"Tarmo Tammaru" wrote in message ...
"Dr. Slick" wrote in message
om...
Someone else suggested that i place the meter
right at the cantenna, to at least exclude the patch from meter to
dummy load.

No, you don't want to do that. Put as much loss as you can between the meter
and the Cantenna. 100 feet of RG58 will have about 6.5 db of loss. It will
make your load a lot closer to 50 Ohms. There are Tables that show SWR as a
function of cable loss for a given mismatch. One chart that I have in
"Reference Data for Radio Engineers" shows that with a load SWR of 1.4, and
6.5 db of cable loss, the SWR at the driving end is around 1.075. Note that
you DO NOT want low loss coax.

Tam/WB2TT


Perhaps you are correct. That you would only want to use the
meter right at the load if you wanted to measure the real SWR of the
antenna, as the coax loss will improve the return loss (SWR).

I'll try a long piece of RG-58 and see how the incident power is
affected.

Slick

W5DXP wrote in message ...
Dr. Slick wrote:
However, in my case, the SWR doesn't change much, while the
INCIDENT power does!!!

Guess I wasn't clear. There are an infinite number of impedances
on a constant SWR circle. The transmitter likes some of those
impedances better than others.


No, i understood you alright, and your statement above is
understood and agreed to.

However, in this case, we only have two impedances to chose from
(two different coax lengths).


Slick

(Dr. Slick) wrote in message . com...
....
What do you mean by "calibrated to the line"?

The SWR meter should read zero reflected power when connected to a
load whose impedance is equal to the line's. Does it? If not, it's
not properly calibrated. Putting it another way, what's the
directionality of the bridge?

....

Not so much a surprise as a disappointment! A difference of 70
watts incident power is totally unacceptable with only 8 feet of coax
length added.

But the load presented to the amplifier is totally different in the
two cases, most likely. Only if the amplifier's output impedance were
the complex conjugate of the line's, and the line were lossless, and
the amplifier behaved as a linear time-invariant system would you
(should you) expect the power to remain unchanged.

The transmitter does not have a mismatch sensor on it for reducing
power at high SWR. It naturally does output less wattage as the
transconductance is reduced with higher temperatures, but my
measurements were done in close proximity to each other.

Exactly how would you measure the output impedance (S22) of a PA?
Drive it with a small signal (class A, linear mode) in one frequency
(F1), and then inject another frequency (F2) into the output, and then
measure the reflected F2 power? If i remember correctly, that's how
someone was trying to measure S22 at my former place of employment.
But all bets are off as soon as you go non-linear, or non-class A.
S-parameters are supposed to be all small signal.

I don't think you can properly measure the S22 of a class C PA.

Tam brought up the suggestion that i try a really long piece of
RG-58 from meter to dummy load, to make the Cantenna more like 50
Ohms. Gonna try it.

Or just tune the load to zero reflected power after you're sure the
meter is properly calibrated...but that can be a catch-22 situation
that the really long RG-58 can help with. Be aware, though, that
"50-ohm" line seldom is -- it can be off 5 ohms or more. It's a cause
of some consternation to those of us involved in calibration of
precision RF test equipment.

Load-pull techniques are commonly used to characterize RF source
impedances. You make known incremental changes to the load, and
deduce from the change in output power what the source impedance is,
assuming it's a linear time-invariant system. Note that adding length
to a mismatched line is one way to make an incremental change to the
load... I wouldn't necessarily say that the output impedance of a
class C amplifier is meaningless, but it may well not be constant for
all loads. and may depend on parameters you'd have trouble controlling
from day to day.

Cheers,
Tom

This means your shield has RF on it .
Also triming cable has nothing to do with SWR. Trimming the cable will
change the impedance the transmitter sees if the transmission line has
standing waves on it.


"Dr. Slick" wrote in message
om...
Hey,

I've been getting different readings on my power meter that
depend on the length of coax that i use to the meter (into a
Cantenna).

This is a push-pull VHF 300 watt transmitter with a coax stub
harmonic filter on the output. My digital Daiwa will read about 310
watts incident power using a 4 foot RG-8X jumper coax (from filter to
meter), and about 240 watts using a 12 foot section of RG-8X!!?? VSWR
stays the same at about 1.4:1 .

