My weekend project this time was to build a cheap magnetic loop antenna. I’m talking really, really, cheap. No variable capacitor, no fancy or expensive parts.
The goal was to build a monoband magnetic loop antenna. I’m tempted to call it “mono frequency” also, since there are no adjustments to it, but given the bandwidth of about 15kHz, it would be wrong to use that term.
I started off by taking a hulahoop my kids used years ago, and inserted a RG213 stub inside. Only the braid of the coax, the shield, is used. The pickup coil is a stub of RG58, about one fifth the diameter of the outer loop, where only the shield is in use.
As a replacement for an expensive variable vacuum capacitor, I simply used a length of RG58 a mock capacitor. I coiled it down the fibreglass pole out of convenience; the shape and position is irrelevant of its functionality.
The mock capacitor was then trimmed off in small sections, maybe around 1-2 cm at the time, while simultaneously keepeing an eye on the NanoVNA to make sure I didn’t overshoot the target resonant frequency. Once I got close enough, I trimmed smaller and smaller segments off, until the antenna resonated at the 20m FT8 frequency.
The antenna was then connected to my FT-950, where I started off by transmitting a couple of short bursts of low power (5w) carriers to monitor the SWR. To my surprise, SWR was about 1.2 all the way from 14.070 to 14.085 – that’s a smashing 15kHz bandwidth!
I carefully tried a couple of contacts on FT4 and FT8 at 20 watts, and got a few in the log. It works!
I also carefully tried increasing the RF power, but at 30 watts the SWR jumped and I didn’t want to risk my equipment. I suspect the RG58 doesn’t like the voltage building up. I decided to stay safe at 20 watts and was more than satisfied with the results!
SWR is stable, regardless of location or direction. It did not change during transmission either. The reception is amazing, compared to the EFHW behind my house.
The total costs for this antenna was $0, since I had all the parts laying around. If I were to build another by purchasing each item, I guess the total cost would be 10 to 15 us dollars, with the SO-239 being the most expensive part.