[SOC] G5RV

[JMcAulay]

At 08:15 PM 06/11/2003 +0100, Paul M3CRQ wrote, in part:

>Would it be correct to say that they're not really baluns at all but rather
>simply common-mode chokes reducing RF currents in the feeder braid? I think
>you said as much in your last post; just trying to get a handle on it...

Your handle seems to be working okay.  In most applications, I'd rather see
a real balun transformer (the last time I looked, about twenty dollars for
one that'll handle a kilowatt!). But for this case the impedance
transformation provided by joining the two lines is an integral part of the
design.  Hence, with the G5RV, the choke is the preferred device. 

>Would it make sense to attach the feeder braid to a 'good' RF ground
>immediately prior to the choke? I.e at the far end of the feed from the TX
>point of view but before the choke.

An RF ground loop can cause problems, and the higher the frequency, the
worse things can get.  Practice at most MW broadcast facilitiess in the US
is to use a broad copper strap (usually four inches wide) to tie together
everything in the RF system:  transmitter enclosure, coupling equipment,
tower ground(s), all that and more.  On the last installation I did some
years ago, I surmised that the two-element antenna system would be stable
without all that added copper (the stuff is terribly expensive).  So RF
ground was carried from the transmitter building to the towers only through
the shielding of the coaxial cables (solid copper jacketed 5/8" Andrew 50R
foam Heliax).  The system worked as planned, and no instability problems
have been reported.  So if you want to ground the far end of the cable, be
sure it is a *good* ground, the *same* ground you had at the transmitter, a
*better* ground than the cable shield is providing.  Otherwise, I'd rather
see it float.  YMMV.

73
John WA6QPL  SOC 263