2007-12-16

# HF Receiving Loop

This experiment was inspired by Peter VK2TPM's receiving loop experiments for 80 metres. I saw his blog post last night and noted that his resonant frequencies looked a bit high, so I emailed him off my back-of-the-envelope calculations for what I figured he should be seeing. This morning I decided to check my own math by building a loop myself.

My loop is a bit smaller than Peter's, a single turn, the circumference is 4 metres + a little for the aligator clip lead, made from medium gauge mains lamp cord ("zipcord" for the yanks). The frame is just two pieces of 25 mm electrical conduit, lashed together with some sash cord. The winding simply taped in place for this quick experiment.

The "ring conductor" inductance formula suggests an inductance on the order of 6.4 uH, maybe a little more because of the relatively thin gauge of the wire. Direct measurements of the inductance at low frequencies using my larger-value inductance meter showed a similar order of magnitude, but the meter has limited resolution, at such values.

To get more accurate measurements I resonated the winding with my C Jig and coupled the loop into my VR-500 receiver with a large ferrite ring and some alligator clip leads. This allows detecting resonance at any frequency the receiver covers by just tuning the capacitance for maximum noise. In the local RF environment (lots of computers and telecommunications gear) there is plenty of noise for this kind of test, but even in the middle of no-where there should be a very discernible noise peak at resonance.

The receiver side of the coupling loop has too few turns I believe. The transformer reflects the receiver input impedance as a transformed value in series with the LC resonant circuit, degrading its Q. The LCR circuit so formed would be optimised by reducing the R (loss plus transformed loading) as much as possible, so the transformer ratio should be large to make the inserted series impedance very small (c.f. typical RF ammeter construction). Alternatively you might couple across the LC circuit instead of in series with it, into a very high input impedance amplifier, like a JFET.

The Q-degrading is easily observed with the receiver, the loop is not near as sharp as I initially expected, but the simplicity of the feed works for the purposes of the experiment.

Low-Q Demonstration
(1.926 Mbytes)

Systematically measuring the capacitance needed to resonate the loop from 4-17 MHz showed an interesting pattern which I can't quite explain. The affective inductance of the loop appears to rise with frequency. Some rise is expected as the distributed capacitance of the loop becomes more significant at higher frequencies (becomes non-trivial compared to the tuning capacitance), but it does seem to take off a bit too fast above 13 MHz.

 MHz pF uH 4 5 6 7 8 9 10 11 12 13 14 15 16 17 >210 150 110 80 67 44 36 29 24 20 16 13 10 <10 <7.5 6.8 6.4 6.5 5.9 7.1 7 7.2 7.3 7.5 8 8.7 9.9 >8.8

Note that the numbers at each end are bounds because of the limits of the C Jig, and the outlier at 8 MHz is likely invalid because of the resolution limits of the C Jig calibration (this is the point where I switch between its scales), but the numbers do show a 6-7 uH inductance which is roughly consistent with the initial inductance estimate. Unfortunately the loop is too big to fit in the lab and measure with my full range of test gear, but from a purely empirical point of view the loop works fine, and the calculations give you figures in the right ballpark.

Still, I was curious just what was going on... At first I thought the receiver input impedance was being reflected into the loop circuit through the transformer (or was so high the transformer primary reactance was being seen directly, 1 fully-coupled turn on a FT240-43 core is about 1 uH and the affective coupling would rise with frequency). I also suspected the ferrite material itself may have been causing the effect, but some checking with the tone dip meter showed consistent results without the coupling transformer in place. I started to doubt the calibration of my C Jig, but after testing that with fixed inductances back in the lab, it appears there is another effect at work. I'm at a bit of a loss, but it might be related to the skin effect, or I've underestimated the distributed capacitance significantly.

Dipping at 40 Metres
(517.605 kbytes)

Rocking Across Resonance at 5 MHz
(1.010 Mbytes)

Confirming Resonance by Dipping at 10 MHz
(4.950 Mbytes)

My capacitor wasn't large enough to take the loop down to 80 metres, at least not without adding more turns, but there is no reason why a similarly sized single turn loop couldn't be used on 80. The normal LC circuit calculator would give you the required capacitance.

This experiment was quite a bit of fun on a rainy Sunday. Very educational, especially thinking about the transformer feed and how I might use it in a transmitting version. I think such an antenna with remote varicap tuning would be a good addition to the shack. I was doing some experiments with varicap tuning yesterday, while fiddling with a VCO circuit. Red LEDs are surprisingly good varicap diodes, but I have a few BB-series diodes for AM broadcast receiver use that should do the trick.