> From: Frank Donovan <donovanf@sgate.com>
Frank's post was excellent, and a very accurate description of a
problem commonly occurring with ANY receiving antenna...not just
Beverages.
Why install a low noise antenna far away from the noisy house wiring,
just to run a transmission line out to the antenna with no isolation
for the outside of the shield? The noise and unwanted signals can
zip right along the shield to the receiving antenna, essentially "turn
the corner", and come right on in!
> My installation uses two ground rods, one at the cold end of the Beverage
> feedpoint 9:1 transformer and a second ground rod connected to the
> feedline shield at the feedline-side of the ferrite bead choke.
I'd like to add something else I do, that Frank mentioned earlier.
I use a completely isolated primary on the transformer if possible.
That removes the direct connection between the antenna system and the
feedline shield. I ground the feedline through a separate ground rod
several feet from the antenna, using a CATV grounding block.
Between that grounding block and the cable from the shack, I use
beads.
By the way, a Faraday shield does not reduce common mode coupling
between the shield and the antenna system when using small
transformers with small windings. (If you are using six inch
diameter air wound coils like they used in the 30's it might help,
however.) As a matter of fact, it actually INCREASES unwanted common
mode ground system coupling in many systems. For a Faraday shield
to be effective in this application, it needs to connect to its own
separate ground system that is not common to either the feedline OR
antenna.
73, Tom W8JI
-
---
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|