On (09 Apr 97) Roy Witt wrote to Jeff Edmonson...
 RW> Hello Jeff.
 RW> 08 Apr 97 17:45, Jeff Edmonson wrote to Roy Witt:
 RW>  JE>
 RW>  RW>> I build VHF/UHF yagi's for a commercial customer.
 RW>  RW>> Fortunatly, I can tune them while on or 1/2 wave above the
 RW>  RW>> ground. If I tune an antenna with the swr bridge at the
 RW>  RW>> antenna and then place the swr bridge at the other end of the
 RW>  RW>> coax, there's a difference. 1.2:1 at the antenna, 1.9:1 at
 RW>  RW>> the tx. No amplifier, No antenna tuner, no etc.. Now what do
 RW>  RW>> I tune to get 1.5:1 or less at the tx?
 RW>  JE>
 JE> tune the coax.
 RW> With what?  A pitch pipe or tuning fork...
I'm not Roy but we have the same initials. "Tuning" the coax amounts to
making it ELECTRICAL multiples of a half wave on the test freq of
choice. Just like cutting a dipole to length but then you have to
multiply by the velocity factor of the coax you are using. My 8214
Belden (foam dielectric), for example has a velocity factor of 0.66.
Ordinary RG-8U is 0.80 as I remember. This makes the input and output
impedence of this chunk of coax a "mirror" of each other (the half wave
sine wave crosses at this point) thus negating the effect of the coax on
the reading.
My feed line for my tri band beam is 8 half waves long on 10 meters, 6
on 15 and 4 on 20. Of course that is only for one freq per band. I
chose 14.250, 21.375 and 28.500. This lets me tune to those freqs on
each of the three bands and measure the SWR of the beam from the
transmitter end of the coax.
See your handbook for more.........
... Energizer Bunny Arrested! Charged with battery.
--- PPoint 2.00
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