TopBand: Isolating Feed Point
Wed, 11 Dec 1996 19:24:47 -0500 (EST)
On 11-DEC-1996 13:49:45.7 wc7m said to GOOSTER
> Howdy gang,
> The Question is: Can I use Ferrite to isolate a feed point on a tower?
> I have a 133 ft tower that is insulated 10 ft above ground. I built a
> collar out of 3/4 inch copper pipe that is insulated from the tower and
> direct ground. I have about 34 quarter wave radials attached to this
> collar. The tower sports a Cushcraft log periodic at 135 ft, and a
> Cushcraft 40mtr dipole at 80ft. Both feed lines cross the insulated
> I loaded the tower, feeding it at the insulated section, and got it "to
> work." I checked my feed lines with a probe and they were hotter
>than a heifer with out a brand. My signal reports were suspicious
>also, when I compared them to what I was handing out to stations with
>dipoles at 20 ft.
> I suspected the feedlines were coupling and shorting out the insulating
> section at RF.
> Prior to the 160mtr contest this past weekend, I unhooked the feed
>lines that crossed the insulator and tied them up onto the tower. The
>tuning of course had to be drastically changed. The feed lines were
>not near as hot. The tower obviously improved and made a good showing
>in the contest.
> My question is, can I load my cables with ferrite and isolate them
>at the insulated feed point? Or will I have to keep disconnecting
>everything to run it on 160mtrs. I could machine a switch that
>remotely diconnects everything completely, but would the ferrite work?
> I have provisions to shunt feed the tower, but I don't want to do that.
> (long story)
> Jay WC7M in Gillette, Wyoming
> On the 1AB Ranch in Campbell County
> FAQ on WWW: http://www.contesting.com/topband.html
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> Sponsored by Akorn Access, Inc & KM9P
Why don't you wind some coils of coax about a foot in diameter and about
17-20 turns in length with a coax width spacing between successive turns.
Then install them right below the insulator on the tower. Tie the shield of
the coax at the top of the coil to the insulated (hot) section of the
vertical antenna and the shield of the same coil of the coax at the bottom
of the coil to the grounded section of the tower. This makes about a
100-150 microhenry coil that will keep you from shorting out the 160 antenna
while still allowing you to use the antennas mounted on the insulated
section without dosconnecting them. We use this technique in the broadcast
industry and I use the same technique on my quarter wave 160 vertical that
also supports my stacked 40's and stacked 20's. It makes a wonderful
isolation coil and works great. There is no 160 energy in these coaxes
when they are hooked to radios in the shack and we can transmit on all three
bands at once. I dont have to use any ICE filters to notch out 160 on the
40 and 20 radios either. Give me a holler if you need more info.
73.....de Goose W8AV (ex- WD8LLD) "firstname.lastname@example.org"
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