That's a good formula for a starting point.
Use an antenna analyzer to get it right.
Quoting Robert Ham <email@example.com>:
> Does anybody have available the formula for the length of the coax
> balun on the old telrex antennas? It was on towertalk a while back
> and I misplaced it. As I recall it was an electrical half wave times
> the velocity factor of the coax used. Say 492(V) / f(MHZ)? Any help
> would be appreciated.
> Robert Ham
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