Howdy all,
I Have a question,,
Will the below set up work with 300 ohm TV type of twin lead?
Joe WB9SBD
i8cvs wrote:
Hi Gus
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 an oscilloscope.
A spectrum analyser is not necessary because any oscilloscope works at 30 MHz
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 /| / open o------------|------------------------| 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 open.
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.
73" de
i8CVS Domenico
----- Original Message ----- From: "Angus" angus@young5769.freeserve.co.uk To: AMSAT-BB@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
as
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
a
piece of feeder (try and keep this feeder as short as possible also but
the
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
down
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
of
the dipole will be a 1/4 wave long). You can use these figures for making
a
1/4wave delay line or if your making a polarity control box you will need
to
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 length.
If you think that the feeder you have used is radiating either do perhaps
6
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
length
of the feeder then the feeder is radiating.
My two pennys worth, should help you get very close to the figures you
need
without the use of a GDO or an antenna analyser.
regards Gus
-- I am using the free version of SPAMfighter for home users. SPAMfighter has removed 3111 spam emails to date. Paying users do not have this message in their emails. Get the free SPAMfighter here: http://www.spamfighter.com/len
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb