[CTSARA] Leonid's Loop Antenna

[Georgi Todorov]

Hi All,

Just wanted to share with you what I was able to find about Leonid's antenna so far. I read a few magnetic loop publications and used a few formulas that seem to be commonly referenced online to come up with the numbers below. 

Overall, the antenna follows a normal magnetic loop antenna design. Inner loop connects to the radio, while outer loop couples with the inner and resonates at a given frequency depending on length of outer (main) loop and capacitance. 

The main loop can be seen as a single 80" loop with another 330" of coax used for capacitance or all 6 loops  (carefully spaced .75" apart) can be combined together for a stronger radiation.

The length of main loop makes it a good antenna for 15-12-10m operation, however the thickness of the copper is only 18awg (~0.04") and that makes it only 10% (-10.5dB) - 20%(-6.8dB) efficient from 15m - 10m.  Usually for such constructions 3/4-1" copper pipes are used. I expected to find a copper pipe inside the PVC, however it was only the RG6 coax.

At 10m the capacitance required to tune the antenna is about 815pF. At 15m - 1447pF

The 5 loops of RG6 at 330" have about 520pF capacitance. That would mean, the two RCAs are probably designed to connect to a variable capacitor for fine tuning.

Since the main loop is so thin, the capacitor needs to handle about 50V tops at 12W transmit power (my radio) and needs to be able to vary from 250-900pF. Not impossible to find for sure. 

If we combine all 6 lops to form one "thick" loop, the copper thickness is increased to 0.2" which makes the antenna more like 35-50% efficient (15m-10m). Still not very good, but much better. The tuning capacitance is now 40-80pF @ max voltage of 670V (again 12W TX). These caps are not that hard to find on ebay…

The biggest problem, and benefit, of the magnetic loop antennas is their bandwidth. At any given point, they only have a couple of hundred kHz bandwidth. That means they need to be retuned all the time. It also means, it will filter out other signals very well.

The box that came with the antenna is a MFJ-1786 controller. Unfortunately it is only the control box for the actual tuner that the MJF-1786 antenna has on it. 
I also opened the box and it has only a few switch ICs in it and a lot of empty space. It can operate with batteries and only needs to be on during tuning (while the variable cap tunes). Unfortunately it isn't very useful without the actual antenna it was designed for. According to the manual, it sends control voltages via the antenna feed to tell a gear motor to turn the capacitor inside the antenna. I tried to search for a used antenna only, but didn't have luck at the usual places…

So that's pretty much it at this point. I'll see if I can find some kind of a variable capacitor that might work with this antenna, and if I do, I'll give it a try and update you. If anyone else feels they can make better use of the antenna, they are welcome to have it.

Here is a photo of the antenna as a reference:
http://joro.geodar.com/images/uploads/leonid's-antenna-1.jpg


73
Georgi
K1GTT