com radio coax length

Stew Hicks wrote:

For radio experts,
I read somewhere that coax should be cut in increments of 1/2
wavelength, for the center of the bandwidth, 1 1/2 meters as I
recall. I can't find the article now can anyone verify this? How
critical is this?
Thanks for any help...............Stew


Stew
It' is true.
But a lot depends on your application as to how critical it is.
The reasoning is this:
impedacne "inverts"every 1/4 wave, so if you make your coax 1/2 wave
increments it "double inverts" which means the impedance is the same at the
input end as at the far end. If you had a purely resistive impedance of
exactly 50 ohms ( for standard radio transmitter/receivers) you really
wouldn't care about length, but being that the impedance of a typical
installation is somewhat unpredictable and is almost certainly a complex
impedance with both resistive and reactive componets it's best to stick
with the 1/2 wave increments.
That being said make sure you figure the 1/2 wave in coax not air. IE if the
coax is 66% velocity factor you figure the 1/2 wave length in air then
multiply by .66 and then make it a multiple of that!
Clear as mud?
John