/Two questions occur to me.
/I have heard that length of coax is critical
/to performance but when I mention this to techs they scoff
/and brush me off with the comment that this is old data
/and no longer applies?!
> Actually, it NEVER applied . . . at least not for any
> good reason. .
//Beg to differ.. it doesn't make a difference only if the
//source and termination are the same impedance as the cable,
//i.e., a flat line. If there are voltage standing waves
//(VSWR) on the line, then the length of the cable matters.
But matters to what and why? There's no way that a 1/4 spike
sticking out of the top of an airplane is going to present a
50-ohm, non-reactive load to a piece of coax over the full
range of communications frequencies. In fact, it will never
present such an idealized load at ANY frequency in that range.
So . . . how would you proposed the coax length be "adjusted"?
//If the line is not flat, the impedance or more correctly the
//reactance varies from point to point along the cable. Assuming
//the transmitter, coax, and the antenna and its installation
//all show an impedance of the same value, the length of the
//cable is not critical, it will only cause an attenuation with
//distance; however if the impedances are not matched all bets are
//off and the length of the line will make a VERY BIG difference.
Everything you say is true from an idealized, textbook analysis
of an antenna/feed-line system. However, in the real world
of working with airplanes and their radios, the words "critical"
and "difference" need definitions. When we advise people that
something has significance, we should be prepared to follow up
with a practical means for achieving the optimal result.
In fact, as the power handling capabilities of RF amplifier
transistors for trasmitters grew, their ability to tolerate
some conditions of mis-match between antenna and coax was lagging
behind. For a brief period in the development of these radios,
there was a flurry of activity to tailor coax feedline lengths
to minimize stresses on the comm transmitters that tended to blow
their brains out under some antenna loading conditions.
Thankfully, those days are long gone and I'm aware of no manufacturer
of a VHF comm transmitter that recommends feed line length optomization
for use of their product. To do so would announce to the world that
they've not done their homework in selection of parts or in the design
of their product.
Regards,
Bob . . .
AeroElectric Connection
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