[600MRG] re. Loop Antennas

Mon Sep 9 21:30:02 EDT 2024

Hi Tom,
   It depends upon the location.  Some areas have crushing loud AM
broadcast that requires filtering out somehow for VLF listening.  Down in
the 10-30 kHz range the Navy transmitters have huge signals compared to
weak signals like the Russian Alpha stations.  This is where a really
selective loop can excel.  On VLF I usually run an AMRAD upconverter to the
80m band.  Currently my 80m IF receiver is a Flex 6600 which is nice due to
the panoramic display.  Depending upon where I am tuning I've seen the
AMRAD upconverter intermod due to Navy transmitters in the 24 kHz range,
WWVB when listening above 60 kHz, nearby NDB beacons if they are a local
transmitter, and AM broadcast band when listening below 500 kHz.  Lowpass
or highpass filter of course can solve the problem.  When one uses a highly
selective loop antenna, overload, desense and intermod problems are
eliminated.
73
Ed, KI6R

On Mon, Sep 9, 2024 at 2:01 PM Tom W8JI <w8ji at w8ji.com> wrote:

> What are you using for a receiver? What is overloading the receiver?
>
> I haven't used a tuned loop for a  receive-only system since around 1970.
> As a matter of fact I avoid them! I either resistor load my loops or
> directly drive a low noise broadband amplifier with impedances set to make
> the system broad band.
>
> 73 Tom
>
> On 9/9/2024 12:10 PM, Dave Riley via 600MRG wrote:
>
> 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>
> <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> 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> 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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