Julio:
Yes, radials are needed. For my shunt-fed tower I have only 7 radials,
only five of which are over 100 feet long. The other two are 30 feet long.
BUT - this tower has a pretty decent lightning ground: three buried #2
tinned copper wires (130 feet total), down about 18 inches and Cadwelded to
twelve ground rods. I can't help but think that these buried radials
contribute to the coupling to the earth and provide some benefit to the
tower on 160M. I've been thinking of experimenting with removing the
lightning grounds and using only the radials for an evening or two, but I
digress...
Someone else on the reflector said if you can't put in
quarter-wavelength radials, it'll do you no good to put in a lot of shorter
ones. Somewhere in my readings on Topband antenna design I ran into this
same rule. If you can only put in short radials, about 20-24 will get you
close to the max efficiency possible with such short radials. No need to
put in 50, 100 or more shorties. So there's hope for us Topbanders who are
confined to small backyards!
73 de
Gene Smar AD3F
-----Original Message-----
From: jperalta@tampabay.rr.com <jperalta@tampabay.rr.com>
To: EUGENE SMAR <spelunk.sueno@prodigy.net>
Cc: towertalk@contesting.com <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Thursday, September 26, 2002 2:47 PM
Subject: Re: [Towertalk] Can my 40' tower be a 160m vertical?
>Aren't radials also needed to make the shunt fed tower work efficiently? If
so
>what would be the correct length and how many for a respectable
installation?
>
>Julio, W4hy
>
>
>
>> Jim:
>>
>> I agree with Phil's recommendations on Jeff's book. You can get it
>> from ARRL. I based my Trylon 64's shunt feed on Jeff's design. The key
is
>> to have something as a capacitive load at or near the tower top to make
the
>> tower appear electrically longer than it is physically. Your Yagi/40M
>> add-on ought to be a pretty good load.
>>
>> Many on this reflector would recommend modeling, but I'd say try it
and
>> let us all know. The feed is nothing more than a wire from the bottom of
>> the tower and connected near the top, keeping it a couple of feet away
from
>> the tower itself (I used PVC tubing as spacer material). Then you feed
the
>> bottom of the wire through a variable transmitting cap of some fairly
large
>> value in series with the coax center; I keep the cap insulated from
ground
>> by mounting it in a plastic weather enclosure (a plastic shoe box from
>> K-mart). The coax shield gets connected to the tower legs and ground
>> radials. Tune the cap for min SWR and call CQ.
>>
>> BTW, the sloper might have an impact on your ability to shunt feed
the
>> tower. If you can't get the tower to load, you might want to remove the
>> sloper and try again. (I haven't modeled this, so I'm guessing.)
>>
>> GL es 73 de
>> Gene Smar AD3F
>>
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