On Sat, 7 Nov 2015 21:57:07 +0000, highlandham
wrote:

Some time ago I acquired the remote control unit of the above loop
antenna ...without the antenna itself.
I now would like to use the control unit for a homebrew magloop .

See photos at:
https://groups.yahoo.com/neo/groups/MagLoop/files/MFJ-1786/
You'll need to join the Yahoo "magloop" group to view them. A little
detail on the motor is provided (12V DC, 0.5 RPM).

The magloop is to be connected to the control unit via coax which also
carries the power for the motor driving the magloop's variable capacitor .

To separate the RF and DC power (to the motor) an inductor is to be
inserted into each lead to the motor .

Oh, so that's what the inductor does. I thought it was part of some
matching network.
https://groups.yahoo.com/neo/groups/MagLoop/files/MFJ-1786/MFJ-1786%20coupling%20loop/
There doesn't seem to be much to the coil. About 10 turns about 0.6"
dia. Here's another photo that can be viewed without joining the
Yahoo group:
http://mirradio.ru/images/mfj-1786-8.JPG
Note the big chokes on the motor leads:
http://joataman.net/radio_ham/info/images/mfj1786x_tuning_motor.jpg
Judging by the photo, I would guess about 2.5 mhy. Why so big? Dunno.

My question : How to size a suitable inductor for this application.
The photo of the original MFJ magloop shows 2 inductors without a core
The magloop to be made willl likely be used for 7 to 30 MHz
So , what inductive reactance would be required for the lowest freqency
, which would then also be OK for the higher freqs .
PS : If necessary /appropriate I do have a range of iron and ferrite
toroids to make any HF inductor for blocking RF to the motor.

For that range, I would consider ferrite, type 31 toroid. The basic
idea is to end up with a reactance that's higher than the 50 ohm feed
impedance. I would guess anything over 250 ohms should be adequate.
Higher is better. Find a core with AL high enough to need a
reasonable number of turns at 7 MHz and that covers the frequency
range. 7 to 30 MHz is on a bit more than 2 octaves, so this should be
easy.

More photos:
https://www.google.com/search?q=mfj-1786&tbm=isch

Frank , GM0CSZ in IO87AT

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558