Hi David,

For a few reasons:

1.) Noise floor is not an issue on low bands because propagated noise power is so high.  Even on 160M in a quiet rural area with narrow filters a -20dBi antenna does not need an amplifier. Noise is even higher on LF.

2.) The system bandwidth determines noise power. The filtering can be anyplace in the system as long as none of the stages before narrow filtering are overloading. If nothing overloaded it would not matter if the filtering was in the audio. As a matter of fact, digital modes using a PC do all the narrow filtering in the audio chain. If nothing overloads or pumps the AGC we can run 3kHz or 250Hz radio filters with no change.

Now if we put an old Sky Buddy receiver on the antenna additional antenna selectivity would help, but that is a receiver shortfall.

My LF and HF system has  broadcast band reject filters (that knock AM BC back 20 dB) because the accumulation of AM stations at night on my Beverage antennas can overload my wide band amplifiers, but any additional selectivity ahead of receivers is completely meaningless. It won't help S/N one bit, because the receiver filters (or computer) set the noise bandwidth of the system. 

73 Tom

Why would you not want the antenna to act as a preselector narrowing the bandwidth of the front end?

The front end noise dominates the receiver performance, and wide band = noisy.

I understand that if it's narrow you need to retune more, and that's a valid trade if you're wanting to be very frequency agile.

On Mon, Sep 9, 2024 at 3:01 PM Tom W8JI <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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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