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I've build a few small loop antennas in the past, but mainly for indoor use. And so eventually I decided to build a more sturdy outdoor loop. With the aim to improve my LF/MF receive capability. 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.
Well, a Hermes loop is a small aperture antenna for receive only. It does require a Low Noise Amplifier (LNA). Basically it is two smallish coils in parallel. So why two loops? Wisdom states that doubling an antenna aperture one would double the performance of the antenna. Now in my book that is an increase of 3dB in performance over a single loop. Additionally, I stumbled over this site https://lz1aq.signacor.com/index.php and found a wealth of information on small loops and loop amplifiers. All that information led me to purchase the AAA-1 LNA eventually.
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 by bringing the inductance of the antenna down we reduce the noise at the LF/MF.
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/inductances (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).
I've had a bit of Aluminium bar lying around and so 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 does correlated with written wisdom.
After a rebuilding 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 4.5m 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.
NOTE: Don't get hangup on the rotator, it is not needed for this type of antenna. It is a hanger on from some VHF/UHF antenna testing.
The Hermes Loop in all its glory.
Well, what about its performance? From the below we can see that the noise at the low and of the spectrum is very low, yet signals in the LF/MF spectrum are strong with excellent SNR! So, for me at my QTH the antenna was (I have moved QTH in the meantime) working very good. Mechanically it withstood plenty of strong winds. However, the PVC started to deteriorate. For the next build I will use some Delrin.
Have a look at the below spectrum plots and judge for yourself if this antenna would suit your requirements. I certainly happy with the low noise the antenna pics up at the LF/MF and below spectrum (down to -124dB). 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)!
LF/MF Spectrum plot using the Hermes Loop with AAA-1 LNA
LF/MF Spectrum plot using LAZY-D antenna.
LF/MF Spectrum plot using ZS6BKW antenna.
One more interesting aspect to note is that (most) small Loops are vertical polarised and with that the antenna would give me the ability to use polarization-diversity reception with some of my dual receiver radios. Which, according to wisdom should improve listening prowess during receiving sessions that are plagued by fading.
As I have a few other antennas, which are designed for reception and transmission at the HF spectrum im happy that I have added the Hermes Loop to my antenna arsenal. And even though those antenna seem receive at the LF/MF spectrum as well, especially the LAZY-D, the ZS6BKW seems to be a rather noisy antenna at the LF/MF spectrum. So yes, I'm happy with the performance of the Hermes Loop at LF/MF.
Should you be inclined to build one for yourself, see the below sketch of my build. I've included a materials list however, it is material that I had lying around. You might find that using smaller aluminium bar will save some weight and wind load. It is not really that critical. Also the distance between the loops have been arbitrarily chosen because it fitted with the left over bits.
NOTE:
Since the antenna is made from Aluminium do not join the the two loops with screws or rivets. Aluminium forms an insulation layer, hence the greyish appearance. And by joining two flat aluminium surfaces you might find that after a while a diode junction will form. As soon as that happened all hell on earth will hit your receiver and your receiving pleasure will deteriode quite quickly.
To overcome that issue I welded/brazed the interconnections together. However, there are other options available (YMMV).
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