[600MRG] re. Loop Antennas

[Dave Riley]

All the time improvements, yea...
Now  I think of a loop just made from old corroded silver plated copper tubing..
Is it really acting like a broadbander or what, silver polish?? Must measure -3db points..
Having fun anyhow...
On 9/9/2024 9:12:13 AM, Edward Larsen <ki6rcm98 at gmail.com> wrote:
Measured -3dB bandwidth at 201 khz using a frequency selective level meter was about 3.22 kHz, thus the Q was about 62. I used quantity-10, 1SV149 diodes in a series/parallel arrangement to achieve a broad tuning range. If there is a problem with over-voltage on the diodes during transmit, one might add a T/R relay to short-out the loop.
When you use a high-Q loop like this it is most interesting tuning the bands since the loop also has to be tuned. It would be fun to try to automate loop tuning to match receiver tuning.
73
Ed


On Mon, Sep 9, 2024 at 2:35 AM david vanhorn <kc6ete at gmail.com [mailto:kc6ete at gmail.com]> wrote:

What sort of Q values were you able to achieve with the loop loaded to 50 ohms?


On Sun, Sep 8, 2024 at 5:01 PM Edward Larsen <ki6rcm98 at gmail.com [mailto:ki6rcm98 at gmail.com]> wrote:

Thanks Dave for the posting regarding loop antennas for VLF. I like your approach and if your loop is out-performing a good active whip it is certainly working.
I did some loop tests a few years ago. What I built was:
13 turns of silver plated AWG 22 stranded/teflon, 79" diameter, wires spaced 1/2" and crossed-over at the bottom (mobius wound).

About 725 uH inductance resonated to VLF with 80-1250pF varactor tuning capacitance. Loop self resonance was about 3.02 MHz. I used a one turn coupling loop inside which I adjusted for 50 Ohm match. I've found that a high-Q tunable loop eliminates any overload or intermodulation problems in a receiver.
I left this loop with KI6MTV when I moved to Arizona. I need to build another. It would tune about 140 kHz to 650 kHz via vari-cap diode tuning.
I have designed/built a pretty good active amplified ferrite rod antenna for 17.2 kHz. I can email you the schematic if you like. It too parallel resonates the high-Q loop for sharp narrow-band tuning. The circuit could be modified for higher frequencies if desired.

There has been a lot of excellent VLF loop design work in the past by several low frequency Gurus who published in the Lowdown magazine. Unfortunately this data is not available anywhere. I have most of the old Lowdowns going back to the 1980s.
Keep on experimenting!
73
Ed, KI6R
SaddleBrooke, Az



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K1FZY (WA4TPW) SK 9/29/37-4/13/15

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