On 4/25/2011 7:35 PM, Sal M. Onella wrote:
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. My curiosity was piqued today as I took some baby
steps into EZNEC. A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

No, the antenna still has the same characteristics. You changed the
impedance at a point remote from the antenna.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. Doesn't seem like it.

It depends on a number of factors. For example, use a .5 wavelength 75
ohm cable to feed a 50 ohm resistance. Your transmitter happily thinks
it is 50 ohms. Because it is.

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. (I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. Thanks.

"Sal"
(KD6VKW)


Transmitters don't have a clearly defined output impedance and, whatever
output impedance is there, doesn't necessarily mean anything.

Your main concern is to provide a 50 ohm load for the transmitter to see.

Actually, it the feed line and load are both 75 ohms, you will only see
a 1.5:1 SWR. Don't worry about it.

Cheers,
John