Antenna help please

[greg vos]

I’m working on a gyro, it seems to have lost its Tx, Rx antenna, it’s equipped with an ATR 500
My calculations using a 50ohm cable from radio to outlet on frame is I need to install a 600mm long antenna?

I have calculated this length based on a flat 125 MHz as being in the center of our frequency range (118-136Mhz) standard aviation frequencies? We accept it’s not perfect and an ideal length would be for every frequency in our spectrum

Can some one with some more experience than me please comment or confirm I’m in the ball park, I would hate to damage the output transistor of the radio by keying the mike, currently I don’t have access to a SWR meter.
Any advise appreciated .... been a long time since I have had to make calks like this

Futher to this I don’t recall seeing an antenna On any aircraft I’ve ever flown with such a long section .... have I got it wrong?

[nomie]

Hi Greg,
600mm is about right, but lots of factors will determine the final length needed, like ground plane size, etch (Unless you are going dipole).
The process usually involves making the antenna longer than needed, then cut a bit off at a time until the VSWR is minimised on the frequency you choose. Doing it without a SWR / signal analyser / network analyser will be a bit challenging. Can you maybe borrow one?
At 600mm nothing should blow up, the VSWR will just not be optimal.