Carl,
Sorry I didn't more clearly describe how to isolate feedline shield
currents from the Beverage feedpoint ground...
Where do these underired feedline currents come from? The coaxial cable
laying on the ground (or even worse, if it is elevated) will pick up
unintended radio signals on its shield in the same way that the Beverage
picks up intended radio signals. Fortunately, the coaxial cable laying on
the ground is lossier than the Beverage, but -- none the less -- the
feedline also acts like an inefficient Beverage antenna!
Topbanders and Towertalkians who have read the fascinating IEEE
interviews (available on the PVRC Homepage: www.pvrc.org ) of H. H.
Beverage are aware that his first Beverage Antenna was simply several
miles of wire laying on a sandy path on Long Island. Only later
was the Beverage Antanna refined to be an elevated wire with resistive
termination.
Okay... Now you are probably wondering if I will ever get to the point!
If shield currents are allowed to enter the Beverage feedpoint ground,
those undesired currents will induce voltages across the loss resistance
of the ground rod (or ground system). This RF voltage will degrade
the inherent sidelobe rejection of the Beverage that is connected to the
same ground rod (or system).
Probably the most certain way to isolate undesired feedline currents
is to use one or more ferrite bead chokes over the feedline. My chokes
consist of 100 ferrite beads over a short length of RG-58. This piece of
RG-58 connects the 9:1 matching transformer to the feedline running back
to the shack. It would certainly be quite practical to use smaller (less
expensive) beads over miniature 50 coaxial cable.
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.
Hope this has been clearer and more helpful...
73
Frank
W3LPL
donovanf@sgate.com
On Thu, 9 Oct 1997, km1h @ juno.com wrote:
> Frank, do you mean seperate ground wires for the feed and antenna
> windings? Explain please. I use a 9:1 xfmr and tie both ground leads
> together in the box and run a single #10 wire to the rod.
>
>
> > - Isolate the feedline shield from the matching transformer with
> >ferrite
> >beads
> > - Install a second ground rod -- isolated from the transformer -- to
> >ground the shield. Some even uise a third ground rod ten ot
> >tweenty feet down the feedline.
>
> As above, I'm not clear on this.
>
> 73 Carl KM1H
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|