[TowerTalk] To get a truly balanced antenna feed

[Jim Thomson]

Date: Tue, 21 Jun 2016 23:21:59 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] To get a truly balanced antenna feed

On Tue,6/21/2016 9:31 PM, Roger (K8RI) on TT wrote:
> Didn't you once say "there is no such thing as a balanced antenna"? 

Yep, but I was speaking more in terms of perfection -- i.e., that in the 
real world, most practical antennas are unbalanced by their 
surroundings, even if we attempt to make them a balanced antenna and 
feed them with 2-wire line and a so-called balance tuner. It is that 
imbalance that causes common mode current. If the imbalance is 
relatively small, and if the antenna is resonant and fed with coax, we 
can choke it effectively.

73, Jim

## OK,  say in the case of a rotary dipole on a freestanding tower, say mounted
part way up a mast,  what exactly is unbalancing it ??    I cant see it being stray C
to the dirt below, since that value is miniscule at best.    However, I can see stray C
between the rotary dipole and what is immediately beneath it, like the boom  + els
of say a yagi.... 5-8 ft below the rotary dipole.   And this is all assuming that the
rotary dipole is  mounted in line with the yagi boom below it. 

##  Ian mentions the current.  If that’s the case, why not just measure the current with a clamp
on ammeter.... at say several random places on the coax feedline ?    Then we have yet another issue 
to deal with, and thats what if the coax braid is  bonded to the top of the tower ?   If a lousy CM 
choke balun  is used at the ant feedpoint, and  braid also bonded to top of tower, we can then have
a case where  CM current is now flowing down the tower.   Then the matter of how well the base of the
tower is bonded to any ground system... IE: just some rods... or rods + extensive radial system, etc. 

Jim    VE7RF