Topband: VF - It is what it is

Charles Bibb zedkay at
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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