Antenna and Coax Length

"ContestID67" wrote in
ups.com:

As others have pointed out, presuming your antenna is
"correct", its RF feed will have an impedance of 50ohms.
Since the coax is also 50ohms, there will be no
impedance mismatch, therefore no standing wave and VSWR=1.

In practice there will be some non-ideal effects:
- sharp bends in the coax will disturb its impedance
- the antenna will only be "correct" at a single
frequency. The antenna design and nearby objects
determine the frequency band over which it is
"correct enough"
In practice, presuming you have a narrowband signal
(say 5% of the carrier frequency) and the antenna
is for that carrier frequency, I doubt that you will
have much trouble.

If you have trouble, how might it manifest itself?
- the RF amp might blow up: that's your problem
- the RF amp might become unstable and start
transmitting on harmonic frequencies: that's
everybody else's problem, and the regulatory
authoratories can become unpleasantly interested
(for very good reasons)

Safest to:
- get the correct antenna
- measure the VSWR at the tx carrier frequency,
and check it is within the RF amp's limits
- ideally, use a spectrum analyser to check the
level of the 2nd and 3rd harmonics

I may be all wet on this subject as I am an electronics engineer (a
bit pusher) and not an RF electrical engineer. Any comments?

Unless you are using obsolete logic, you ought to know
about RF effects! Consider that modern logic has
sub-nanosecond rise and fall times, which is equivalent
to frequencies above 1GHz. At such frequencies, PCB
tracks are designed as RF transmission lines. Any more than
0.5" long have to be considered as transmission line
stubs which will set up standing waves which will corrupt
the digital signals. The result would be reduced operating
margins and intermittent pattern-sensitive malfunctions.