On 11/21/2011 2:37 PM, Jim Bowman wrote:
> I have seen various and differing articles regarding baluns for
> verticals but most seem to indicate that it is good practice to use them to
> avoid the feed line from behaving like another radial.
Yes, it is a VERY good idea.
> But what about this
> situation? Wouldn't baluns conceivably mess up the element phasing?
A so-called "current balun" is really a length of coax, and a GOOD one
for 80M is a lot of turns on a ferrite core. So yes, the phase shift in
that length of coax MUST be considered. No, the ferrite core does NOT
affect phase shift.
>
> Along the same vein, the coax length from the transmitter to the phase box
> will be about 60' or 70'. Would a common mode balun at the transceiver end
> or phase box end serve any purpose?
Yes, for this length, which will look like a resonant quarter wave with
a ground on one end, a choke at both ends would be a very good thing.
Also, remember that for COMMON MODE current (that is, the coax shield
acting as part of the antenna), the velocity factor is about 0.98
(thanks to the properties of the outer jacket and the shield diameter),
while the phase shift inside the cable depends of the velocity factor of
the coax itself, typically 0.66 for solid dielectric, or between 0.78
and 0,85 for foam dielectric).
There are detailed designs for excellent common mode chokes in the Choke
Cookbook that is part of http://audiosystemsgroup.com/RFI-Ham.pdf I
find that I can get about 7 turns of most RG8-size cables through 1.4-in
i.d. cores before I put the PL259 on, and five turns with a connector
already installed.
There are several major advantages to using chokes like this. First, the
length and velocity factor of the cable is a known quantity (assuming
you have measured it at the frequency where you are using it). Second,
it is far better than any choke you can buy. Third, it is far less
expensive than any choke you can buy that won't be as good. You should
be using #31 cores on 80, which cost about $5 from a good distributor
(see Appendix One in the link above for part numbers and good vendors).
73, Jim Brown K9YC
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