On Apr 25, 6:07*pm, Jim Lux wrote:
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.

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.

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)

I suspect that most ham transmitters do NOT have a 50 ohm output
impedance. *What they do have is a specification that they will
adequately drive a 50 ohm load (and some sort of internal circuitry that
detects an "unacceptable" output condition and turns down the drive).

After all, your transmitter could have an output impedance of zero ohms
(a "stiff" voltage source), and adequately drive your transmission line
and antenna at 50 ohms (yes, this is not the optimum power transfer, but
nobody ever said that ham transmitters are designed for optimum power
transfer... maybe they're perfectly happy with less transfer, but still
operating within their safe area)

ON9CVD made some simple measurements using a couple of resistors and
foudn that a TS440 has a Zout somewhere around 15-40 ohms (depending on
frequency and output power).http://sharon.esrac.ele.tue.nl/~on9c...impedantie.htm

Grant Bingeman also has words on this:http://www.km5kg.com/loads.htm

I agree with Jim. While it's true that if a source (transmitter) is
tuned for maximum output, the output impedance must necessarily be the
conjugate of the load impedance, it is NOT generally the case that the
transmitter is tuned for maximum output. Rather, the transmitter is
tuned for an output that won't destroy the output devices and will
result in acceptable distortion (in the case of a linear amplifier).
There are plenty of cases of sources designed to be loaded with an
impedance far different from their output (source) impedance: the AC
power line, audio amplifiers, ... .

A while back, I set up a couple precision high power directional
couplers so I could measure the output impedance of a couple different
ham rigs. In the case of the rig with vacuum tube output stage, if I
operated the output stage with limited grid drive and tuned the plate
tank for maximum output power, indeed the output impedance was 50
ohms, within the tolerance of my ability to adjust the output for
maximum. But if I increased the grid drive for solid class-C
operation and tuned for the rated output power (which is no longer the
maximum possible power), the impedance seen at the output dropped. If
you work through the pi-network transformation back to the vacuum tube
plates, it's apparent that the plates under those operating conditions
represent a considerably higher source impedance than when things are
tuned for maximum available power (as first described).

But coming back to "Sal's" original question, it's always made sense
to me given the availability of inexpensive 75 ohm line with low loss
to go ahead and use it to feed antennas that have a feedpoint
impedance closer to 75 ohms than to 50 ohms. If you need to provide a
bit of matching at the transmitter end so that the transmitter is
operating correctly, it should be straightforward to do that. But
whether the actual source impedance of the transmitter is one value or
another is really of very little importance. The only time I can
think that it would matter is if you're trying to transmit a very
broadband signal and you don't want power that's reflected at the
transmission-line:antenna interface to re-reflect from the
transmitter:transmission-line interface and go back to the antenna,
delayed by enough to cause a "ghost" (in a television picture), for
example. In such a case, you'll be well served by insuring that the
antenna is well matched to the transmission line so there is an
insignificant reflection there anyway.

Cheers,
Tom