLing and single transistor regenerative receivers

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Many thanks to SWLing Post contributor, Bruce (VE3EAR), who shares the following:

I live in the village of Saltford, ON, Canada, near the eastern shore of Lake Huron. It’s a quiet location signal wise, and I’m lucky that I have enough property to erect some big antennas. My two favourites are a 1200 foot long terminated Beverage, aimed at 50 degrees true, which targets Europe and the UK. The other is a 333 foot perimeter delta-loop, apex up and oriented north-south. Both antennas are fed with RG-6 coaxial cables and impedence-matching transformers.

I use the loop with a recently acquired Airspy HF+ Discovery SDR and the Gqrx SDR software, in my iMac. I like to listen to amateur activity on 160, 80, and 40 metres, along with the few shortwave broadcast station that are still on the air. I also like to listen to the trans-Atlantic air traffic control stations in Shannon, Ireland and Gander, Canada.

I once heard a U2 spy plane returning from a mission over Russia!

My other hobby is designing and building simple, one transistor regen receivers, most of which tune the AM broadcast band, although I have built a couple covering the lower portion of the HF broadcast bands as well, just for a challenge. All my receivers are built breadboard style.

My favourite of them is one based upon the Vackar oscillator, with the addition of a diode detector and “Benny”, as is used in crystal radios.

Here is the schematic of the Vackar circuit:

The diode and “Benny” connect to the collector of the transistor, then to a pair of home made headphones using two telephone earpiece elements installed in a pair of hearing protectors. The receiver is both very selective and very sensitive. Here is a pic:

Most of the electronics are on a proto-board, which allowed easy component substitutions during the build. When I had it optimized, I decided to leave it that way! The controls, left to right, are on-off switch, regen, fine tuning, main tuning, and range selector switch, hidden behind the reduction drive. Audio is taken from the DET OUT jack, to either the headphones described above, or to an audio amplifier for listening with a speaker.

Bruce, it sounds like you certainly have an excellent spot and excellent antennas for DXing! I love regen receivers as well and radio design can hardly be more simple.

Thank you for sharing!

More about hacking VGA cables to make binocular ferrite cores

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A few days ago, we posted an a short article showing how Oscar hacked a VGA cable to make a binocular ferrite core for his homebrew NCPL/Youloop antenna. Many thanks to SWLing Post contributor, Grayhat, who explored this clever hack a little further:

Hi Thomas, Having some time in my hands Sunday afternoon I decided to try pulling out the ferrite chokes from a VGA cable I had around, and while doing so, I decided to coarsely document the process with some pics.

Figure 1

The first thing to do is use a cutter to carefully cut around the “washer” shaped plastic at the connector end of the choke (fig.1, #1,#2, #3 above), then on the same side, after cutting the plastic also cut the inner conductors (fig.1, #1).

Move to the other side of the choke, gently cut around the “washer” w/o cutting the inner conductors, now pull the cable to extract it from the choke (fig.1, #3), repeat the process for the other choke.

Now look at the “cans” containing the chokes, one side of those will show a “cap” (fig.1, #4), insert a small screwdriver into the center hole and gently ply to one side to raise the cap and extract it (fig.2, #1).

Figure 2

The result will be as in fig.2, where #1 is the closing cap, #2 is the ferrite choke and #3 is the “can” containing the choke. Repeat the process and you’ll have two ferrite chokes as shown in fig.3 (the VGA connector is there to give an idea of the dimensions):

Figure 3

At this point, use some tape (duct tape will be a better idea, I used clear tape just to make an example) to tie the two ferrites together as in fig.4 and you’ll have your “binocular ferrite”:

Figure 4

Willing to use whatever you have there to wind the transformer, you may now extract the tiny insulated wires from the VGA cable (fig.1, #3, see wires) and use them for the windings.

Notice that other cables may use different choke “cans” which may need to cut a larger portion around the flat faces at the ends. But remember that in any case, those are just “snap-in” cans containing the ferrites, so with a bit of attention and patience, it shouldn’t be difficult extracting the ferrites.

