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

August 10th 03, 05:17 PM

(Dr. Slick) wrote in message . com...
W5DXP wrote in message ...

Remember the maximum power transfer theorem? The transmitter probably works
better into some impedances than into others. With an SWR of 1.4:1, the
transmitter will see a resistance between 35.7 ohms and 70 ohms with a reactance
between zero and about plus or minus j15. Knowing the frequency and VF of the
RG-8X, you should be able to estimate the two impedances seen by the transmitter.

Actually, i read off of my Smith Chart about +/- j20, but you are
close.

Anyhow, my point is that the ham "wife's tale" of adjusting the
coax length for lowest SWR may be truly just a tale.

However, in my case, the SWR doesn't change much, while the
INCIDENT power does!!! To me, this may be due to the fact that the PA
isn't 50 Ohms at the output (I'll bet not many really are), and so a
swing of +/- j20 may improve or degrade how close you are to a
conjugate match.

What do you dudes think?

Instrument it properly and answer your own question. Measure the
power at both ends at the same time. Then, assuming you have good
power meters, calibrated to the line you're using, you'll know if the
input end of the coax also experiences the same power difference. If
it does, then it's presumably the output of the transmitter/filter
that changes. You've already offered a very reasonably hypothesis
about why that happens, and could prove/disprove that through some
measurements, too. Is the transmitter a linear time-invariant system?
(Or does it have a load sensor that reduces power output for some
loads?) Can you determine the actual source impedance of your
amplifier, and correlate that with the readings you've gotten?

I trust that the readings you got weren't much of a surprise...

Cheers,
Tom