RadioBanter - View Single Post - Length of Coax Affecting Incident Power to Meter?

August 13th 03, 05:15 AM

(Tom Bruhns) wrote in message m...
(Dr. Slick) wrote in message . com...
...
You're right about this, and it reminds us that if there is any
loss at all, we theoritically move away from a purely resistive
characteristic impedance into a complex one. This furthers the
complexity on the problem, as we must expect reactance in our coax.

It's not just theory, it's practice and measurable. But it's not a
requirement that loss introduce reactance to Zo; it's just that if the
line is lossless it must have a non-reactive Zo. Clearly if L/C =
R/G, the impedance will be non-reactive.

True. Never considered this before, thank you. Makes sense that
if the ratio of the series resistance and shunt conductance are the
same as the ratio of the series inductance and shunt capacitance, that
the transmission line will still be non-reactive.

Also, consider what reactance you do get in practical lines, at what
frequencies. What effect does frequency have on the reactance? Why?
Under what conditions will it really be a problem? Might the reactive
part be so small that it will be totally swamped out by variations in
the real part? Think a bit about how much it will or won't mess up
the measurements you want to make.

Cheers,
Tom

In general, using an MFJ antenna analyzer to get a rough idea of
what the series equivalent complex impedance is (these are not $80,000
vector network analyzers!), it seems that cheap 3' RG-8x jumper coax
cables mainly add series inductance to the system, as the reactance
gets higher with increasing frequency.
We try to design 9 element chebychev low-pass filters, which is
not difficult at very low power levels, as two 1/4 watt 100 Ohm
resistors make a decent dummy load all the way out to 200 megs or so.

The problem is characterizing insertion loss using higher power
transmitters, when we know that the 1000 watt cantenna swings from 40
to 70 Ohms (with reactance too) as you get above 80 megs or so. It
become difficult to know if you are moving in the right direction or
not.

Slick