Based on a little online research, it sounds like the ferrites used to choke the VGA cables (HDMI ones too) are generally type #31.

Looking at some #31 datasheets it appears that while #73 is works fine at frequencies below 50MHz, the #31 is best suited for the 1-300MHz range.

This means that #31 won’t be the best pick for mediumwave, although if one doesn’t have another choice… well, go for that! Also notice that the ferrite permeability is different:1500 for #31 and 2500 for #73. This means that we’ll need to increase the number of windings to achieve acceptable signal transfer, otherwise the transformer loss will make our antenna deaf.

One might try increasing the number of windings to say 8:8 or 16:16; as long as the winding
ratio will remain the same, there won’t be problems (although the resulting bandwidth will become narrower).

Thanks for documenting and sharing this, Grayhat! Since most of us have more time on our hands at home, I think it would be worth experimenting with the number of windings to see how it affects the antenna performance. That’s a clever thought, too, to use the VGA wires to wind the Balun. As long as the cable is long enough for the amount of turns, it’s certainly the most efficient use of resources!

DIY: How to build a Passive Resonant Transformer-Coupled Loop Antenna for HF reception

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We recently posted a tutorial on building a simple Noise-Cancelling Passive Loop (NCPL) antenna. This prompted SWLing Post contributor, Bob Colegrove, to share his excellent article on building a Passive, Resonant, Transformer-Coupled Loop (PRTCL) Antenna:

A Passive, Resonant, Transformer-Coupled Loop Antenna for Shortwave

By Bob Colegrove

Over the years I have resisted the level-of-effort necessary to construct and maintain outdoor antennas. Rather, I have focused on squeezing out all of the microvolts I could get inside the house. Many years ago I had access to a well-stocked engineering library, and used my advantage to gather information about the theory and development of loop antennas – a daunting undertaking for an English major. Ultimately, by adhering to a few basic rules, some of them dating back 100 years, I found quite acceptable performance can be had with an indoor passive antenna intersecting just a few square feet of electromagnetic energy.

Theory

There are a couple of advantages of resonant loops as opposed to non-resonant ones. The first is the fact that the signal dramatically increases when you reach the point of resonance. The second follows from the first in that resonance provides a natural bandpass which suppresses higher and lower frequencies. This gives the receiver a head start reducing intermodulation or other spurious responses. The downside of all this is that the resonant loop is, by design, a narrow-band antenna, which must be retuned every time the receiver frequency is changed by a few kHz. On the other hand, there is nothing quite as rewarding as the sight (S-meter) and sound you get when you peak up one of these antennas – you know when you are tuned in.

There is nothing new about the loop antenna described here. It’s just the distillation of the information I was able to collect and apply. There are a number of recurring points throughout the literature, one of which is the equation for “effective height” of a loop antenna. It basically comes down to the “NA product,” where N is the number of turns in the loop and A is the area they bound. In other words, provide the coil with as much inductance as possible. Unfortunately, for resonant loops, the maximum coil size diminishes with frequency.

With this limitation on inductance, the challenge becomes minimizing unusable capacitance in the resonant frequency formula in order to get the highest inductance-to-capacitance (L/C) ratio possible. Some of the unusable capacitance is built into the coil itself in the form of distributed capacitance, or self-capacitance between the coil turns. This cannot be totally eliminated, but can be minimized by winding the coil as a flat spiral rather than a solenoid, and keeping the turns well separated.

The second trick is with the variable capacitor. Even with the plates fully open, there is residual capacitance on the order of 10 to 20 picofarads which can’t be used for tuning purposes. A simple solution is to insert a capacitor in series, about ¼ the maximum value of the variable capacitor. This effectively decreases the minimum capacity and extends the upper frequency range. In order to restore the full operating range of the variable capacitor, the fixed capacitor can be bypassed with a ‘band switch.’ With the series capacitor shorted, the variable capacitor operates at its normal range and extends coverage to the lower frequencies.

