Alan,

The best way to answer is to quote from section 5.11.2 of the Trig
installation manual. They state that excessive cable loss will degrade
both transmitter output power and receiver sensitivity. Then they
define the three qualities of an acceptable cable:
(1) less than 1.5 dB loss for the run length,
(2) a characteristic impedance of 50 ohms, and
(3) double braid screens, or a foil and braid screen.
They then say that their table of maximum usable lengths for common
cable types is a guide only, and tell you to refer to manufacturer’s
data sheets for your specific chosen cable. This is clearly permission
to choose your cable type, as long as it meets Trig’s three acceptance
criteria above as backed up by specific manufacturer’s data.

As for “knowing” about cables, like anything else you need to do
research and get educated. I’ve tried to let this forum know that
there are much better choices for microwave cable than RG-58 and
RG-400. Naturally, people will do as they wish, even to the extent of
tossing away transponder output power and getting less receiver
sensitivity. It may work out OK, or it might possibly be a link in an
accident chain someday. I have the satisfaction of knowing that in my
transponder installations pilots are getting 175 W transponders that
meet or exceed their performance specifications for just $20 extra in
cable.

-John

On Feb 8, 2:04 am, (Alan) wrote:

The problem is that "better" isn't just lower loss per foot when the cable
is new. There may be a lot of other factors that the manufacturer took into
account with their cable selection. "Better" needs to be better in all of
these factors.

The transponder may need a minimum amount of loss in the coax to ensure that
the SWR seen by the transmitter is low enough in case something gets near the
antenna. (This would protect both the transmitter, and the receiver, as a mismatch
at the duplexer often reduces the isolation between the transmitter output and the
receiver input.)

I suspect it LMR 240 would work better than the other cables, both in loss and
service life, but I don't know it. As JJ notes, he doesn't know it for sure, and
and would need to.

Alan