There is the old ham wife's tale that you can tune your
transmitter for a better match if you adjust the legnth of your coax
to the antenna. And from a theoretical point of view, it _should_ be
only a tale, because if your antenna is not a perfect 50 Ohms, the
length of the coax should still not matter, because the constant VSWR
circles around the center of the Smith Chart have just that, the same
SWR independant of the wavelengths away from the antenna (assuming 50
Ohm transmission lines are used).

OTOH, the actual series equivalent complex impedance will be
alternating from inductive to capacitive, every 1/2 wavelength.
So perhaps this will tune/detune the amplifier?

Any non-bullsh** advice/explainations appreciated.


Dr. Slick

"Dr. Slick" wrote in message
om...
W5DXP wrote in message
...

Remember the maximum power transfer theorem? The transmitter probably
works
better into some impedances than into others. With an SWR of 1.4:1, the
transmitter will see a resistance between 35.7 ohms and 70 ohms with a
reactance
between zero and about plus or minus j15. Knowing the frequency and VF
of the
RG-8X, you should be able to estimate the two impedances seen by the
transmitter.


Actually, i read off of my Smith Chart about +/- j20, but you are
close.

Anyhow, my point is that the ham "wife's tale" of adjusting the
coax length for lowest SWR may be truly just a tale.

I hope this is a wifes tale that has long been discredited. Trimming cable
changes the impedance seen by the transmitter with no change to SWR. It is
true that there are a lot of people who dont understand the difference.

However, in my case, the SWR doesn't change much, while the
INCIDENT power does!!! To me, this may be due to the fact that the PA
isn't 50 Ohms at the output (I'll bet not many really are), and so a
swing of +/- j20 may improve or degrade how close you are to a
conjugate match.

What do you dudes think?


Dr. Slick

That would give you 13db of return loss which would be an SWR less than 2:1
even if the far end was a dead short.
Given his current SWR I woud say this would result in a return loss of
around 30db. Numbers are a lot of by guess and by golly but think I am not
too far off.
"Tarmo Tammaru" wrote in message
...

"Dr. Slick" wrote in message
om...
Someone else suggested that i place the meter
right at the cantenna, to at least exclude the patch from meter to
dummy load.

No, you don't want to do that. Put as much loss as you can between the
meter
and the Cantenna. 100 feet of RG58 will have about 6.5 db of loss. It will
make your load a lot closer to 50 Ohms. There are Tables that show SWR as
a
function of cable loss for a given mismatch. One chart that I have in
"Reference Data for Radio Engineers" shows that with a load SWR of 1.4,
and
6.5 db of cable loss, the SWR at the driving end is around 1.075. Note
that
you DO NOT want low loss coax.

Tam/WB2TT

(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com...
...
What do you mean by "calibrated to the line"?

The SWR meter should read zero reflected power when connected to a
load whose impedance is equal to the line's. Does it? If not, it's
not properly calibrated. Putting it another way, what's the
directionality of the bridge?


You mean the directionality of the couplers inside the meter? I
don't know.
You can adjust the needle on the analog Daiwas to read zero watts with
nothing hooked up to them.



...

Not so much a surprise as a disappointment! A difference of 70
watts incident power is totally unacceptable with only 8 feet of coax
length added.

But the load presented to the amplifier is totally different in the
two cases, most likely. Only if the amplifier's output impedance were
the complex conjugate of the line's, and the line were lossless, and
the amplifier behaved as a linear time-invariant system would you
(should you) expect the power to remain unchanged.


I'm not sure i agree with your statement as you have written it.

The power would remain the same only if the load and the line
were perfect 50 Ohms and also lossless, then the length of the coax
should not matter at all.

This is certainly not the case on our bench, which is a problem,
because i want to have an approximation of how it will perform
attached to the antenna before i ship it out to the customer. A
difference of 70 watts is not acceptable.




Tam brought up the suggestion that i try a really long piece of
RG-58 from meter to dummy load, to make the Cantenna more like 50
Ohms. Gonna try it.

Or just tune the load to zero reflected power after you're sure the
meter is properly calibrated...but that can be a catch-22 situation
that the really long RG-58 can help with. Be aware, though, that
"50-ohm" line seldom is -- it can be off 5 ohms or more. It's a cause
of some consternation to those of us involved in calibration of
precision RF test equipment.


