On 26 abr, 02:35, "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)

Hello Sal,

Make yourself up for a long discussion (as we had such a thread
earlier).

Most amplifiers are designed just to provide the desired amount of
power into a certain load. The actual output impedance of the
amplifier is not important in many cases. Changing the load impedance
(for example in case of a solid state amplifier) may result in less or
more heat generated in the active device(s). You can see this by
changing the load (for example with an external matcher between power
meter and PA) and watching the output power and current consumption.

If you have an amplifier with tunable output section (vacuum tube PA),
you are within the range of the tuner, and you tune it for maximum
output power (given certain drive), the output impedance equals the
load impedance (or conjugated value in case of non-ohmic load). If
you change the drive (so adjust the output power), the output
impedance may change (due to saturation issues). Same is valid for the
load, if you change the load, voltage across and current through the
active device may saturate, hence changing the output impedance.

Non-tunable amplifiers (for example a 3…30 MHz balanced amplifier)
will mostly not present 50 Ohms to your load (unless specially
designed for that using feedback).

Virtually all high-efficient switching amplifiers do not show 50 Ohms
to the load. If you add an external tuner and match to maximum output
power, you will very likely destroy the amplifier in case of no
supervisory circuits present.

You can do some experiments with your own amplifiers. Just change the
load impedance and see what the forward power indicator on your
reflectometer/VSWR meter does. If it doesn't change, your output
impedance is very close to 50 Ohms.

Measuring the output impedance (for relative small change in load) is
possible, but is not a simple task. Very likely other people will
comment on this.

With kind regards,

Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely.