[CTSARA] Leonid's Loop Antenna

[ranceiii at aol.com]

Georgi,
 
Don't order anything yet, becaue we have a loop antena tuner that Leonid also left us that we are sure was intended to work with that magnetic loop antenna.  We can loan it to you in the short-term.
 
I will arrange for you to get it from Jon Perelstein, who has it along with most of Leonid's stuff donated from his estate.

I'll be interested to see the test results once the 2 are put in action together.

Terry




-----Original Message-----
From: Georgi Todorov <terahz at geodar.com>
To: Stamford Amateur Radio Association <ctsara at mailman.qth.net>
Sent: Fri, Sep 7, 2012 11:46 pm
Subject: [CTSARA] Leonid's Loop Antenna


Hi All,
Just wanted to share with you what I was able to find about Leonid's antenna so 
ar. I read a few magnetic loop publications and used a few formulas that seem 
o be commonly referenced online to come up with the numbers below. 
Overall, the antenna follows a normal magnetic loop antenna design. Inner loop 
onnects to the radio, while outer loop couples with the inner and resonates at 
 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 
or capacitance or all 6 loops  (carefully spaced .75" apart) can be combined 
ogether for a stronger radiation.
The length of main loop makes it a good antenna for 15-12-10m operation, however 
he 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 
onstructions 3/4-1" copper pipes are used. I expected to find a copper pipe 
nside the PVC, however it was only the RG6 coax.
At 10m the capacitance required to tune the antenna is about 815pF. At 15m - 
447pF
The 5 loops of RG6 at 330" have about 520pF capacitance. That would mean, the 
wo RCAs are probably designed to connect to a variable capacitor for fine 
uning.
Since the main loop is so thin, the capacitor needs to handle about 50V tops at 
2W transmit power (my radio) and needs to be able to vary from 250-900pF. Not 
mpossible to find for sure. 
If we combine all 6 lops to form one "thick" loop, the copper thickness is 
ncreased to 0.2" which makes the antenna more like 35-50% efficient (15m-10m). 
till not very good, but much better. The tuning capacitance is now 40-80pF @ 
ax voltage of 670V (again 12W TX). These caps are not that hard to find on 
bay…
The biggest problem, and benefit, of the magnetic loop antennas is their 
andwidth. At any given point, they only have a couple of hundred kHz bandwidth. 
hat means they need to be retuned all the time. It also means, it will filter 
ut other signals very well.
The box that came with the antenna is a MFJ-1786 controller. Unfortunately it is 
nly the control box for the actual tuner that the MJF-1786 antenna has on it. 
 also opened the box and it has only a few switch ICs in it and a lot of empty 
pace. 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 
ctual antenna it was designed for. According to the manual, it sends control 
oltages via the antenna feed to tell a gear motor to turn the capacitor inside 
he antenna. I tried to search for a used antenna only, but didn't have luck at 
he usual places…
So that's pretty much it at this point. I'll see if I can find some kind of a 
ariable capacitor that might work with this antenna, and if I do, I'll give it 
 try and update you. If anyone else feels they can make better use of the 
ntenna, they are welcome to have it.
Here is a photo of the antenna as a reference:
ttp://joro.geodar.com/images/uploads/leonid's-antenna-1.jpg

3
eorgi
1GTT
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