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

August 13th 03, 06:47 PM

(Dr. Slick) wrote in message om...

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.

So if you have 200 feet of RG-213 and 200 feet of RG-58, put those in
series to the cantenna. Coil them loosely and cool them with a fan if
needed, if you are running high power (or start the chain with larger
coax). That should get you close to 15dB attenuation in the coax at
100MHz and a 1.02:1 or better SWR at the input end for a 2:1 SWR at
the cantenna. At lower frequencies where the line loss is lower, the
cantenna is good enough that you don't need the line loss as
compensation. Do I have you worried about what the actual impedance
of the coax is from earlier postings? If so, good! Now you're in a
position to think about what's good enough, and measure what you have
to see if it meets the desired goals.

If that's all too kludgey for you, go buy a good load. Or just tune
the cantenna for bands of interest. You could make a set of small
boxes with L networks, each of which compensates the cantenna on a
particular band. Or look up one of the articles about how to make a
better "cantenna", probably starting with the parts you have. There
are lots of options. Why settle for a setup which will only lead you
deeper into confusion? Short of really precision measurements,
there's a lot you can do trading off time and careful thought for the
high cost of commercial equipment.

Cheers,
Tom