Oh, definitely, especially the cheap stuff. But if the RG-58 went
around the world a couple of times, the end could be open or shorted,
and you would still measure 50 Ohms, most likely.


Load-pull techniques are commonly used to characterize RF source
impedances. You make known incremental changes to the load, and
deduce from the change in output power what the source impedance is,
assuming it's a linear time-invariant system. Note that adding length
to a mismatched line is one way to make an incremental change to the
load... I wouldn't necessarily say that the output impedance of a
class C amplifier is meaningless, but it may well not be constant for
all loads. and may depend on parameters you'd have trouble controlling
from day to day.

Cheers,
Tom


Right, they did plenty of load-pulling on FETS at my former place
of employment. I think they may have been more meaningful for class A
linear CDMA PAs. But finding the max power transfer impedances with
small signal class A measurements doesn't guarantee that these will be
the optimum impedances the transistor wishes to see in class C mode.

But i don't think there is a practical way to measure the S22 of a
high powered Class C amp. Most companies don't really seem to care
how close or far from 50 Ohms it really is, all they care about is the
%PAE and Pout and harmonics dBc going INTO a 50 Ohm system. But even
ensuring that your bench will match your customer's bench is a
difficult thing.


Slick

(Dr. Slick) wrote in message . com...
....
You mean the directionality of the couplers inside the meter? I
don't know.
You can adjust the needle on the analog Daiwas to read zero watts with
nothing hooked up to them.

:-) The meter-needle zero generally not a very big part of the
problem! Yes, it's the coupler (bridge) inside that you want to make
sure is calibrated for your reference impedance. If it's not
adjustable, at least learn what it is calibrated to, or how good it is
for your reference impedance.

....

But i don't think there is a practical way to measure the S22 of a
high powered Class C amp. Most companies don't really seem to care
how close or far from 50 Ohms it really is, all they care about is the
%PAE and Pout and harmonics dBc going INTO a 50 Ohm system. But even
ensuring that your bench will match your customer's bench is a
difficult thing.

So you've come back around to what some of us here have been saying
for years. A transmitter or amplifier may be designed to work
optimally into some particular impedance (say 50 ohms), but have a
source impedance vastly different from that. Whether it's a
measurable impedance or not is moot: what matters is that you present
a reasonable load to it. There ARE cases where a matched source
impedance is important, but they are rare in ham transmitter work.

The easy way to insure proper operation is to present the
transmitter/amplifier with the right load. A properly calibrated SWR
meter (or as Reg would have it, a "transmitter loading indicator")
will tell you when you have such a load. Yours is telling you that
you do not. In such a case, I'd work on a match to the antenna (or
the Cantenna or whatever) first--well, right after making sure the
meter was telling me the truth or something close to it. When you get
that correct load, then the length of coax won't matter noticably,
assuming the SWR meter is calibrated to the line impedance.

You generally don't have control of your customer's bench, but you can
specify some particular load impedance and then make sure you test
with that, and leave it to the customer to insure that they provide
that load. It's reasonable (if not common) to specify a range of load
impedances over which some perhaps slightly reduced performance is
achieved. You may, for example, get more power but also higher
distortion products, at a non-optimal load.

Cheers,
Tom

On 11 Aug 2003 09:54:47 -0700, (Tom Bruhns) wrote:

In such a case, I'd work on a match to the antenna (or
the Cantenna or whatever) first--well, right after making sure the
meter was telling me the truth or something close to it.

Hi Tom,

This is kind of a chicken and egg problem.

Calibrate the meter against the load.

Calibrate the load against the meter.

It would seem that discussion would eventually get around to bringing
in a known and stable reference. The length of lossy line would
suffice for a rough cut solution but still leaves a bench full of
rather sloppy instruments.

73's
Richard Clark, KB7QHC

Jimmy wrote:
I hope this is a wifes tale that has long been discredited. Trimming cable
changes the impedance seen by the transmitter with no change to SWR.

However, a 50 ohm SWR meter reading will change as one changes the
length of the ladder-line. When the 50 ohm SWR meter reads 1:1,
the transmitter is seeing 50 ohms. That's how I tune my dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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