Hi Carl and all.
I designed my Bev stystem to use 75 Ohm CCTV cable . The stuff I use is
Motorola SDN6201 which is similar to Belden Triax. It is about .6 inch in
diameter and has a 13 GA copper center and two completly seperate copper
shields. The 9:1 transformer is wound with and isolated secondary, leaving the
antenna end ground connection going only to the cold end of the primary and to
the outer shield only. The outer shield is NOT connected to anything at the
other end. The inner shield and center conductor carry the signal back to the
hub which is 1000' away, and connect to the hub through a common-mode filter,
which is nothing more than adout 50 turns of small 75 Ohm Teflon cable wound on
a stacked torroids.
I also have a ZJ built BCI filter which can connect in line after the hub
switch but find that it is not needed, even with a 5KW MW station on 1070 KC
fust down the street. BC RF is so strong here that I can't use my MFJ antenna
analyzer, or even the Millen RF bridge due to full scale readings, yet in the
shack there is zreo interference/splattter/AGC pumping.
with two shields you get electrostatic as well as electromagnetic shielding,
and the common-mode filter rejects any residulal energy which is INPHASE
(induced) on the shild and inner conductor, yet freely passes OUT-OF-Phase
(desired) signals.
My hub switch also provides lightning chokes and fusible links for the antenna
side and lightning shunt on the shack side, and handles switching 5 Beverages,
one remote Beverage (the one described above) which is fed back by coax, and
the bi-directional feed/termination of the above beverage. This E-W wire is
1000' long and can be fed/terminated (uni) from either end or
fed/unterminated(bi) from either end. Makes for several interesting
possibilities.
The whole system consists of 3- 1000', 2- 500' and 1-875' pieces of wire giving
it a total of 8 directions.
---73 GEO (K0FF)-------
> 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/topband.html
> Submissions: topband@contesting.com
> Administrative requests: topband-REQUEST@contesting.com
> Problems: owner-topband@contesting.com
--
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
|