The Outstanding Passive Loop Antenna by Airspy
Airspy, the maker of the HF+ Discovery SDR receiver, also makes the YouLoop, the passive, so-called "2-turn" mobius loop sold for use especially with Airspy's SDR receiver lineup.
I bought a YouLoop three or so years ago when I bought my Discovery, tried it for a short time, and put it away, not fully realizing its potential. Sometimes, as in real life, there's more just under the surface than what meets the eye at first. If you don't give a second look, you might miss it. For starters, you have to get your mindset pointed in the right direction, that is, towards ultra-low noise floors and signal-to-noise ratio improvement. Don't stay focused on S-meter readings and brute signal strength. The YouLoop is perfectly matched to present an ultra-low noise floor with maximized signal pickup, and particularly for the mediumwave band.
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| Factory stock YouLoop |
From the factory, the YouLoop has a circumference of 2 meters, comprised of two, one meter sections joined at both the top and bottom, presenting a diameter of 0.64 meters, or 25.2 inches. The top ends are joined through a simple crossover network, interchanging the coax shield with the inner conductor. At the bottom we have a tiny voltage balun to match the loop's low impedance and provide additional noise suppression and isolation. A low noise, highly sensitive receiver is required to get the best out of the YouLoop, to wit, the HF+ Discovery, which has a noise floor on the order of -142 dBm and a minimum discernable signal (MDS) sensitivity of -140 dBm (0.02 uV / 50 ohms at 15 MHz). That is an extremely faint signal. The YouLoop will work minimally with the SDRPlay RSP series of receivers, but they lack the low noise floor and sensitivity to get full value from it. A preamp might help there, mounted right at the loop itself.
In the mediumwave band, the stock YouLoop's sensitivity falls off gradually below 1000 KHz. It was evident that a slightly larger loop might help with this. In attempts to correct this I've done experiments by adding to the length of the YouLoop, first adding two short 1/2 meter lengths of RG-402 (one per side), then two 1 meter lengths in a similar fashion. The 1/2 meter lengths increase the total circumference to 3 meters, the 1 meter lengths increase it to 4 meters. In order to add to the existing YouLoop coax, you will need to purchase two coax pigtails of the same type (RG-402) and two female SMA unions, both readily available from Amazon.
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| SMA unions and RG-402 pigtails |
Let's Do Some Testing
Testing was performed with both extension modifications, comparing each to the stock 2 meter loop while noting the signal strength and the signal-to-noise ratio differences. The standard 6 ft. coax feeder was used between the SDR and the YouLoop and the SDR was connected to the computer through a 15 ft, high quality shielded USB cable to get it as far away from digital hash as possible.
Two distant stations were used as reference and checked during mid-daytime hours, CHLO-530 KHz, Brampton, Ontario (1 KW) at 195 km distance and WWKB-1520 KHz, Buffalo, NY (50 KW) at 111 km distance. Both put in readable but not overly strong signals at these distances.
Results were very interesting, and somewhat surprising, indeed. See the table just below. SNR values are measured to include the carrier + the sidebands.
Freq Stock 2-meter YouLoop 3-meter YouLoop 4-meter YouLoop
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530 -84dB (snr=30, nf=-114) -76dB (snr=30, nf=-106) -68dB (snr=31, nf=-99)
1520 -85dB (snr=37, nf=-122) -85dB (snr=38, nf=-123) -83dB (snr=39, nf=-122)
*signal strength values in dBFS (dB Full Scale)
*snr = signal-to-noise ratio, in dB
*nf = background noise floor, in dBFS
Signal strengths at 530 KHz improved by about 8 dB when the loop circumference was extended from 2 to 3 meters. A 16 dB improvement (530 KHz again) was seen when the loop was extended to 4 meters. To my surprise, signal strengths at the high end of the band (1520 KHz) remained essentially the same for all three loops, rising only about 3 dB (half an S-unit) with the 4 meter loop. The 2 and 3 meter loops produced identical signal strengths at 1520 KHz.
Perhaps more surprising, the SNR, or signal-to-noise ratio of both stations did not vary more than 1-3 dB across all three loop lengths at each frequency. At 530 KHz, all three loops realized an SNR of 30-31 dB. At 1520 KHz, the SNR hovered between 37-39 dB. The relatively unchanging SNR is significant and will allow us to choose the best length loop for our receiving purposes.
The last phase of the testing was null depth. Null depth did not change across all three loops. Differences were undetectable from the original product. I am able to get 23-26 dB null differential across the MW band on semi-local, low angle (60 miles distant) groundwave signals.
So which loop would be the best choice for mediumwave?
The 4 meter loop had only a small gain change over the original 2 meter version at the high end of the band, some 3 dB, but realized about 16 dB gain at the low end. The 4 meter loop seems the logical choice. However, our noise floor at 530 KHz rises from -114 dBm (original YouLoop) to -99 dBm, an increase of 15 dB too! The new -99 dBm noise floor is the equivalent of S-5 on the S-meter. The original YouLoop's noise floor at -114 dBm is about S-2 and much quieter, and with the same SNR and same copyability.