Construction

I have constructed similar loops covering long wave, medium wave, and shortwave all the way up to about 23 MHz. I wanted to optimize this loop for the most active portion of the shortwave spectrum. Consequently, it covers approximately 2.6 to 12.3 MHz. See Figure 1.

Figure 1. A Passive, Resonant, Transformer-Coupled Loop Antenna for Shortwave

Figure 2 is a schematic diagram of the antenna. Cd (in red) is the distributed capacitance of the primary coil, L1. This is not tunable capacitance, but it still contributes to the resonance; likewise, the 15 pf minimum capacitance of C1. By adding C2, the minimum total capacitance can be lowered to greatly increase the upper range of the antenna. S1 is the ‘band switch.’ It shorts out the series capacitor, restoring the maximum low frequency.

Figure 2. Schematic Diagram

Frame – The frame is made from 3/8”-square basswood or poplar dowel (see Specialized Parts). Two pieces, each 36” long, have been predrilled at ½” intervals to accommodate the primary and secondary coil wire (think of a tennis racket). It is a good idea to drill holes along the length of each dowel – more than you will need. You may decide to change things later on, and drilling holes in an assembled antenna is not easy. Also the two dowels are notched in their centers to fit together. See Figure 3 and Figure 4. The clear plastic disk in Figure 4 is a packing disk from a spindle of CDs; it is cemented to the square dowels, and used to hold them at right angles. Any rigid, light-weight material will do.

Figure 3. Square Dowel Showing 1?2” Hole Spacing and Lacing of Secondary Coil

Figure 4. Cross Members Notched and Square Dowel Reinforcement

Primary Coil – With a coil size 36” in diameter, you likely won’t be able to get more than two turns of wire to resonate at frequencies up to 12 MHz. This takes into account the precautions described above to minimize unusable capacitance. AWG 22 stranded, insulated wire was used to lace this coil; ensure the dowels remain at right angles with one another. Note that one set of holes in the dowel is skipped between the first and second turn.

Tuning Capacitor – Almost any salvaged variable capacitor can be made to work. For a typical 2-gang unit, the gangs can be connected in series through the common rotor sections and metal frame with the stator terminals of each gang used as the outer terminals. This will create a lower minimum capacitance as described above.

For the antenna described here, a single-gang, 365-pf capacitor (see Specialized Parts) was used with a fixed mica capacitor in series. The minimum capacitance of the variable capacitor is nominally 15 pf. Figure 5 shows the capacitor assembly for the primary circuit. Components are mounted on a perforated circuit board, which, in turn, is mounted to the bottom of the vertical square dowel. A portion of the base can be seen at the rear. A large diameter tuning knob is suggested, as the peak tuning for a properly constructed loop will be very sharp and require a delicate touch. As an option, I have used a planetary reduction mechanism on other antennas to give an 8:1 ratio with the capacitor shaft.

You may notice at high frequencies that the antenna is somewhat unstable with body contact of the knob or around the tuning capacitor. This is because the resonant circuit is operating at a very high L/C ratio with capacitance at just a few picofarads. Body capacitance will tend to detune the antenna. It may be useful to extend the knob 2 or 3 inches from the tuning capacitor with an insulated shaft.

Figure 5. Capacitor Assembly

Secondary Coil – The secondary coil operates at low impedance to feed the lead-in. There are two extremes governing the size of the secondary coil. A coil which is too small will not pick up much of the magnetic field generated by the primary circuit at resonance. On the other hand, a secondary coil which is too large will overcouple or load the primary circuit. This will reduce the Q, or sharpness of the tuning.

