[TowerTalk] Vertical Antenna Question

[Howard Hoyt]

Rob,

I've been a broadcast engineer for 40 years, and of course radials are mandatory with a vertical radiator, but one of the main reasons broadcast facilities do not need CM chokes at the antenna feedpoints is: they are tuned to one frequency only so the match can be optimized to the point where there is little to no CM current.  Some transmitter manufacturers like Nautel specify a CM choke at the transmitter itself, but this is largely to provide an inductance to delay a potential lightning surge on the feedline itself until the stout grounding system found at commercial transmission facilities drains most of the charge.

Amateurs are frequency agile and a perfect match is seldom found on a single antenna on all frequencies used.  In this situation a CM choke forces radiation by the antenna instead of the feedline and the ham will see the true antenna system match with the feedline out of the equation.  Many hams think their antenna is a good match over a wide range of frequencies when the is feedline radiating until they install a good CM choke at the feedpoint...then reality can rear it's ugly head.

Cheers!
Howie / WA4PSC


>>Not if you use enough radials.  I don't think I've ever seen ferrite
>>cores out on heliax at AM broadcast towers.  If you use enough
>>radials, the return current will be so divided down that the current
>>on the outside of the feedline (burying it helps but isn't necessary)
>>will be negligible.  If you have to use ferrite cores to keep RF off
>>your feedline exterior, you don't have enough radials down.