Honey, I shrunk the loop

Like most amateur radio antennas, magnetic loop do not receive positive looks from relatives, neighbors and by-passers. My 20m magloop on the balcony could be seen by us and very few other people, but would still hurt the eye when looking out in that direction.

So, I shrunk the loop, from 2.3 m circumference to 1 m. Of course it now operates on a different frequency: 50 MHz. It has been chosen on purpose, since May and June are the Sporadic-E ("ES") season. Thanks to the lockdown and work-from-home situation I stand more chances to be in the shack when ES hits, unpredictable as usual.

I changed the tuning capacitor from an air variable to a RG-58 coax stub, meant to withstand higher voltage, thus input power. It did work on the 20 m loop, but it doesn't on 6 m.

This is what happens. On the VNA the antenna is perfectly matched. On the transceiver the SWR meter is happy.  Then, if you keep the key down, SWR increases to well over 10:1. On the 20m loop the stub tip would produce a bright white spark when I raised the RF power above 30-40W, which was solved by making a bit more space between the braid and the inner conductor. On 6m there was no spark: the RG58 stub would heat up and change the impedance.

IMPORTANT. How to check if it heats? Either use a thermal camera or touch the stub once that both these conditions are met: you have produced the SWR increase AND you have have switched off the transceiver completely (so that it doesn't transmit while you sense the temperature with your fingers).

This is a sign that the voltage at the open loop side is high, so the match is good. Also, being the circumference close to lambda/4, the efficiency is higher and the same holds true for the voltage.

The maximum power the loop can withstand depends on the environment temperature, but the heat produced with 5-10W is negligible and has no effect. At least at 20 °C!

NanoVNA as Signal Generator

If you are in a hurry and need a quick & square wave signal between 50 kHz and 300 MHz the NanoVNA can come to the rescue. Just set the centre frequency of the stimulus at the value you need and a span of 0 Hz. The signal is then available at port 1.

Be very careful not to overload the output or feed a voltage/signal to it, else the NanoVNA would be damaged! If the procedure and implications are unclear to you, do not try it.

Why does it not work all the way to 900 or 1500 MHz? Because the NanoVNA(-H) uses some clever tricks using harmonics at frequencies above 300 MHz, but you always get the basic signal 0-300 MHz at port 1.