[TowerTalk] Pattern Distortion caused by faulty balun?

[Steve Hunt]

Tony,

I made some azimuth plot measurements on my hexbeam with various CM 
choke arrangements, including "none":
http://www.karinya.net/g3txq/hexbeam/common_mode/

The most sensitive indicator of CM issues was the shape of the rearward 
lobe. Although the effects on that web page look small, I should 
emphasise that they will vary massively depending on the impedance of 
the conducted CM path. If you happen to have a coax feedline which is an 
electrical half-wave long (or multiples), and which is well grounded at 
the shack end, the effect would be much much greater.

On the other hand, the requirements on choke impedance in this 
application are not so demanding. The impedance looking into one half of 
the beam is just a few tens of Ohms, so for a choke to be effective we 
don't need the several thousand Ohms of CM impedance that we would in 
some other applications. Take a look at the table at the bottom of this 
page:
http://www.karinya.net/g3txq/chokes/

You'll see that with the worst-case feedline length, a choke resistance 
of just above 1kOhm was sufficient to reduce the CM current to 30dB less 
than that in a half-wave dipole leg; you'll also see that for some other 
feedline lengths, no choke at all is needed to achieve the same result. 
Finally, the same table also illustrates why reactive choke impedance is 
undesirable and can actually *increase* CM current with some feedline 
lengths.

73,
Steve G3TXQ