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Tuned loop antenna idea, will it work?

I am thinking about trying a loop antenna idea. The loop will be tuned by applying a tuning voltage to the coax center lead and tuned by two varactor diodes, 30 to 550pF, in series.

Any chance this arrangement will work? (Or is it a waste of time and effort).

As for the material for the main loop, I've got a few meters of old copper fuel line I could use. Or, will a normal electrical wire do?

PS:

Should I connect a couple of diodes directly across the gap in the main loop to short out any voltage spikes that might be induced?

30 comments

30 Comments

coildog

Oct 03, 2022

I've done some more work on my loop project today, built a new remote loop amplifier and installed a bigger loop. Approximately 20m wire in a horizontal circular shape, a couple of meters above ground. Schematic of the amplifier was inspired by this video:

My amplifier draws only 1.2mA. I think that makes the input impedance higher, but the circuit uses less current and could possibly be run off of a battery. Instestead of the broadband transformer, I've used tuned circuit.

EDIT: (A day later...) There was an error on the schematic. It has now been corrected.

Seems to work well on higher frequencies, say from 12MHz and up. But it seems to be lacking sensitivity at lower frequencies, 49 and 41m bands. Or maybe it is the solar storm that is to blame. I'll have to listen a bit more. (Yes, there is a geomagnetic storm at the moment)

qrp-gaijin

Sep 15, 2022

Here's a link to an old NASA paper by J. F. Sutton showing that regeneration applied to the antenna actually increases the physical area (volume) of the EM field with which the antenna interacts, thus improving antenna performance above and beyond the voltage gain of the Q-multiplying amplifier. I like to think of regeneration applied to an antenna as an invisible way of physically enlarging the antenna. It especially helps with really tiny antennas such as hand-held loops or ferrite rod antennas.

https://ntrs.nasa.gov/api/citations/19940020710/downloads/19940020710.pdf

Selected quotes from the article (emphasis added):

We wished to confirm by measurement the principle that antenna current causes an interaction with the signal field such that energy is intercepted from an effective area greater than the antenna geometrical area. Accordingly, a preliminary experimental test was performed on a tuned, regenerative loop antenna-amplifier receiver circuit similar to the standard regenerative receiver circuits of the 1930's. This Q-Multiplier circuit was designed to maximize the antenna coil current consistent with the requirement of circuit gain stability and tuning stability. The circuit diagram for this receiver is given in Figure 7. For comparison purposes, a simple voltage amplifier, Figure 8, was also constructed. This amplifier was designed to have a high input impedance to keep coil current to a minimum so that antenna-field interactions would be minimized. These two amplifiers were used to monitor the 24.3 kHz sideband generated by the U.S. Navy radio station in Cutler, Maine.

[...]

The results were: Voltage Amplifier: -80 dBV, and Q-Multiplier: -40.5 dBV. That is, with the same ambient signal field and the same pickup coil in the same position, the measured output signal of the Q-Multiplier circuit was 39.5 dB greater than that of the simple voltage amplifier.

SPICE analyses (see Appendix) were performed on the two circuits to determine voltage gains. [...] the Q-Multiplier had 26.3 dB more voltage gain than the voltage amplifier. The difference of 39.5dB - 26.3dB = 13.2dB represents an output from the Q-Multiplier circuit which is more than 4.5 times greater than it should be on the basis of circuit voltage gain alone. This means that the effective area of the coil in the Q-Multiplier configuration was over 20 times greater than the geometrical area of the coil and that, therefore, more than 20 times more power was intercepted from the plane wave front, resulting in the 13.2 dB greater output voltage. This is sufficient to demonstrate the principle of plane wave-dipole interaction. The increased output is real signal and therefore represents a real 13.2 dB improvement in signal to amplifier-generated noise ratio.

qrp-gaijin

Sep 15, 2022

Replying to

Probably the most troublesome component in such a setup is the varactor, which as we know is prone to hum problems when used in a loop antenna. Hum seems to get worse when regeneration is applied. However, your success in eliminating hum in your non-regenerative, varactor-tuned loop has me wondering if the same strategy will work with a regenerated, varactor-tuned loop. Somehow I am still worried that the lack of loop balance (due to grounding at one side, not at the center), plus the increased antenna currents with regeneration will still make the system prone to hum pickup.

On the other hand, I never experienced that kind of hum when using a variable capacitor to tune my loops -- even regenerated loops.

