My first tests using the 24″ aluminium bicycle rim did not work out well. This was, as you may have already read elsewhere, due to problems with the Faraday coupling loop that I was using at the time.
Having had extremely encouraging results using the LDF4-50 based loop I decided to try the bicycle rim again but this time I simply used the working coupling loop from the LDF4-50 loop.
So here’s a photo of the 24″ loop:
I made two cuts in the rim opposite the hole that was left by the inner tube valve to form a small 1/2″ (12.5mm) wide gap. A small section of Perspex was then used to bridge this gap to maintain shape and rigidity. The same method of tuning was adopted as used on the larger loop i.e. a large split-stator capacitor for the coarse tuning plus a small butterfly capacitor in parallel for fine tuning. The coupling loop was a very simple single turn of copper micro-capillary tubing as used on boiler thermocouples; this was formed into a 5″ (125mm) diameter loop.
To tune the loop, I normally set the small (butterfly) capacitor so it’s half in mesh and then adjust the large capacitor to peak the received signal, then using a very low output (typically 1W) I adjust the small capacitor for minimum VSWR on transmit.
This loop tunes approximately 20 – 35MHz and is used mainly on the bottom end of 10M for digi-mode contacts and experiments (WSPR, QRSS etc)
The results with this size loop, setup on a stand, indoors, have been very encouraging, running WSPR tests with less than 2W output have produced the following results:
WARNING: Because of the very high Q, some capacitors can arc over at power levels as low as 10 watts. Remember also that even with only a few watts of RF power, magnetic loop antennas produce very high voltages across the capacitor(s) and can cause nasty RF burns if touched while transmitting.