[600MRG] re. Loop Antennas
Tom W8JI
w8ji at w8ji.com
Mon Sep 9 22:27:10 EDT 2024
Absolutely Ed.
I have to use AM BCB reject filters here or my distribution amps with
overload from the accumulation of hundreds of strong AM BCB signals at
night.
A 12V 50mA panel lamp will glow dull red connected to the distribution
amplifiers output if I do not use a BCB reject, but that is all the
filtering I need and I only need it on antenna that rock the input like
the Beverage arrays.
My point is simply that filtering can be anyplace before the overload
point of the system. There is no advantage to making an antenna narrow
when the same thing could be done in an easier less complicated
location. Filtering just has to be ahead of the overloaded stage, and
there is no advantage in noise figure by making an antenna narrow rather
than doing it downstream. Filtering can be as far back as the audio if
nothing overloads.
73 Tom
On 9/9/2024 9:30 PM, Edward Larsen wrote:
> 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>
>>> <mailto: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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