I have been active lately with field ops and collecting my observations on these great antennas
I have various loops for reception for vlf , mw and the 160m/80m bands
My first magnetic loop for transmitting was the AlexLoop which was a great antenna
I decided to build my own and I went with the Chameleon Antenna as I wanted several mounting options specific to my needs
They offer various versions and like all their stuff its milspec grade. Avoid all copy cat versions and just buy Carls’s stuff and you will not be disapointed and he offers the best customer service anywhere
So why these mag loops?
– small and lightweight
– ideal for field ops
– multiband in a small footprint
– rapid deployment
Comments:
1. Very narrow bandwidth. You must use a gear reduction in order to tune it and you still need to go slow. The BW can be as narrow as a few KHz.
2. High degree of front end selectivity which can help with reducing interference and noise
3. Noise reduction due to the loop responding to the magnetic components of the signal only it can be beneficial on 80m (urban areas are full of plasma tv noise). The noise component of a signal is predominantly near-field and not magnetic.
4. Quieter than dipoles or monopoles which can help make signals more readable (withouthe noise) especially on cw. For receive only use in a hybrid antenna configuration a 15-20db preamp will be useful.
5. Ability to null out noise by rotating the antenna. This antenna has directivity properties.
6. Mounting this antenna on a tripod with vertical polarization seems to not be affected by height above ground effects. This is because the magnetic field lines run parallel to the ground. I didnt notice any differences at 3, 8, 12 and 16 feet.
It worked well on 40m during a NVIS sprint so the radiation angle is higher than 45 degrees I believe.
Hence the use of mag loops for military NVIS communications.
7. Horizontal mounting now has the magnetic field lines that are affected by the ground. Looking at the radiation angles with the antenna tools the radiation pattern starts to resemble a vertical. Height above ground is a factor.
I confirmed the radiation patterns using a field strength meter in both polarizations and with WSPR data
8. High voltage and currents are present and rf burns are possible.
I will test using an rf current probe at one point to confirm
9. These loops are inefficient so expect less than 5% on 80m. 20m offers around 68% efficiency.
10. Easy to make. Once you have the capacitor and gear drive sorted out (hint Chameleon Antennas kit is available) the build is straightforward.
I tried a double loop version to improve the 80m performance with a jungle rat build (think bamboo and paracord) and a non-geared Johnson variable capacitor and it was frustrating to get it to resonance as hand capacity affects will vary the sweet spot. This needs to be boxed up.
So the AlexLoop is a great mag loop antenna that is highly portable and very well made.
The Chameleons are the most rugged and offer various designs based on coax or aluminum tubing elements.
MFJ has several mag loops and I do have the wire based non-mag loop – 9232 model for qrp that seems to work ok but I will field test shortly. I have heard good things about the 935 series and the rf current meter makes adjustment easy. My short time playing with the MFJ years ago seemed promising but I got distracted by the cu,t of Buddipole. There are several models and offer the ability to use 100watts but these are wire based and not magnetic based.
I confess I personally met Alex several times and have been a happy Chameleon customer for over ten years. I have the original MFJ kit from the 70s and have never met Martin.
Recommendation:
I highly recommend a magnetic loop for field ops and in use where having a proper dipole or vertical can be challenging. It is not a DX antenna by any means but you will make contacts with some skill and patience because remember we are at low power levels under 10 watts.
If you want to run 100 watts then MFJ has many options.
good luck and cu on 40 cw
ve3ips john
Oops
11. Great for apartment/condo cliff dwellers