Length of Coax Affecting Incident Power to Meter?
 

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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"Tarmo Tammaru" wrote in message ...
You don't say what VHF frequency, but the Cantenna is probably unsuitable at
that frequency. Your amp is spec'd to put out 300W into 50 Ohms, not 35 or
70. It could put out more or less power into a different impedance. Your
best bet is to build a little L network right at the Cantenna, adjust it for
1:1 SWR, and then measure the power. Another approach would be to string all
the RG58 you own in series with the 8X.

Tam/WB2TT

Less than 150 MHz. Someone else suggested that i place the meter
right at the cantenna, to at least exclude the patch from meter to
dummy load.

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

Slick

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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"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

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