com radio coax length

[Stew Hicks]

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

[John]

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

[RST Engineering]

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

[Stew Hicks]

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
>