Magnetic loop measurement – the proof of the pudding is in the eating.
On the left of the above picture is the heart of my magnetic loop. This is a resonant circuit formed by a 5 foot copper loop and a home made butterfly capacitor The loop and capacitor are joined mechanically (clamping) and electrically (solder) with a short copper strip.
After connecting an antenna analyzer to the loop and running an SWR scan, I was thrilled to see a match at roughly were I expected one. The SWR showed around 1.1:1 at 11.4 MHz. This indicates both a resonant frequency for the loop, and a decent match to a 50 ohm coaxial cable.
However, all is not well. What worked in theory is not doing so in practice.
My design of this loop was mainly for 10 MHz ham band, as well as covering from roughly 7.0 to 15 MHz. The design software tells me that a 5.2 foot diameter of 1/2 copper tube has an inductance of roughly 4.7 microhenries. Thus, to cover the frequency range I seek requires a variable capacitor of around 20 to 100 Pico farads. (If you want to check my work, you can find the formula for LC resonance here, and a magnetic loop calculator here.)
Previously, I had measured my capacitor as approximately 20-45 pF. So I knew that I would not get enough coverage down to 7 MHz, but might be okay from 10 – 15 MHz which would be a good start.
However, when I did my magnetic loop measurement tests, I could only detect resonance and low SWR between 10.3 to 12.5 MHz – a very narrow range. This tells me that my capacitor not providing a wide enough range, or the copper loop has a significantly different inductance than I though it would have. Or, maybe both.
Magnetic Loop Measurement – Time to Dig Deeper
Some things to try include substituting a different capacitor and making a more stable arrangement for mounting and adjusting the coupling loop. Oh, and as expected, the plastic mount at the top cracked under pressure from the copper tube, so that needs to be replaced.
So, mixed feelings at this point. But what the heck. I have never done this before and there is a lot to learn.