[amsat-bb] Re: Polarity questions
domenico.i8cvs at tin.it
Sun Sep 21 14:15:51 PDT 2008
To know the velocity factor of a coax (known or un-known ) with good
accuracy I make the measurement at about 30 MHz because the velocity
factor do not change with frequency and a low frequency measurement
make the error the smallest as possible.
Get a N coax T and connect the male port of it to the vertical Y channel of
A spectrum analyser is not necessary because any oscilloscope works at
Connect one female port of the T connector to a RF generator and set it
to 30 MHz
If you don't have a RF signal generator use a low power 30 MHz TX at
1 or 2 watt output power or less using few suitable 50 ohm attenuators.
Connect the other female port of the T connector to an estimated 1/4
electrical wave long of coax cable under test and live open the other
end of it.
| | | 30 MHz |
| Oscilloscope | | signal generator |
| | | or low power TX |
|___________Y_ | |_______________|
1/4 Lambda /\| \/
end T connector
If you suspect that the velocity factor is 0.66 like for solid polyethylene
(PE) make the 1/4 electrical wave at 30 MHz 1.65 meters i.e. 65" long
The measurement plane of the coax cable under test is the middle of
the T connector just between both female ports where the generator
and the coax under test get in contact each other so that the real lenght
of 1/4 electrical lenght must be taken from the above measurement
plane including the male N connector up to the open end of it.
Because of the theory of transmission line and following the Smith
Chart we see that a 1/4 electrical wave long transmission line becames
a short circuit at his resonating frequency when the other end of it is
While looking at the CRT of the oscilloscope adjust the frequency
of the signal generator until the amplitude of the signal on the CRT
falls abruptly to zero.
The frequency shown by the signal generator is the resonant frequency
of the 1/4 electrical wave.
Knowing the frequency and the physical lenght of 1/4 wave it is
easy to calculate the real velocity factor of the coax cable under test.
No special instruments are necessary because about everyone own a
signal generator for 30 MHz and a CRT oscilloscope.
Using a GDO or an antenna analyser the measurement can be wrong
because it is difficult to know if the above instruments made for
amateur use are well calibrated or not.
----- Original Message -----
From: "Angus" <angus at young5769.freeserve.co.uk>
To: <AMSAT-BB at amsat.org>
Sent: Sunday, September 21, 2008 7:57 AM
Subject: [amsat-bb] Re: Polarity questions
> Hmmmmm might get shot down in flames for this but.
> If you want to know the velocity factor of a coax (known or un-known) try
> getting two eaqual lengths of coax which are physically 1/4wavelength long
> i.e. 50cms for 2mtrs.
> Short out ONE end (join the inner and outers together with a short a link
> possible) and do this for both pieces of coax.
> We are going to make a coaxial dipole so these two shortened ends go on to
> piece of feeder (try and keep this feeder as short as possible also but
> dipole needs to be somewhere clear but does not to be 100 feet up either).
> Yes you will need to support the dipole elements as they will just hang
> so perhaps a piece of garden cane or some other RF invisible material.
> Trim back both OPEN ends of the dipole until you get the lowest possible
> VSWR (on the centre frequency you want to use) and you should have very
> close to an electrically 1/2wave dipole for the coax you used (each side
> the dipole will be a 1/4 wave long). You can use these figures for making
> 1/4wave delay line or if your making a polarity control box you will need
> know the lengths of both 1/2 and 1/4 of the coax you are going to use, the
> dipole will be considerably shorter than a normal wire dipole for the same
> frequency because we have used the velocity factor to help reduce its
> If you think that the feeder you have used is radiating either do perhaps
> turns around a small 1" tube to make a balun at the feedpoint or alter the
> length of the feeder slightly, if theVSWR alters when you change the
> of the feeder then the feeder is radiating.
> My two pennys worth, should help you get very close to the figures you
> without the use of a GDO or an antenna analyser.
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