Mobile 20-meter band antenna mounted on a speaker stand
Today I mounted my Diamond HF20FX (a mobile 20-meter band antenna originally bought to use with my mag-mount on the car) on a speaker stand in the spare bedroom, along with counterpoises (or radials).
I found a counterpoise calculator on the website of M0UKD ( https://m0ukd.com/calculators/quarter-wave-ground-plane-antenna-calculator/ ) which suggests a counterpoise length of 5.67 meters for a frequency of 14.070Mhz. In fact, there is some discussion online about the number of radials and the length of radials necessary for an effective quarter-wave vertical antenna ( https://www.radioenthusiast.co.uk/news/radials-for-quarter-wave-verticals-an-overview/ ). I should first mention that the length of the Diamond HF20FX is adjustable and that I had previously tuned the antenna for a minimum SWR on a frequency of 14.070Mhz on the car mag-mount.
My initial experience with the mobile 20-meter band antenna using a single 5.67-meter long counterpoise was disappointing as it did not produce a strong reduction in the SWR across the 20-meter band. However, a second counterpoise just 5 meters long did produce a strong dip in the SWR across the 20-meter band centred on 14.070Mhz, when measured with a NanoVNA-H4. An additional third counterpoise, just 4 meters in length, left the centre of the dip in SWR centred at 14.070Mhz with the SWR further reduced to about 1.1:1.
By this point, I was using up scraps of wire from my junk box. A fourth counterpoise, 3 meters in length, increased the SWR and seemed to be capacitively coupled to my body, as the SWR fluctuated every time I moved. So this counterpoise was removed.
Now satisfied that the antenna was working in principle, I replaced the NanoVNA-H4 with my Sotabeams WSPRLite antenna tester, running WSPR between 14:30 and 15:00 today. When you consider that the mobile 20-meter band antenna was mounted indoors on a speaker stand in the spare bedroom with two counterpoises hanging out of windows and the third counterpoise running down the stairs, the results were surprising. I was transmitting just 200 milliwatts and yet my WSPR signal was heard in Fuerteventura, the Canary Isles; Otradalur, Iceland; and Urbino, Italy.
I now look forward to better weather, when I shall repeat testing on the patio and perhaps further afield.