John:
If you can de-mount your C-3 and get at the 20M parasitic elements at
the ends of the boom, you don't need to install a choke to provide top
loading for 160M. Natan W6XR, the designer of the early F12 Yagis, posted
on TowerTalk a few years back that the insulated parasitic elements could be
shunted directly to the boom at the center without affecting the antennas'
performance. The center of each element is a voltage node (minimum).
I tried that with my (freakishly similarly designed) Bencher Skyhawk
and was able to load my 64 foot tall tower on 160M with little trouble. I
attached pieces of aluminum flashing to all the parasitic elements with a
#10 stainless self-tapping screw through the insulator into each element,
and with a stainless hose clamp and NoAlOx on the boom. My flashing pieces
were 3/4 inch by 5 inches, IIRC.
I have a photo of one of these grounded elements at:
http://s83.photobucket.com/albums/j282/ersmar/?action=view¤t=Elementgroundstrap.jpg
.
And if I can get my tower to work well on 160M, anyone should be able to.
73 de
Gene Smar AD3F
P.S. In case it wasn't obvious, DON'T install the flashing on the driven
elements, just the parasitics.
----- Original Message -----
From: "Herb Schoenbohm" <herbs@vitelcom.net>
To: "John Clark" <n5er74@yahoo.com>
Cc: <topband@contesting.com>
Sent: Friday, February 12, 2010 8:30 AM
Subject: Re: Topband: Inverted L
> John,
>
>
> You don't need to put up anything since your 75 foot tower with a C-3 on
> top will make a much better TX antenna on 160 with a shunt fed or cage
> feed. There is one thing about the Force 12 C-3 is that the elements
> are insulated from the boom so you lose a bit of top loading. However
> you do some trick with it and it will work well for you. The best bet
> is to put some a small choke from the insulated to the boom that will
> trap of 20 meters and up but provide a good RF path on 160. A half inch
> coil of 12 turns of # 144 solid electrical wire will work fine for that
> purpose. If you have any doubts you can first at them to each side of
> the driven element to the boom. You can measure any changes and the
> only ones you will see are on 160 where the self resonant frequency of
> the tower will be lowered. There is not much reason to add any more
> than you need except if you run extreme QRO and are afraid the
> insulators on the outer elements need break down protection.....then
> they need the coils anyway.
> SNIP<
>
>
> 73
>
> Herb Schoenbohm, KV4FZ
>
>
> John Clark wrote:
>> Looking to build an inverted L can anyone point me toward a good set of
>> plans.
<SNIP>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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