Figure 1. A Passive, Resonant, Transformer?Coupled Loop Antenna for Shortwave
Many thanks to SWLing Post contributor, Bob Colegrove, for the following guest post:
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.
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 1?4 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. Continue reading →
As promised, some more pictures of my antenna mounted on a rotatable stand. I have used standard (in the UK) 20mm electrical plastic conduit and fittings to make the frame.
Fitted a small plastic box to house the balun and have put a BNC socket on the underside of it for connection the coax cable to my Rx.
I have also been experimenting with cheap low cost amplifiers (LNA) found on eBay (see picture) which do seem to improve the general strength of signals by 10-15db, but the baseline noise also rises.
I did manage to hear a QSO on 160M using one of these which was inaudible without the LNA but I cannot say for sure yet if they are worth the extra noise introduced.
Thank you for sharing your update, John! The plastic conduit support is simple and effective! Indeed, it looks very professional. What I love about your NCPL build (loop, stand, and LNA) is it that it’s all incredibly affordable as well.
I think this passive loop antenna project has been so appealing because (1.) most of us around the world are sheltering at home due to the Covid-19 pandemic and (2.) this project is simple and you likely have all of the components in your tool shed or junk box at this very moment.
A number you have written to tell me about your antenna builds and some of you have agreed to allow me to share your projects with the SWLing Post community.
Below, you’ll find three fine homebrew examples of the NCPL antenna–all of which were made with what these fine radio enthusiasts had on-hand:
Jerome van der Linden
Jerome’s NCPL antenna
Well, I took up the challenge and built a NCPL antenna pretty close to your instructions.
Unfortunately, the coax I had available used (had aluminium shielding, and too late into the project I discovered solder would not take to it. My solution was to cannibalize a coax cable joiner (see photos attached), where – normally – the centre conductors are joined / held by a plastic centre piece and screw fittings.
The braid / shield for the two bits of coax is clamped / squeezed by an outer metal piece. My cannibalising effort involved removing the plastic centre bit which joins the two centre cores, and keeping just the outer metal component which I used (after completely cutting through the coax) to clamp the two metal braid sections, while the two centre copper bits were far enough apart for me to solder the leads for the ferrite balun.
Of course, I could not do the same at the top of the loop where the internal and external conductors need to swap over. I soldered some quite thick copper wire (perhaps 2mm in diameter) to each center core, pushed the center core into the opposing coax and coiled the 2mm thick copper tightly around each end of the coax.
Once it was all taped up it looks no worse than yours, and it does indeed WORK! [see photo above]
Here in Oz, I could not source the identical ferrite, but I think it’s pretty close. Best performance for me is on 11MHz, where the Radio New Zealand signal on 11725 is markedly better using the loop than the internal whip on my Tecsun PL-880. Other bands not quite so significant, but the Noise level is definitely lower.
As you say, Jerome, once all packaged up, it looks great! Sure, the mixture of materials you had on hand wasn’t ideal (aluminium shielding, etc.) but you found a way to make it work from the resources you had in your home. And I love the fact it’s lowed your RFI level! Thanks for sharing!
Giuseppe Morlè (IZ0GZW)
I’m Giuseppe Morlè (IZ0GZW), from Formia, central Italy, on the Tyrrhenian Sea.
I wanted to build your noise canceling loop seen on SWLing Post …
seems to work well especially from 40 meters. upward…
the diameter is 50 cm.
I will do other tests soon.
You can see the initial test on my YouTube channel via this link:
Thanks for the nice idea and a greetings from Italy.
73. Giuseppe IZ0GZW
Thank you, Giuseppe! What an amazing view you have there from your balcony! I’m quite impressed your PL-660 can take advantage of this design so well. We look forward to your other tests! Grazie mille!
My idea was to use a fitness hoop 75cm diameter bought off eBay. I removed the flashy striping to reveal a plastic like hoop that was joined in one spot with a plastic insert.
