Topband: VF - It is what it is
zedkay at bellsouth.net
Wed Mar 28 21:21:44 EST 2007
Thanks to all for the many responses to my post questioning the
velocity factor of foam-dielectric coax.
After re-verifying (for the third time) the physical length of the
piece I was checking, and after verifying the accuracy of my Autek
RF-1's frequency readout against the station receiver, my
measurements are confirmed. VF for this cable is 0.84.
Moral of this part of the story in not to blindly follow coaxial
cable tables or data sheets or even labels. Verify the specs for yourself.
Also, thanks to W5UC for pointing out that John, ON4UN, warns against
using foam-dielectric cable for phasing or delay lines because the VF
will change (decrease) over time.
But... the foil-shielded, so-callled RG-6 used in satellite TV
systems is good low-loss cable, and it's cheap, and already on hand.
My solution to the changes-VF-over-time problem in a crossfire phased
system is: To first use equal lengths of it from each Beverage to
reach a common point ahead of the un-un. (Change of VF in these
sections will not matter, since both pieces are the same length of
the same cable, cut from the same spool. Both pieces will remain the
same electrical length as their VF changes identically.) Then, to
add the phasing line to the line from the rear staggered Beverage as
a separate piece of cable. This way the section of line being used
to achieve the proper phasing angle of the two antennas can be
disconnected periodically, it's VF checked, and be replaced if need
be. This would also allow the null to be easily moved for any other
reason by simply substituting the appropriate length line.
Charles - K5ZK
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