On Jun 5, 12:00*am, Richard Clark wrote:
On Thu, 4 Jun 2009 22:05:00 -0700 (PDT), wrote:
On Jun 4, 12:18*pm, Richard Clark wrote:

Hi Jim,

Searching and measuring are worlds apart.

In the context of discussing on a newsgroup, I'm willing to spend a
few minutes searching. I'm not willing to spend hours measuring.
Others might. I seem to have recalled seeing some data a few years
ago, but I couldn't find it with google.



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.

Now, this is the most curious statement of them all. *Every LPF that
is mounted in any Ham grade HF rig is designed with both a 50 Ohm
input Z and a 50 Ohm output Z. *This is easily verified through the
same page that does the calculations, or through trivial math for the
individual components' Z.

Uh huh... and all manufacturers use high precision components, and the
impedance at one end of the filter isn't affected by the impedance at
the other end?

My original point is that, barring measurement, you don't KNOW.
(which is sort of your argument too, eh?)



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). *

Why not? *

Because the MMICs are a totally different design model. To start
with, they're also Class A, while most ham rigs run Class AB. They
also tend to be "detuned" for broadbanding, at the expense of
efficiency. (not all MMICs are this way.. I'm talking about the MAR-n
series, for instance)

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.

And for those same 30+ years of HF solid state rigs, their power
transistors have had (and still do) output "native" Z of several Ohms.

Would that the active device has a Z that is constant, but it's not.
Sure, the MRF454 data sheet says the output Z is 1+.2j ohms (or
something like that) at 30MHz, but is it still that at 1MHz?

Looking at a more modern power FET for amplifier use, the IXZ210N50L..
There's a whole page of S parameters, and S22 goes from 0.88@-51deg at
2MHz to at 14.32 MHz to at 30 MHz... that's
at Ids =200mA.. bump Ids to 500mA, and the magnitudes stay about the
same, but the phases change, by tens of degrees.

Having actually worked on an amplifier design with similar parts, I
can also say that the datasheet is only a "get you in the ballpark for
the design" tool. The "real parts" (especially when packaged on a
board and attached to the heat sink) are substantially different.

No simple transformer is going to make that look like a constant 50
ohms.

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

Mine (Drake TR-7 and Kenwood TS-430s) exhibit values that vary around
50 Ohms with a low of 35 Ohms and a high of 70 Ohms in the margins.
Those rigs also suffer in those margins. *

so the VSWR looking back from the tuner into your transmitter is
1.4:1? A return loss of around 15dB... what's that work out to... an
error of about 10-15% in the "measuring impedance with a tuner"
technique... not bad, but not great, either, especially stacked up
with the other uncertainties..

Good enough to give a "cross check" on another measurement? Maybe...
if the tuner technique showed I had a load Z of 100+50j, and the MFJ
gave a result of 90 + 40j.. yeah, I'd say it is consistent.



Measurements were done by
pull, by substitution, by looking into the antenna connector with an
RF Bridge and all confirmed by simple reverse design principles.
Variations between any method rarely departed from one another, and
never from the values above. *

Although you have to admit that a 2:1 impedance variation isn't a
particularly outstanding "constant impedance load"


*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. *

This apology condemns the hobby to the lowest common denominator. If
it were meaningful, we would be reading yet another miracle antenna
claim without hint of skeptical enquiry braced with theory, hammered
with models and test gear behind it. *

Not at all.. just because *I* don't want to spend the time measuring
it doesn't mean that the information is of no value to the community.
I would venture that of all the data that hams, collectively, could
measure, this is actually not as useful as some other data.. It just
doesn't have that much impact on day to day operation. Very few hams
adjust their tuner by calculating L and C based on measured data, or
else there wouldn't be a plethora of articles and posts about
"tuning", "pruning", "trimming" and the techniques for doing this, and
arguments about whether a Brand X meter is better than a Brand Y
meter, etc.

Hams, by and large, adjust their tuners by minimizing the reflected
power, and don't much care what the actual component values are. (e.g.
what ham tuner actually has accurate dial calibrations in pF or uH? )

Professionals, on the other hand, do CARE, and do make the
measurements, particularly if they're doing phased arrays, or
designing circuits for mass production, or have to document that their
system will work over wide ranges of temperatures, aging, and other
effects. But, because they're getting *paid* to do it, they're more
than happy to do so. It makes the rest of the job easier.



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.

Every problem is reduced to those two options?

Obviously not, but I'll bet that it covers over 90% of hamdom (and a
lower percentage of the folks reading this thread).


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

Can you supply a known mismatch? *It is inputable at that page;

This is a substitution method.

Ahh.. I misunderstood.. I thought you were pointing to process for
doing the measurement and/or some measured data. The cited page is
just the calculator for part of the problem.




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. *

I've heard that platitude far too many times. *Of course it is an
active device. *Of course the output Z changes with load. *Do you have
anything more to offer than simple qualitative musings?

Sure.. check out the Ixys data sheet. Plenty of grist for "Z varies
with load and frequency"
Phase of S22 varies 40-50 degrees with Ids. That's in your 10%
ballpark
On the other hand, I have worked with high power Transistor circuits
that have acted exactly as resistors, inductors, and capacitors and
output Z was exactly like an antenna at a given frequency (or rather
input Z, as one design was an active 100W load).


Yes.. but were those run-of-the-mill amateur transceivers? (the
original question).. I have no doubt that it is possible to build
amplifiers with constant Z (to any degree of constancy desired.. heck,
a 1000W amp and a 60db pad gives you a 1mW amplifier with very good
output Z, regardless of what the amp does).

But, does a "designed for mass production and cost target" transmitter
fall into that category?
It's not a published spec
ARRL doesn't measure it when they review rigs

So it's left to someone who cares to do so.