I have wrapped the whole hoop in tin-clad copper foil tape that has a conductive adhesive backing, but to be sure I have soldered all the overlapping seams. I drilled two holes opposite each other for the upper foil connections and the lower exit to the Balun.
Hopefully the three pictures will be helpful, I did the 4 turn design on Airspys website and it works really well connected to my RSPdx.
Thank you, John. What a fantastic way to build the NCPL antenna without using a coax for the loop. Indeed, since your plastic hoop has a small insert in the middle, you’ve an ideal spot to make the shield to center conductor cross-over. Very clever! I also like how you mounted the 1:1 Balun (or Unun) on a small board. Thanks for sharing this.
Post readers: If you have your own unique NCPL antenna design, please consider sharing it with us! Contact me with details and photos. I’ll plan to publish at least one more post with examples here in the near future.
I’ve gotten an number of inquiries from SWLing Post readers asking for a step-by-step guide to building the passive loop antenna I’ve mentioned in a number of previous posts. This antenna is the homebrew version of the commercially-available Airspy Youloop.
It works a treat. And, yes, folks…it’s fun to build.
There are a number of loop designs out there, and to distinguish this one, I’m going to henceforth refer to this loop as in the title above: the Noise-Cancelling Passive Loop (NCPL) antenna.
Before we start building, a little antenna theory…
I’m neither an engineer nor am I an antenna expert, so I actually turned to Airspy president and engineer, Youssef Touil, to learn how, exactly, this passive loop works. Youssef was the guy who experimented with several loop designs and ultimately inspired me to build this loop to pair with his HF+ Discovery SDR and the SDRplay RSPdx. “The main characteristic of this loop,” Youssef notes, “is its ability to cancel the electric noise much better than simpler loop designs.” Got that! [See loop diagram below]
“The second characteristic of this loop antenna is that it is a high impedance loop, which might appear counterintuitive. This means it can work directly with many receivers that have a low noise figure, in order to mitigate the impedance mismatch loss.
Note the resonance lobe near 4MHz. The resonance frequency is controlled by the diameter of the loop, the parasitic capacitance of the cable, and the loading from the transformer. It happens to be located right where we need it the most.
The transformer is basically a 1:1 BALUN that covers the entire HF band with minimal loss. Our BALUN has typically 0.28 dB loss.
[…]By connecting the center of this outer shield to the ground of the transmission line, you effectively cancel all the electric noise. The BALUN is required for balancing the electric noise, not for adapting the impedance.
[…]If you want to boost the performance in VLF, LW and MW, you can try a different impedance ratio, but this will kill the higher bands.”
What makes this loop so appealing (to me) is that it can be built with very few and common parts–indeed, many of us have all of the items in our junk boxes already. As the name implies, it is a passive design, so it requires no power source which is incredibly handy when you’re operating portable.
When paired with a high-dynamic range SDR like the Airspy HF+ Discovery or SDRplay RSPdx, you’ll be pleased with the wide bandwidth of this antenna and noise-cancelling properties.
Enough coated magnet wire for a total of eight turns on the BN-73-302
Heat-shrink tubing or some other means to enclose and secure the cable cross-over point and balun. (You may be able to enclose these connection points with PVC or small electrical box enclosures, for example)
A cable stripper, knife, and/or box-cutter
Soldering iron and solder
A heat gun (if using heat shrink)
*A note about loop cable length: Vlado and I made a loop with 1.5 meters of cable. The Airspy Youloop ships with two 1 meter legs that combine to give you an overall loop diameter of about 63.6 cm.
When I first decided to build this loop, it was only a day prior to a trip to the South Carolina coast where I planned to do a little DXing. I didn’t have all of the components, so I popped by to see my buddy Valdo (N3CZ). Vlado, fortunately, had all of the components and was eager to help build this loop. As I’ve mentioned in previous posts, Vlado is an amazing engineer and repair technician, so when I say “we” built it, what I really mean is, Vlado did! But I could’ve done it myself.
