I've build a few small loop antennas in the past, but mainly for indoor use. And so eventually, with the aim to improve my LF/MF receive capability I decided to build a more sturdy loop for the outdoors. Perusing the World Wide Wait (WWW) system I stumbled over a loop design with two closely spaced loops. According to the WWW this antenna design is called a Hermes Loop.
So why two loops? Well, wisdom states that doubling the aperture of an antenna one would double the performance of the antenna. In a logarithmic sense this is a 3dB improvement in performance over a single loop.
I stumbled over this site https://lz1aq.signacor.com/index.php and found a wealth of information on small loops and loop amplifiers. Chavdar, LZ1AQ has done quite a lot of testing with small loop antennas, he has documented his findings and made them available for our perusal. In this particular document wideband_loop_ndb_band_11.pdf he has analysed small loops using ltspice and writes that "the noise peak moves to lower frequencies when the loop inductance gets higher". The way I understand this is that if we reduce the loops inductance, the noise peak moves away from the lower frequencies. Which basically means that by bringing the inductance of the antenna down we reduce the noise at the LF/MF. All this information led me to purchase of the AAA-1 LNA eventually.
Now according to wisdom, the inductance of a 1m loop is about 3µH. And physics (Kirchhoff's Law) dictate that if we are to parallel two coils/inductance (aka the loops) we would halve the overall inductance. Wow, so not only do we get more gain by paralleling two small loops no, we also lower the antennas inductance to help with noise reduction (sounds like a win win situation to me).
As I had a bit of alumina bar lying around I decided to build two loops of 1m in diameter. The picture below shows a quick knock-up and measurement of the antennas inductance.
As we can see from the above, the inductance measurement of the two 1m loops in parallel is about 1.4µH. Unfortunately I didn't measure a single loop, but I believe that my result seems to correlated with written wisdom.
After a rebuild of the antenna using PVC insulators, I mounted the loop on to my antenna test pole. The pole is about 4m in length, plus the height of the rotator with its mounting brackets. All up the loop was about 5m above the ground and 2.5m above a corrugated iron shed. You can see the AAA-1B LNA just mounted underneath the loop connected to 30m of STP cable.
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| The Loop at it's final resting place. |
Well, so what about its performance? Unfortunately I don't have any comparison between other loop's however, I have compared the loop against some of my bigger wire antennas. Comparing it to my Lazy-D which, at MF and LF is a BIG aperture loop or a ZS6BKW to the Hermes Loop shows that the Hermes Loop doesn't have to hide behind those antennas and is actually better at VLF compared to both wire antennas.
The below pictures show that the difference in noise performance at the low end of the spectrum. It is very low, yet signals in the LF/MF spectrum are strong with excellent SNR! At the beginning of the MF band we can see that the LNA has a bit to much gain which does cause a bit of an issue, but that is easy to rectify (attenuate).
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| Hermes Loop with AAA-1 LNA |
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| Lazy-D |
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| ZS6BKW |
So, for me at my QTH the antenna was (I have moved QTH in the meantime) working very well. Mechanically it withstood plenty of strong winds. However, the PVC spreader started to deteriorate. As such I will use some Delrin spreaders for the next build.
Have a look at the spectrum plots and judge for yourself if this antenna would be a suitable antenna in your environment. I'm certainly happy with the low noise performance of the antenna at the LF/MF and below spectrum (down to -124 dBm). Just don't forget that my QTH is in Suburbia. I'm surrounded by houses with all the usual noise makers (animals, humans (big and small), oh and of course the electrical kind)! However, taming those (noises) is a different kettle of broth.
Here are a few more comparisons between the Hermes Loop and some of my other, wire antennas.
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| RX1 (top half) antenna Lazy-D and RX2 (bottom half) Hermes Loop |
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| RX1 (top half) antenna ZS6BKW and RX2 (bottom half) Hermes Loop |
We still can see some noise coming from the strong (666kHz) MF BC station on the ZS6BKW and none of that on the Hermes Loop.
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So if you are interested in building one, below is a little sketch with dimensions.
NOTE: The dimensions are really not that critical, use what you have, but don't be deceived as to bigger is better! This is not always the case with active antenna systems.
Oh, and one more note, the joints should be welded (aka bracing). If you use screws or rivets you will find that those joints eventually form a diode junctions, and as soon as that happens you'll be very (very) unhappy about the poor noise floor of the antenna.
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