The secondary coil is 16” diagonal at the largest turn and consists of 7 turns of AWG 20 buss wire. Buss wire was used so the coil can easily be tapped after the 1st, 2nd, 3rd, 4th, and 6th turn. The 7th turn is not currently used. A tapped coil will provide better impedance matches to the lead-in when the antenna is used through a wide frequency range. The taps are selected with a rotary switch. The taps are connected so that the outer turns are used first, and inner turns connected as needed. It is important that unused turns remain unconnected (free) rather than shorted. See Figure 6.

Figure 6. Secondary Coil Switch

Lead-in – A twisted pair of AWG 22 stranded wire is used as the lead-in. This will be more flexible than coax. The lead-in should be kept as short as possible and twisted tightly so it will not pick up any signal by itself. This is important at shortwave frequencies. A twisted pair can be fabricated from two lengths of wire with one set of ends anchored in a vise, while the remaining ends are twisted in the chuck of a hand drill. Most portable radios are equipped with a standard 1/8” phone jack at the external antenna connection point. So, this antenna is terminated with a 1/8” phone plug.

Base – There is nothing special about the base. Your only guidance should be to make it as stable as possible. Since the frame is light, most of the weight will be at the bottom with the capacitor assembly and other parts. That helps stability. This antenna uses a 5” plastic jar lid for the bottom. Keep the base small, as the antenna will likely be operated on a desk or table.

Operation

The antenna is intended to operate in close proximity to the radio, such as on a desk or table. There must be sufficient space to rotate the loop laterally. As described, this antenna has a range of 2.6 MHz through 12.3 MHz with a band overlap around 8 MHz. Depending on your selection of capacitors, your range and overlap may be slightly different.

Tune the receiver to a desired frequency.
Set the band switch on the antenna to the corresponding band.
Tune the antenna capacitor to resonance (peak signal).
Rotate the secondary switch to the position of maximum signal strength. Begin with the fewest turns (generally one) in the secondary.
It may be necessary to repeak the primary circuit.

Repeat the procedure to test operation of the upper or lower band.

Unlike similar loops for long and medium wave reception, this antenna is not especially responsive to direction for peak or null signal reception. However, you will find it very useful to reduce or possibly eliminate locally produced noise. Simply rotate the antenna on its base.

Modification

The basic concept for this antenna can easily be extended to higher or lower frequencies. Removal of the inner turn of the primary will significantly raise the upper frequency; whereas, adding turns will increase the lower range. Note that the lacing of the primary coil skips one set of holes in the square dowels between the first and second turn. This minimizes distributed capacitance between turns. This separation should be maintained if additional turns are added to lower the operational frequency.

Specialized Parts

Some sources for square wood dowel and single-gang 365 pf variable capacitors are listed below. The author does not endorse any of them. Prices for similar capacitors vary widely.

Square wood dowel:

Variable capacitor (365 pf):

Amazon
Walmart
Tubes and More
MTM Scientific
eBay (Multiple sources for both single- and double-gang capacitors)

Bob, thank you so much for sharing this excellent, detailed tutorial. Although I don’t have the exact same variable capacitor, I have all of the other components to make this antenna. I will have to put this on my Social DX bucket list! Thank you again!

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Hack a broken VGA cable to make a binocular ferrite cores for your NCPL antenna

13 Replies

Many thanks to Oscar (EA3IBC) who shares this simple hack.

I’ve gotten quite a few emails from Post readers telling me that the only thing holding them back from building a NCPL antenna is the BN-73-302 binocular ferrite core needed for the 1:1 Balun. While so much of the world is sheltering at home due to Covid-19, it’s less convenient to purchase one just for this purpose.

But almost everyone has an old VGA or USB cable with ferrite cores they can cannibalize for this very purpose. Oscar shared this super-simple hack on Twitter:

1. Cut the cable and remove the ferrite cores

2. Tape the two cores together

3. And wind four turns on both sides

Admittedly, this 1:1 balun might not have the same properties as the BN-73-302 from our tutorial, but it seems to be working for Oscar. Check out this screenshot he shared from SDR# while hooked up to his NCPL antenna:

Thanks again!