Microwaves are cool, but difficult to work outside of the NAC contest once a month... And what if it rains that day?
My earlier experiments with OCFDs and verticals suggest this is not exactly easy thing to do.
The first criterion is satisfied by this seld-made magnet loop antenna. Its circumference is ≤ 10 × λ at 14 MHz so it is truly an electrically small antenna (a magnet loop) in the 20 m band [1], which is the band where our guide to becoming true radioamateurs explicitly tells us to work [2]. Incrementally reducing the circumference of the small feeder loop, a really good return loss of ≤ 30 dB and an impedance match to 50Ω coax was achieved, these measurements were calibrated to the antenna connector. The Q factor at 14.2 MHz was approximately 600, which for the physical dimensions implies an efficiency of 30 % and 60 mΩ of additional losses, most probably from the tuning capacitor assembly [3]. I say this is very good!
On a calm days the antenna can also be mounted on top of a lightweight Manfrotto camera tripod. But the rotator is quite handy as the antenna has sharp nulls which can be be pointed towards interference sources...
So far I have worked Japan, Australia, U.S.A. and many European countries on 20m band with FT8 from the rooftop. Changing bands is quite easy just by tuning the radio to the target frequency and turning the knob until a peak in the noise is heard, and then fine adjusting for lowest SWR.
The loop works really well, actually way better than I hoped, however the Q is so high on 20m that the heating caused by direct summer sunlight affects the tuning...! It was arduous (also too unsafe!) to climb up ladder every now and then and turn the tuning knob by few degrees when working on the same frequency. So I built an constructed an tuning motor, seen in this picture and a fancy controller for it, the dial was salvaged from old junk. The motor assembly has a DC motor with a reduction gear, 10 kΩ 10-turn potentiometer connected by cheap timing belts from a well-known online market. I was lucky that the capacitor covered 14-30 MHz in less than ten turns...!
[1] F.E. Terman, Radio Engineers' Handbook, Mcgraw Hill Book Company Inc., New York and London, 1943
[2] Mikko, OH2MP, "Ohjeet kuinka tulla oikeaksi ja uskottavaksi radioamatööriksi", http://oh2mp.ham.fi/oikea_amatoori.html
[3] B. A. Austin, A. Boswell and M. A. Perks, "Loss Mechanisms in the Electrically Small Loop Antenna [Antenna Designer's Notebook]," in IEEE Antennas and Propagation Magazine, vol. 56, no. 4, pp. 142-147, Aug. 2014.
The second criterion may be fulfilled with FT or JT65 modes
Updated 21.9.2018
Disclaimer: This webpage is for general information purposes only. It is not professional advice and contains opinions and most probably inaccuracies. Viewer discretion is advised.
