[TowerTalk] Six Meter Antenna Feedline/Choke

[Ian White GM3SEK]

Lee Buller wrote:
>
>  I've been researching 6-meter antennas and found a lot of nice 
>construction articles out there for 3 and 4 element antennas.  Well, 
>running out of time, I purchase a Cushcraft A50-3S for a good price. 
>In my research, alot of people used a 5-inch diameter roll of coax with 
>5 turns as a choke balun.  Is that needed?  I understand what it is 
>doing on the antenna, but gee.....its 6 meters.  The Cushcraft has a 
>gamma match feed.  I just don't see the coax radiating that much with 
>just a 100 watt transceiver.
>
>  Just wondering?

Just wondering what you mean by "Gee...  it's 6 meters"? Radiation from 
the coax when transmitting is only a small part of the story on this 
band.

On receive, 6 meters is a lot quieter than most people can begin to 
imagine - they're not aware of how badly their receiving capability has 
been affected by pickup of local noise, computer spurs and other 
man-made garbage. They think that's just how the noise level is on 6 
meters.

A feedline choke, along with everything else you can do to prevent RF 
currents where they don't belong, can produce amazing results in some 
situations.

As for the gamma match, it just doesn't seem a good idea to make a 
deliberate connection between the coax shield and the boom, the mast and 
the tower. On the contrary, it seems like an open invitation for RF 
currents to flow all over the metalwork; and on receive, to pick up 
unwanted noise and feed it into the coax.

There are many other ways to match a yagi, that also avoid noise pickup 
on the feedline and other metalwork.



-- 
73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek