When I erected my 80m aerial I
delberately made it longer so I could trim it later. The usual
way to do this is to use a transmitter and SWR meter and, by
trial and error, just chop off short lengths of wire until the
performance is optimum for one's favourite frequency.
Having acquired a spectrum analyser
I wondered if this could be used to look at the aerial's performance
Looking on the Net it seems a directional
coupler could be used to do this. This device is basically the
main part of an SWR bridge. Add a couple of diodes and a sensitive
meter and you can measure forward and reflected power. The greater
the difference between these two values should provide the resonant
frequency of the aerial.
The picture below shows a directional
coupler I made in an hour or so from junk box bits and pieces.
The tracking generator feeds in at the
lower left and the 80m aerial upper left. The spectrum analyser
input is fed from the right.