So if you want a regenerated, remotely-controlled loop, a motor-controlled variable capacitor is probably the best option for tuning the loop. For controlling the regeneration, I would say that a cross-coupled pair right at the loop, with a battery at the loop, should work, with regeneration being controlled for example by a light-dependent-resistor in the emitter of the cross-coupled pair.

coildog

Sep 15, 2022

Replying to

hm ... funny you should mention LED

a couple of weeks ago I toyed with a LED, wondering about what I could use it for. I made this:

Just a small 12V 40mA light bulb and a LED, mounted on a piece of polystyrene foam. It fit snugly in an old coil can, the coil itself is 'used up' long ago.

I think I'll give it a try using an armstrong type feedback coil to replace the pick-up coil in my existing amplifier.

But it needs to stop raining!

qrp-gaijin

Sep 16, 2022

Replying to

Looks great. I'd be very interested in hearing if you manage to get a remotely-controllable, regenerative loop antenna working -- especially if you succeed in making it work with varactors!

qrp-gaijin

Sep 10, 2022

Since you are experimenting with both loops and whips, and have also expressed interest in regeneration, have you seen this design? It describes itself as a "universal LF/MF preamplifier" and also has regeneration. It can surely be adapted for HF as well. I didn't build it, but it looks interesting.

https://www.transkommunikation.ch/dateien/schaltungen/diverse_schaltungen/radio_circuits/Universal%20LF-MF%20Preamplifier.pdf

As I live in an electromagnetically-noisy environment, I have found that loops are much better than whips in my case, because the whips tend to pick up local electrical noise more readily, whereas the loops are more immune to electric-field noise in the very near field area. Also, I did some experiments with regenerating a whip antenna and it was very touchy with a lot of hand capacitance. Loops are easier to regenerate in my experience. This makes sense, because with regeneration, the whip antenna becomes much more sensitive to the local electrical field, which is easily disturbed by your body. On the other hand, the loop antenna, when regenerated, becomes much more sensitive to the local magnetic field, which is less easily disturbed by your body, meaning less hand capacitance.

coildog

Sep 20, 2022

Replying to

Update to the IF-filter:

I removed the ceramic filter and replaced it with a black IF-can instead. That allowed me to chose a different IF frequency. I moved it up to 520 kHz, I guess I could still go a bit higher as there is still a tiny bit more room to tune the IF cans.

I must say, I think the IF-cans sounds better than the ceramic filter.

Circuit:

w5jag

Sep 20, 2022

Replying to

Did you match the impedance of the input / output of your ceramic filter to the TDA1072?

The IF input to the TDA1072 is 3K, but my Murata ceramic filter (+-4.5 KHz) is 2K, so it required matching to the TDA1072.

The Murata literature states:

"This example shows the changes produced in the frequency characteristics of the SFZ455A ceramic filter when the resistance values are altered. For instance, if the input/output impedances R1 and R2 are connected to lower values than those specified, the insertion loss increases, the center frequency shifts toward the low side and the ripple increases." (my emphasis added).

If your filter was looking into a higher resistance, that could explain the apparent upward shift of the IF center frequency to 458 KHz, Or they could just be crap filters 😀 .

I just added a parallel resistance between pins 3 and 4 to bring it down to 2K - that seemed to work as my filter is right on the specified 450 KHz center frequency.

73,

Win W5JAG

murata piezo filter introduction

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Download PDF

CFU455H2

.pdf

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coildog

Sep 21, 2022

Replying to

You are probably right regarding the filter! I didn't know that the input impedance of the TDA1072 was so low, so I have probably mismatched the ceramic filter.

Picture of my cheap chinese filters:

I think I had a 1.8k resistor (or maybe it was 2.2k?) in parallell. And a 1.8k in series from the mixer to the filter. The filter seemed to give two maxima. The most prominent responce at around 458 kHz. A weaker peak at a lower frequency.

The result was that I could hear strong stations before they came into the passband. The resulting impression was that the responce was broad, about 20kHz. With two IF cans tuned to the most prominent filter responce, I was more or less able to eliminate the false responses.

In the end, I think the receiver sounded better when I used three IF cans without the ceramic filter. But that is probably subjective to my ears. The passband skirts are probably gentler and work better with music.

I am tempted to try different coils and capasitors and see how high up I can move the IF and still get an acceptable selectivity out of it. There are so many things to try!

coildog

Sep 10, 2022

Update saturday, september 10th:

I'm a bit confused about exactely where the comme