Topband: T Vertical feed

[Joe Subich, W4TV]

On 1/27/2012 12:32 PM, DAVID CUTHBERT wrote:
> Charles, Joe listed some symptoms of excessive feedine common-mode
> current such as RF bites and RF feedback. If you do not suffer from
> any of these effects a balun may be of no use.

I listed the most obvious symptoms - ones which old timers will
remember very well.  They are certainly not the only symptoms and
others can be even more dangerous resulting in intermittent problems
to serious damage to transceivers, computers and accessories.

Common mode issues are real and even more common on unbalanced antenna
systems - particularly verticals of all kinds - due to their inherent 
imbalance.  Anecdotal information simply does not cut it when it comes
to protecting your equipment.  Either build a device for measuring
pico amp levels of current on the shield of the feedline or use good
common mode chokes to make sure the currents do not exist.

73,

    ... Joe, W4TV


On 1/27/2012 12:32 PM, DAVID CUTHBERT wrote:
> Charles, Joe listed some symptoms of excessive feedine common-mode current
> such as RF bites and RF feedback. If you do not suffer from any of these
> effects a balun may be of no use.
>
> If one does experience CM problems that might be cured by a balun (CM
> choke) by all means give it a try.
>
> In my NEC model a CM choke having an impedance magnitude of 300 ohms
> reduced the coax CM current by one half.
>
> My anecdotal evidence from a dozen vericals is that no balun has been
> needed. Even in the case of a 12' top loaded 1.8 MHz vertical having a base
> current of 10 amps.
>