This is actually a very simple build––something even a beginner can do, as long as they’re okay with using a soldering iron. It does take patience preparing the loop cable properly. Take your time as you start, and you’ll be on the air in an hour or two.
1. Strip the ends of the loop cable.
Although your cable type and diameter may vary, strip back the cable ends roughly like this. To make finding the middle of the cable easier, we taped off the ends.
2. Make an opening in the middle of the cable to attach Balun leads to center conductor.
This is the trickiest part of the whole operation. The goal is to create an opening to tap into the center conductor of the cable.
You need to open a hole in the middle of the cable by
1 cutting away a portion of the outer jacket;
2 carefully separating and opening the shielding;
3 digging through the dielectric core, and finally
4 exposing the center conductor of the cable
Try to make an opening just large enough to gain access to the cable’s center conductor, but no bigger. Don’t allow any piece of the shielding to touch the center conductor.
When you reach the center conductor, expose enough of it so that you can clip it in the middle and create an opening to solder your balun leads to both conductor ends.
Once you’ve finished with this step, your cable should look something like this…
In the photo above, note that the shielding is completely pulled away, the dielectric core has been cut through, and we’ve clipped the center conductor, leaving a gap large enough to solder.
3. Make a 1:1 Balun
Grab your BN-73-302, and with the coated magnet wire, make four windings on one side, and four on the other. It should look like this:
Don’t have a binocular ferrite core like the one above? If you have a broken cable with ferrite cores, you can hack one! Click here to learn more.
4. Connect the Balun to a feed line.
Vlado just happened to have a BNC pigtail in his shack (he’s that kind of guy), so we cut and stripped one end, then connected the center conductor and shield to one side of the balun. We then enclosed the balun in heat shrink tubing to make it a little easier to attach to the loop later:
Of course, you could also create this junction in a small enclosure box or short cross-section of PVC. There are a number of ways you could secure this.
Youssef also added the following note about the feedline:
To use the NCPL antenna without a preamp, it is recommended to keep the length of the cable below 10 meters. The supplied Youloop 2 meter cable [for example] is sufficient to keep the antenna away from the magnetic interference of a computer or a tablet, and has very low loss and parasitic capacitance.
5. Connect Balun to the coaxial loop.
To make a solid connection, tin both sides of the center conductor. Next, attach the other end of the balun leads to each portion of the center conductor, as seen below:
Update: Note in the loop diagram near the top of the page that the ground wire on the output connector connects to the loop coax shielding on the primary side of the balun. I don’t recall that we did this in the build, but I would encourage you to do so. This should result in even lower noise, although admittedly, I’m very impressed with the performance of ours without this connection. Thanks to those of you who pointed out this discrepancy!
6. Secure the Balun/Coax junction.
Since this loop is intended to be handled quite a lot in the field, make sure the junction point of the balun and coax loop is secure. Again, we used several layers of heat shrink tubing since we had some in the shack.
7. Solder and secure the cross-over point.
Next, create the cross-over point of the loop by simply attaching the center conductor of one end of the cable to the shielding on the other end…and vice versa.
Before you grab the soldering iron, however…if, like we did, you’re using heat shrink tubing to secure the cross-over point of the loop in the next step, you’ll first need to slide a length of tubing onto the coax before you solder the ends together. Vlado, of course, thought of this in advance…I’m not so certain I would have!
Take your time soldering this connection and making it as solid as you can. If you solder it correctly, and you’re using a high-quality cable as we did, the cross-over point will be surprisingly durable. If you’re using a thinner cable, simply make sure the connection is solid, then use something to make the junction less prone to breaking––for example, consider sealing a length of semi-rigid tubing around this point.
Vlado cleverly added heat shrink tubing around the cross-over point to protect and secure it.
That’s all, folks! Now you’re ready to put your loop on the air.
