That's because there are as many "rules" as there are thumbs. ;-)
I don't know about anyone else, but I can tell you about the highly
scientific method I use.
I start with a multiple of 1/2 electrical wavelength and trim as
necessary. I'd stay away from an odd-multiple of 1/4 wavelength in this
application... no good reason, just because (black magic and all that).
Try cutting the transmitter-to-duplexer line using the receive frequency
length, and vice versa. If that doesn't work out, you can swap them.
I know, on a 2m amateur system, the length difference is about 1/4". In
that case, make a cable 1-2" shorter and see what happens.
An alternative is to use multiple short lengths of coax connected
together to find a "happy" length, then replace with a single coax cut
to that length.
As I said, highly scientific. :-P
73, Russ WB8ZCC
On 8/14/2010 2:59 PM, Ross Johnson wrote:
So will someone post a simple rule of thumb... If you have the option
of optimizing cable length from PA to first cavity, IE you haven't
made them yet what's the best "simple" rule of thumb to follow to
build them to avoid reactance. 1/2wl if allowed minus coupling loop
depth? Or is that past a simple thumb. Also, This will obviously not
work well for 220 or 440 or a most vhf repeater setups. So what would
the next ideal cable wl be? And so forth... The reason I ask, if your
building new cables why not? Answers on here seem to range a lot...
Ross kc7rjk
