RadioBanter - View Single Post - Using Tuner to Determine Line Input Impedance

Richard Clark · June 5th 09, 08:00 AM posted to rec.radio.amateur.antenna

On Thu, 4 Jun 2009 22:05:00 -0700 (PDT), wrote:

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).

Hi Jim,

Searching and measuring are worlds apart.

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.

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? Those same HF rigs have switched LPFs for each octave. This
has been a staple of solid state design for 30+ years. Consult a
schematic.

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.
This is not a remarkable deviation by the progression to FET, the FET
is simply a different "native" Z with a different transform to get to
the same 50 Ohms. Consult any schematic where the Z transform in the
output transformer is clearly in a Z step-up in the proper ratio. This
stuff has been slam-dunk for decades.

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. 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. Walt Maxwell has reported his own data
(tube set albeit, but the principles of transformation and exhibited
Source Z are not dependant upon technology).

*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.

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?

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.

That is arguably so, and my experience described above about operation
in the margins would suggest so. But it would be a very atrocious
Source Z that would lead one to that observation. My experience was
noted only by close examination, not a smoking finals deck. The
complainants experience would argue that the rig is flat out broken
with the drivers pushing 5 or 10 W through incapacitated finals.

More likely the complaining is related to issues outside of the rig.

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.

Op. Cit.

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.

Can you supply a known mismatch? It is inputable at that page;
Can you supply a known C from a tuner that has matched that load? It
is inputable at that page;
Can you supply a known L from a tuner that has matched that load? It
is inputable at that page;
Can you adjust the source Z when the tuner has matched that load? It
is inputable at that page.

This is a substitution method.

These steps reveal the common design criteria for building an LPF for
an output stage. Consult "Filters, Image-Parameter Design" in any
copy of "Reference Data for Radio Engineers." Consult, further,
"Filters, Modern-Network-Theory Design" in the same source. Read the
caption of Fig. 2 in that section:
"The Generator and Load must be considered part of the filter."
In the supporting text:
"The generator and load resistors can be assigned any value
between zero and infinity."
Fig. 25 shows the mathematical impact of Source Z (as R or G) to the
Network response. It also shows the Load Z as a mirror R or G in
symmetry to the source. These commonplace considerations are repeated
in Fig. 27, 28, and 29.

Consult Fig. 34 where both ends can be "interchanged." And repeated
out through Fig. 56.

In the section entitled "Low-Impedance Generator and Load" observe:
"A low impedance generator and/or load may be used with
the above filter by ... an effective turns ratio that transforms
the low impedance to the value required..."

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? One could as
easily dream that output Z varies with room temperature, with
humidity, with time of day, with elevation, and if the total of those
variations were capped off with a quantitative measurement of an
accumulated ±10%, then the whole list of objections would get hooted
off the stage. Give me a metric instead of looking under the bed for
spooks.

It's not like
an antenna, where the feedpoint Z at a given frequency is pretty much
constant, regardless of the incident power.

It is also not like a resistor, or a capacitor, or an inductor. Nor
is it like an audio amp or its speaker. The output Z is not like so
many things that to start this list only leads into an infinity of
trivial comparisons that wander the landscape.

So, the output Z is not like the load Z. What significance does that
bring?

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).

73's
Richard Clark, KB7QHC