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| 3 meter YouLoop with added pigtails |
The 3 meter loop had virtually no gain change over the original 2 meter version at the high end of the band but showed about 8 dB gain at the low end. This time our noise floor at 530 KHz rises from -114 dBm (original YouLoop) to -106 dBm, an increase of 8 dB too! The new -106 dBm noise floor is the equivalent of S-3.5. The original YouLoop, still at -114 dBm (S-2) and still quieter, has the same SNR and same copyability.
The 3 meter loop, the one with the two 1/2 meter extensions, may be the best compromise here. We get signal gain, as it raises signal levels at the low end of the band by about 8 dB. Our noise floor is kept in check at the low end as well, rising only to -106 dB, near a one microvolt signal. The larger 4 meter loop sets the noise floor too high at -99 dB, possibly masking very weak signals at the microvolt level (-107 dBm).
The YouLoop at 3 meters circumference starts to become a little unwieldy indoors, but can be arranged in a vertical oval so that it can be rotated a little easier and not hit other objects in a room. Arrange it in a 24 inch wide by 48 inch tall configuration. Little difference was seen in its nulling ability or signal degradation when configured in this shape.
Conclusions
Extending our original YouLoop to 3 and 4 meters increased our signal levels, but the background noise levels increased by the same amount. Kicking in the preamp on SDR# would accomplish the same thing, amplifying the signal and noise the same amount. The HF+ Discovery has a 15 dB preamp.
The HF+ Discovery's published sensitivity is -110 dBm for 6 dB signal-to-noise ratio. That is a signal some 6 dB, or one "S" unit above the noise. You're going to have to do a bit better than that, however, in that you need a better SNR than 6 dB to extract meaningful audio out of a signal. 9-10 dB might give results, 12-15 dB even better. The signal must be modulated adequately too - low modulation levels will require a higher SNR to make sense of. We can hear a 1 microvolt signal (-107 dBm) if our background noise is enough below that, our bandwidth just right, and our signal is modulated properly.
You need to get the signal strengths up there, closing in on the -107 dBm level to copy audio from signals. In that signal range, it would be best to have a base noise level of not much more than about -115 dBm actual measured noise level.
Note again, we also have the preamp at our disposal. A low noise preamp can make up the difference we need in signal strength. Remember, though, generally the background noise will be amplified equally with the signal. Not to worry if it brings our signal up to a listenable level. Example: Rising out of a -120 dBm noise floor we have a -110 dBm signal we are struggling to get copyable audio out of. Kick in the preamp and we raise that -110 dBm signal to -95 dBm, even though our noise floor is also raised to -105 dBm. The signal is now copyable.
Bonus Test
While I was at it, I put together a tiny 1 meter circumference loop using the two 1/2 meter pigtails. It is the size of a pie plate. One would think this would be so small a loop as to be totally ineffective. Not so.
Here are the test results for that YouLoop configuration as compared to the factory YouLoop:
Freq Stock 2-meter YouLoop tiny 1-meter YouLoop !!
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530 -84dB (snr=30, nf=-114) -97dB (snr=27, nf=-124)
1520 -85dB (snr=37, nf=-122) -89dB (snr=35, nf=-124)
Signals were lower but adequate, and signal-to-noise levels were in the same ballpark of the other loops. Surprising, even a tiny YouLoop is effective!
Some Low Noise And Signal Improvement Tips
1. Position the YouLoop as far away from your computer and monitor as possible. Monitors in particular are terrible electrical hash generators.
2. Position the SDR itself as far away as possible from your computer for the same reasons.
3. To keep the SDR as far away as possible from your computer, you will need a long USB cable. The USB 2.0 standard allows for cable lengths of up to about 15 ft. You MUST use a good quality, properly shielded USB cable. Stay away from the latest rage, the cotton-covered USB cable. The ones I've tried have inadequate shielding and let all kinds of trash in. I've had great luck with the Monoprice line of cables, available on Amazon. If you attempt to run farther than 15 ft., be aware that you may incur data transmission loss. Your SDR will suddenly drop out of "Play" and into "stop" mode. It may not even start. Trust me, I've tried it. They do make USB amplifiers. I haven't tried them or know if they'd even work.
4. The YouLoop works best when the connecting coax to your SDR is the shortest length possible. Operated passively (without a preamp), you will not get good results or maybe even any results by feeding it with 50 ft. of coax and placing it out in the yard. Try to keep the coax feed under 12 ft. Three to six ft. of coax is optimum. SDRs, like the HF+ Discovery, can also be directly connected to the YouLoop feed point for outstanding results.
5. Used indoors, place the YouLoop next to a window for maximum signal pickup. This really does make a difference.
6. Don't be sloppy with the circle or oval you form. Make a wooden form in the shape of a cross if you like, and fasten the loop to it, ensuring the coax lies in a flat plane. This will maximize the loop's balance and nulling abilities on mediumwave. A child's plastic Hula Hoop also makes a wonderful circular form and can be used in pairs for bigger loop sizes.
