In the proper sense, small magnetic loop antennas are not considered a compromise. With a properly sized loop, they have the efficiency of a properly mounted half wave dipole*. The Chameleon F-Loop 3.0 antenna unleashes that power with a magnetic loop design that is easy to deploy and offers a broad range of frequency support between 10 and 80 meters on the HF bands.
Chameleon CHA F-Loop 3.0 Magnetic Loop Antenna: https://chameleonantenna.com/shop-here/ols/products/cha-f-loop-30
Konsait Black Camera 323 Quick Release Plate with Special Adapter (200PL-14) for use with Manfrotto tripods: https://amzn.to/49fTAEo
(*) An Overview of the Underestimated Magnetic Loop HF Antenna
Leigh Turner, VK5KLT, 2015 (accessed February 12, 2024)
https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf
Magnetic loop antennas consist of three components. The first, and most noticeable is a large primary loop. This loop circumference is typically within ⅓ to ⅛ the operating frequency range of the antenna. Inside this primary loop is a smaller conducting loop. RF energy is fed into the conducting loop and the electromagnetic coupling between the two are what creates the RF radiation. The third part is a tuning capacitor, which is connected to the large primary loop. Loop antennas have a low radiation resistance and the purpose of the capacitor is to eliminate reactance so that the loop becomes a resonant radiator. Since it is a resonant antenna, no antenna tuner or transmatch is required.
Since magnetic loop antennas have a low radiation resistance, high currents flow through the primary loop. These current flows are what radiates the RF energy in the form of electromagnetic coupling. These current flows are a limiting factor of magnetic loop antennas as if you give them too much transmit power, the capacitor will arc and short circuit.
With that said, magnetic loop antennas are a great choice for QRP or lower power operating and we’ll talk about those benefits and features in a bit, but first, let’s look at the Chameleon F-Loop kit.
Tuning a magnetic loop antenna
Now we get to the fun part, tuning your F-Loop antenna. Magnetic loop antennas have a very high Q, or quality factor. What that means is that their 2:1 bandwidth is very sharp and pronounced. As you go lower in frequency, the bandwidth shrinks. This loop will have 210 KHz of bandwidth on the 10 meter band, but on 80 meters, that bandwidth drops to 6 KHz. This is due to the function and size of the radiating loop. To a certain degree, a larger loop will give you more bandwidth on the lower frequencies, but even at that there are limits. Basically if you want to change frequencies, you will have to readjust the antenna.
There are two methods of adjusting the loop. The first is to use an antenna analyzer. Connect the analyzer and turn the tuning knob until you see a dip on the meter. As you are tuning, you will notice that touching the unit will slightly detune it, so as you are tuning, you will need to step away from the unit to get a more accurate reading. The knob is very sensitive and the 6:1 reduction drive lets you do precise adjustments. When you get in the ballpark, it will take very slight movements of the knob to hit the sweet spot.
The second method to tune the loop is to listen to your transceiver. As your turn the knob, you will hear the noise level increase and then decrease. This point of maximum noise is the sweet spot of the antenna and the result of your tuned frequency. After adjusting for maximum noise, make a test transmission to check the SWR and make any minor adjustments until you are at the minimum. You may not always hit 1:1 match, but anything under 2:1 SWR is fine and the antenna will operate well.
With magnetic loop antennas, you should never use a tuner or transmatch. These antennas are designed to work without a tuner and should only be adjusted to their resonant frequency using the tuning knob.
Once the antenna is adjusted, go ahead and transmit. If you need to change frequency, you will need to readjust the antenna.
My experiences with magnetic loop antennas
So what are my experiences with the F-Loop magnetic loop antenna? Generally, I had a lot of fun with this antenna. It sets up and tears down quickly, it is reasonably efficient, and by virtue of its use of the H or magnetic fields, it is highly resistant to man made noise. That makes these antennas great choices if you are in an urban environment or a residential area that is prone to RF interference.
I took this antenna out for a few Parks on the air activations to test its performance. This antenna paired really well with my Yaesu FT-817 QRP transceiver, but I would think this is an excellent companion also to the Icom IC-805, Elecraft KX series or the Xiegu G90 or 6100 low power rigs.
Magnetic loop antennas are often characterized as QRP antennas and the F-Loop can handle up to 25 watts sideband and 10 watts CW or digital. If you use more power than that, the air capacitor inside the tuning unit will start to arc. And therein lies the misconception that magnetic loop antennas are poor performing antennas.
In reality, they are not. A magnetic loop will work just as well as other antennas. At their best, a magnetic loop on the upper bands is comparable in performance to a dipole. At their worst, they are 6 dB or 1 S unit lower than an optimized three element beam. Which brings me to my on air test.
Looking at the data, the magnetic loop performed quite favorably when compared to my customary ¼ wave vertical antenna. On FT8, the signal reports where pretty equivalent. Using my Yaesu FT-817 set to 5 watts, I ran both antennas on the 20 meter band for about 45 minutes each and in that time period received about 18 contacts on each antenna. With the magnetic loop I gave an average signal report of -9 and received a report of -11 dB. With the vertical, I gave a signal report of -13 and received a -12 dB. To me, that isn’t much of a difference. In fact, the only difference that I can spot is that in looking at the maps of contacts, the distances achieved with the vertical were further than the magnetic loop. But that could be attributed to the vertical antenna propensity for having a lower RF angle of radiation.
So what are the down sides of Chameleon F-Loop antenna. I don’t think there is really anything negative about this antenna on it own, this is a highly engineered and well constructed piece of equipment. But the negatives relate to magnetic loop antennas in general. First is its low power capability. I know a lot of people aren’t into QRP operation so the 25 watt limit on sideband may be a turnoff. Chameleon does offer an optional power compensator that allows you to use up to 60 watts sideband with the loop, but still these are categorized as low power antennas.
The second downside is the relatively narrow bandwidth. If you change frequencies, you will need to re tune the antenna. You certainly have more bandwidth on 10 and 15 meters, but 20 meters and lower, the bandwidth approaches paper thin ranges. That can be great for rejecting adjacent noise and signals, but if you are the hunt and pounce type of person, you will find the bandwidth highly limiting.
I think these two limitations of magnetic loops really lend themselves to digital and CW operation. The weak signal modes like FT8 and the narrow bandwidth of CW really make these antennas shine. And being that we are at the peak of the solar cycle, we can also take advantage of their enhanced performance on the higher HF bands.
Since magnetic loop antennas also exploit the H or magnetic field, they can be successfully used indoors with a minimal performance loss. H fields have better penetration through walls and glass, so If you live in an apartment, condo, or have HOA limits, this may be the type of antenna that gets you on the air at home. In a high RF noise environment, their enhanced signal to noise ratio will also be of a benefit, and you may be able to hear things that you miss with other antennas.
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Timestamp
00:00:00 Introduction
00:00:30 Chameleon F-Loop 3.0 Magnetic Loop Antenna
00:01:02 How do magnetic loop antenna works
00:02:26 Chameleon F-Loop 3.0 Kit Options
00:04:38 Assembling the Magnetic Loop Antenna
00:07:04 Tuning your Magnetic Loop Antenna
00:08:59 Chameleon F-Loop 3 80 meter band
00:09:51 Personal Experience with the F-Loop
00:14:15 Magnetic Loops are NOT compromise antennas
00:16:35 Who are magnetic loop antennas good for
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