com radio coax length

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 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

Old wive's tale. You will get "experts" in here talking about rotation
about the Smith chart for every half-wavelength and that is true. However,
what you are simply doing is a rotation about a "constant VSWR circle" and
all extra coax buys you is whatever loss that extra coax has in that extra
length.

Cut it to fit.

Jim





"Stew Hicks" wrote in message
news:Ewv5f.232579$1i.88992@pd7tw2no...
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

Thank you very much for the answers...............Stew





"RST Engineering" wrote in message
...
Old wive's tale. You will get "experts" in here talking about rotation
about the Smith chart for every half-wavelength and that is true.
However, what you are simply doing is a rotation about a "constant VSWR
circle" and all extra coax buys you is whatever loss that extra coax has
in that extra length.

Cut it to fit.

Jim





"Stew Hicks" wrote in message
news:Ewv5f.232579$1i.88992@pd7tw2no...
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