In a message dated 7/14/2005 10:12:08 A.M. Central Standard Time,
k6rix@earthlink.net writes:
Hello John, how do you test your lines and what equipment do you use?
I've experimented with the MFJ 259B for stubs and phasing lines, but if it
is critical, I'll order lines from M2. I'd sure like a good method that I
could employ myself. Also, do you short the end of the coax or leave it
open. The MFJ book lists both methods.
>...but I even measure the actual phase shift for each delay line of solid
>dielectric coax, if the application is critical.
>
>And with foam coax it is MANDATORY to TUNE (trim) each delay line to the
>correct electrical length.
>
>--John W0UN
Dino...k6rix@earthlink.net
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If you connect a Tee to the 259 analyzer with a 50 ohm Dummy load to one
side of the tee connector, you can then connect your coax to be tested to the
remaining side of the tee connector.
If the coax is open at the far end, you will see a null in the swr reading
at each frequency that is a half wave multiple of the coax length.
If the coax is shorted on the far end you will see a null in the swr reading
at each frequency that is an odd multiple of a quarter wavelength of the
coax length.
If you want cables to be the same electrical length then the swr null should
occur at the same frequency for each cable.
If you want the cables a multiple of half wave length then the cables are
trimmed for a null at the actual operating frequency.
Most tee connectors will add about one inch to the cable length so if you
are a perfectionist you will want to adjust your test frequency to compensate
for the length difference.
If you don't have a 50 ohm dummy resistor, one is easily made with a PL259
and three 150 ohm half watt resistors connected in parallel inside. Use minimum
lead lengths to reduce the series inductance. The PL259 load is not
wonderful at 144MHz and higher but is just fine for lower frequencies.
Happy tuning!
73/k5gw
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
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