Mag-Loop Antenna Tuning Indicator
I really enjoy using the Chameleon F-Loop 3.0 magnetic loop antenna in my indoor ham shack (as without it I would not be on HF here in my HOA restricted QTH). It covers 80-10m and is easy to tune while being used. The only challenge is that I need my IC-705 radio's display to be showing the Meter screen on the lower half of the display to be able to see the VSWR reading to fine tune the antenna to resonance. This setting reduces the waterfall display area and makes it harder to see signals on the bands at that moment.
I have seen that AlexLoop antenna at one point sold a magnetic loop clip-on LED tuning indicator (AlexTune) which glows brightly at the resonance point and dims when away from the tuning point. It has a clip on the backside to attach to the shell of the PL-259 loop coax connector. Looks pretty neat! The challenge is it costs $50 USD and is not locally available (i.e., needs to be ordered via the internet).
Let's review the functions needed for this device. It has 1) a mounting feature to hold it to the loop, 2) some sort of induction function or coil inside to detect the antenna's field, 3) a LED indicator, and 4) packaging to hold it all safely together. How hard could it be to DIY an indicator like this?
With that goal in mind, a brief internet search was made to see what others have already done. This yielded a few videos about how to build your own loop antenna and didn't turn up what I was really looking for: a schematic of the AlexTune or a picture of the inside of one. One video author showed how he wound a wire coil and connected a holiday light bulb across it to yield a simple tuning indicator for his mag-loop antenna. His unit also had a pair of ~ 6" straight wires connected across the indicator. Was this a coil loop detector, or a small dipole with a coil in the middle? The video showed it worked, but the smaller AlexTune device does not have these protruding wire elements.
Another video showed a clamp-on ferrite core with a few turns of wire through it, connected to a LED indicator (no detector diode or other parts, just core, wire, and LED). This was used to detect the RF current flowing on a wire antenna element. Maybe it would work on a loop as well?
Grabbing a ferrite core, a 12" clip lead, and an LED yielded a working tuning indicator as shown below. Can't get much easier than that, right? A 5 minute solution...
I initially tested this up on 10 meters in the evening after the band had closed. It was easy to see on the LED turn bright when the lowest VSWR tuning point was achieved. Looking good! The next test was down on 40 meters. The band seemed pretty quiet for that time of day, but the tuning LED worked there as well. Suspicious about the "quiet band", I unclamped the core from the antenna loop wire and the band came alive! Looking into this further, I came to the conclusion that the ferrite core was doing its job and dampening RF current flow through the interior. With or without the LED and clip-lead wire inserted, the ferrite core was definitely dampening the loop performance when clamped over the loop wire.
If the ferrite core would not work, how about just a small coil connected to the LED with no magnetics? I found a gallon water bottle cap and wound the clip lead wire around it with some electrical tape to hold it all together. Holding it up to the loop antenna showed that the LED indicator lit up brightly when oriented along side the loop coax. Another bit of tape held it to the antenna coax wire as shown below.
Wow! This worked even better than the first prototype and didn't appear to degrade the loop receive performance at all, on any of the bands tested (40m to 10m). For my indoor antenna setup, this prototype works just fine - and is now in daily use here at the station.
DIY Mag-Loop Tuning Indicator Tip: At one point I found that this specific location for the indicator worked best for me (and that I had issues making it work on the right side of the loop). Don't be afraid to move the loop around until you find the spot that works best for you!
Achievement Unlocked: Build a DIY magnetic loop tuning indicator. It works!
Next steps are to gather up some more water bottle caps, LEDs of different colors, and some wire and look into building a more compact, durable version for future field use. Will it all fit inside of a plastic cap and just hang onto the coaxial loop with a-bit of velcro strap? More fun ahead!
All author photos taken with an iPhone 16e.