[Lowfer] LF receiving loop

Tue Nov 6 11:14:23 EST 2018

Zack:  I have a small 17 inch passive  loop antenna wound on a circular band of fiberglass (FR4 with no copper) at about 1.2mH total inductance. I use it in passive coupling mode with various

portable LF receivers which have ferrite rod  antennas and the 17 inch loop is very sharp tuning---enough so that it pulls on the rx tuning a khz or 3 at resonance when over-coupled. I would say it makes a good 30dB

imporvement over a portable radio with a ferrite rod antenna alone.   I would guess your 5 ft loop is potentially even sharper and difficult to tune. My 17 inch loop will overload a portable receiver's AGC on medium to strong signal level AM BC stations

and have the opposite effect of the received station level decreasing.   Using AGC "off" may be necessary, but not many portable LF receivers have that feature. Some of the LF/MF RDF radios turn off the AGC or use fast AGC (old Zenith 780 and 790, Benmar 555, etc.)

You might try it as a passive loop antenna first to make sure the loop is working correctly and have a base line reference for captured RF levels.  Perhaps some different values of resistance (5,10 ohm, etc.) in series with the loop to lower the Q until you find a reasonable resonance bandwidth.

I seem to recall a loop antenna in QST for 160m years ago that added some series "R" for bandwidth adjustment....but I do not think I could easily locate it.  May have been in Tech Correspondance vs a feature article in the last 30 years....

If the loop works well in passive mode, then I would guess the issue is impedance matching at the input....(could you have blown the gate on the fet open when connecting the loop?)

73 Mike wa3tts

-----Original Message-----
From: Zack Widup <w9sz.zack at gmail.com>
To: Discussion of the Lowfer (US, European, &amp, UK) and MedFer bands <lowfer at mailman.qth.net>
Sent: Tue, Nov 6, 2018 10:37 am
Subject: [Lowfer] LF receiving loop

I finally got done with a project that has taken me a couple years in my
"spare time." I made a 20 turn LF loop, 5 feet on a side, with turns spaced
1 inch apart on a pvc "X" form. And I built this remote tuner/preamp:

http://www.lowfer.us/k0lr/preamp/preamp.htm

It's the second schematic, with the four varactors, that I built. I used
MVAM109 diodes. I also added a set of switchable parallel capacitors to the
circuit. allowing me to add capacitance from 100 pF to 6400 pF in powers of
2. I also placed taps on the loop, at 3, 5, 10, 15 and 20 turns so I could
select smaller inductances.

I got around to trying it out last night. I was not happy with the results.
The tuning appears to occur where I expected it based on some calculations
I made. But the signal strength is on the weak side. I tuned part of the AM
band and saw signal strengths far below what I'd hoped for. Only a coupe
local stations moved my s-meter. I am getting stronger signals from an HF
vertical (MFJ 1796).

I am using 100 feet of Micro-8X coax cable between the remote tuner box and
the receiver in the shack. I don't know if that's part of the problem or
not.

Does anyone have any experience with this?

73, Zack W9SZ

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