Depending on what type of cable you used for this loop, you might require or prefer some sort of dielectric structure to support the loop so that it maintains the ideal round shape. My loop maintains its integrity pretty well without supports. I’ve supported it a number of times with fishing line/filament from two sides (tying on at 10 and 2 o’clock on the loop). That seems to work rather well.
In this setup, I simply used the back of a rocking chair to hold the antenna. As you can see, the loop maintained its shape rather well.
If you’d like to see and hear how this antenna performed on its first outing, check out this post.
Show the Post your loop!
If you build a NCPL antenna, please consider sharing your design here on the SWLing Post! Considering that there are a number of ways this loop can be built, and likely even more optimizations to improve it or make its construction even easier, we’d love to see your designs and/or construction methods. Please comment or, if you prefer, contact me.
And many thanks to my good friend Vlado (N3CZ) for helping me with this project and allowing me to document the process to share it here on the Post. Got a radio in need? Vlado’s the doctor!
Before I start talking Youloop, I have a little confession to make up front:
At the Winter SWL Fest when I gave a presentation about Portable SDR DXing, not only did I give attendees the wrong name of the Airspy Youloop antenna, but I also configured it incorrectly, hence the poor performance via my Miscrosoft Surface Go tablet PC.
I had assumed the the crossover component of the antenna was the transformer component. I realized the mistake I made when I saw some of the first promotional photos of the Youloop antenna a few weeks ago.
The crossover connects both sides of the loop while the tee junction contains the transformer.
Doh! I’m trying to forgive myself for making such an obvious mistake, but in my defence–and in the spirit of full disclosure–my antenna was a very early sample prototype without instructions, diagrams, etc. so I set it up imagining it being similar to the homebrew loop Vlado and I built. (FYI: When I say “Vlado and I built” it, I really mean, “Vlado built it.”)
So obviously I made a poor assumption.
Once I assembled the antenna correctly? Wow. Just. Wow!
Youloop: The ideal travel antenna for high dynamic range SDRs
The Youloop, Airspy HF+ Discovery, SDRplay RSPdx, and all cables easily fit in my Red Oxx Lil Roy pack.
The Youloop is truly the travel antenna I’ve always wanted for portable SDR DXing. Here’s why:
I’ve also used it numerous times with the new SDRplay RSPdx while using SDRuno in High Dynamic Range (HDR) mode. With the RSPdx, I can make spectrum recordings of the entire AM broadcast band. Note that HDR mode is only available on the RSPdx at 2 MHz and below, using the SDRuno app.
I have not tested the Youloop with other SDRs yet. I will soon test it with my WinRadio Excalibur.
So how well does the Youloop perform?
Listen for yourself!
I’m doing a little cargiving family members today. Their home is swimming in RFI (radio interference/noise). In the past, I’ve struggled to make good mediumwave recordings at their home–certainly an ideal situation for a mag loop antenna.
This morning, I wanted to record one of my favorite local AM stations (WAIZ at 630 kHz), so I set up the Youloop in the middle of a bedroom, hanging off a large bookshelf set against an interior wall. In other words: not an ideal situation.
When I plugged in the Airspy HF+ Discovery and loaded the Airspy SDR application, I fully expected to see a spectrum display full of broadband noise.
Instead, I saw signals. Lots of signals:
Sure, there’s some noise in there, but it’s low enough I could even do proper mediumwave DXing on most of the band if I wished.
In fact, if you’d like to experience the HF+ Discovery/Youloop pairing in this compromised, less-than-ideal DXing setup, why not tune through one of the spectrum recordings I made?
The recording was made on March 30, 2020 starting around 10:50 UTC. You’ll need to open this file inAirSpy’s free application SDR#or a third party SDR app that can read AirSpy .wav files.
I can’t wait to try the Youloop in other locations. Since we’re in lock-down due to Covid-19, I won’t be able to try the Youloop in a hotel any time soon. Almost all of my 2020 travel plans have been canceled.
If you have one of the SDRs mentioned above, go grab a Youloop. At $35 USD, it’s a fantastic deal.