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

With a non-resistive load, the length of the coax and placement of an
inexpensive (e.g. non-Bird) meter will cause different readings.

"Dr. Slick",
Sure, for the CB crowd, it's in increments of 3 feet.
'Doc

Dr. Slick wrote:
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 .

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.
--
73, Cecil http://www.qsl.net/w5dxp



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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.

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

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.
--
73, Cecil http://www.qsl.net/w5dxp



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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

On 9 Aug 2003 17:49:36 -0700, (Dr. Slick) wrote:

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).

On 10 Aug 2003 03:11:06 -0700, (Dr. Slick) wrote:

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

On 10 Aug 2003 03:05:16 -0700, (Dr. Slick) wrote:


Unfortunately, as i have mentioned before, the cantenna is not the
greatest 50 Ohms at anything above 10-20 MHz.

A classic re-creation of Mismatch Uncertainty already discussed at
great length in the thread "The Cecilian Gambit, a variation on the
Galilean Defense revisited."

Search google for details.

73's
Richard Clark, KB7QHC

(Dr. Slick) wrote in message . com...
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.

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?

Instrument it properly and answer your own question. Measure the
power at both ends at the same time. Then, assuming you have good
power meters, calibrated to the line you're using, you'll know if the
input end of the coax also experiences the same power difference. If
it does, then it's presumably the output of the transmitter/filter
that changes. You've already offered a very reasonably hypothesis
about why that happens, and could prove/disprove that through some
measurements, too. Is the transmitter a linear time-invariant system?
(Or does it have a load sensor that reduces power output for some
loads?) Can you determine the actual source impedance of your
amplifier, and correlate that with the readings you've gotten?

I trust that the readings you got weren't much of a surprise...

Cheers,
Tom

"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