Yesterday, I ventured back onto the north end of the Long Mynd, well outside the AZ, but with clear takeoff towards the Stoke repeater / beacon, to carry out a more objective antenna comparison.
The first surprise was the level of QSB, which made accurate comparison difficult, as it takes time to swap antennas. At the start of the test, the signal was often low, around S2, peaking to S9 for several seconds at a time. After an hour, this pattern had almost reversed, the signal being often around S9, sometimes sinking right down.
I’m guessing this is likely to be multi path phasing, with the Wrekin as a possible reflection suspect. I have a lot to learn about this band, though!
Anyway, the setup was an FT290 connected to the SG Labs transverter via a switched attenuator 0 to 15dB in 1 dB steps. I connected an analogue multimeter across the S meter, to give an easy to read level reference point.
Method - connect PCB antenna, note meter reading. Swap antenna, adjust attenuator to give the same reading. Repeat.
(I did move the antennas relative to each other, and to the car, in case there was interaction. None discernible)
I repeated the comparison a number of times over the course of an hour, noting in each case the QSB peaks. Not ideal, but by the end I was reasonably happy with the result. I may experiment with different locations, to see if I can achieve a steady signal.
So, the main comparison was between my home brew 15 ele, and the SG Labs supplied PCB antenna. I haven’t found a spec for the latter, but it is 2 elements, both fed. So, I’m assuming similar performance to the HB9CV at around 4dBd.
Result: My 15 ele appears to have 8dB gain over this, so approx 12dBd.
The theoretical gain (from the K7MEM calculator) is 13.9dBd.
A couple of dB off. Is that measurement error? Deficiency in my construction? Absorption in the PVC boom? Difference between theoretical gain and what can be achieved in practice? Feeder loss? - though 1m of RG223 should be a lot less than 1dB.
Don’t know yet, but I’m happy with it for now!
What of the old J Beam 15 over 15? Well, I didn’t have a way of mounting that vertically polarised, so I couldn’t spend the same time watching the QSB on the meter. I did hold it in my hand at the same height as the others etc for a while, and it seemed very similar to the HB Yagi.
All good fun
All good fun