> The best approach is to bond the braid AND use a good choke right at
> the feedpoint. Remember, we want to take the feedline completely out
> of the game, and the typical length of coax between the feedpoint and
> the tower can make a pretty good RX antenna.
If the braid is bonded to the top of the tower (it should be bonded
both top and bottom for lightning protection), one particularly
needs and effective balun. Without it, current in the "shield"
side of the drive element will divide unpredictably between the
boom/tower and the driven element half.
In addition. if the tower itself is close to "resonant" on the
band of the antenna, it will provide significant noise and omni-
directional RF to the "shield" side of the driven element by
common mode connection. A common mode choke is important to
prevent pattern deterioration here.
All in all, ground the shield of the feedline at the top and bottom
of the tower and use a high quality balun/common mode choke at the
feed point of the antenna.
73,
... Joe, W4TV
On 6/4/2010 12:04 PM, Jim Brown wrote:
> On Fri, 4 Jun 2010 08:40:41 -0700, Jim Thomson wrote:
>
>> Rauch advocates grnding the braid of the coax at the top of the
>> tower and or mast asap.
>
> This is partially for lightning protection, but it also has the
> advantage of minimizing noise pickup on the feedline. Tom's
> absolutely right. It is a VERY good idea. It's also great if you want
> to load the tower on 160 or 80.
>
>> Why not just make the coax an electrical odd multiple of a quarter
>> wave.. and then bond the braid to the tower.
>
> The best approach is to bond the braid AND use a good choke right at
> the feedpoint. Remember, we want to take the feedline completely out
> of the game, and the typical length of coax between the feedpoint and
> the tower can make a pretty good RX antenna.
>
> All of that screwing around with coax length is a PITA. The right
> choke at the feedpoint (see my Cookbook) always works and the
> feedline length doesn't matter.
>
> 73, Jim Brown K9YC
>
>
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>
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>
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