On Jun 4, 12:18*pm, Richard Clark wrote:
On Thu, 04 Jun 2009 11:47:31 -0700, Jim Lux
wrote:

By the way, the assumption that the run of the mill ham rig has a 50 ohm
resistive output impedance is not necessarily valid.

By the way, this comment above is another assumption in that it lacks
a quantifiable. *I find it offered quite often as a negative assertion
to which the several many posters who offer them never provide an
actual value to prove what the run of the mill ham rig is, much less
is "not."

Actually, I did a casual search for such data, but couldn't find any
for the "run of the mill" solidstate 100W ham rig . There is a fair
amount of data for one tube rig or another). There is some data in
the Moto Ap notes by Granberg, etc, that's reasonably representative,
but it doesn't include the effect of the inevitable LPF on the output.

So, looking at things with which I have practical experience and
measurements.. MMIC amps tend to be be pretty flat over octave
bandwidths, but I don't think they're representative of ham rigs with
either FET or Bipolar output stages (which have to cover multiple
octaves, in any case). Hot microwave FET amps have output impedances
that are anything but 50 ohms, and designing the output networks keeps
lots of RF engineers employed, especially over temperature and device
parameter variation.

I'd love to see some real data for ham rigs.



*Rarer, indeed, is any effort put forward by those posters
to show they have attempted to quantify their own equipment.

Perhaps that's because this is, after all, "rec. radio", as in, nobody
is paying people to comment here, and unless you have a particular
need to know the output Z, it's not worth it to spend the time to
measure it. As previously commented, either you're in the "no tuner"
category, and you tolerate whatever mismatch there is on both ends of
the transmission line, or you have a tuner, and you tune for "best
match", with whatever the output Z is.

For all we know, the folks that complain about not getting a good
match on a Brand X antenna, when everyone else does, have a rig with a
bad match on the output.


As there are posters here who have performed this work, shown their
data, and such data follows conventional design considerations (which
is easily revealed within the page cited athttp://www.wy2u.com/);

Indeed? I'd love to see the data.


then these assumptions dressed in denial are rather unprofound proofs.
As this topic has been visited many times, and as it quickly descends
into equally unsupported claims (although often annotated with vague
references and citations that are quickly demolished); I doubt
anything said here is going to sway those assumptions.


My original contention is that if you're going to measure Antenna Z by
using an autotuner and seeing where it tunes, one of the underlying
assumptions is that the other side of the tuner is 50 ohms.

In reality, having actually done this (e.g. use LDG AT200PC tuners to
measure the mutual impedance matrix of an array), I think the
resolution/step size of the tuner is a bigger problem with the
technique. Given the availability of low cost VNAs for the ham
market, that's a MUCH better solution to measuring antenna impedances.

As an amusing exercise (I anticipate none will tread down this path),
the page athttp://www.wy2u.com/offers a means to test your own rig's
Source Z - if, in fact, you can cope with translating your tuner's
settings into picofarads and nanohenries, and if you can obtain a
known mismatch. *These impediments are Herculean to most,
unfortunately.

Looking at that page, I don't see an obvious link.

Measuring the output Z of the transmitter would be an interesting
exercise.. for microwave circuits, one uses a load-pull setup..

The challenge is, of course, that the amplifier is an active device,
so the output Z probably changes depending on the load. It's not like
an antenna, where the feedpoint Z at a given frequency is pretty much
constant, regardless of the incident power.



73's
Richard Clark, KB7QHC