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.
I had a C-3 on 75 foot tower form many years until a hurricane had
other ideas) and it worked very well on 160.
If it is not convenient to make mods to the C-3 you can just feed the
tower as is as the C-3 boom will give you at least top load equivalent
to a 90' vertical. This will require a LCL or L network to easily match
the cage feed...or you can try an Omega match to avoid the trips up and
down the tower to try and find the sweet spot so you will have a 50 feed
impedance plus some reactance which you can tun out with a series
capacitor. As in AM broadcasting the tower there is no real magic to
achieving self resonance in the tower, if you can match the base
impedance with an ATU and have at least 16 radials or more at the tower
base.
The inverted L is not a good deal as too much radiation goes up to the
clouds...and the metal tower can give undesirable coupling that can work
for or against you. It is easy construction but also easy to fool you
that you have something that is really working well for DX. After
calling something rare for a week with the inverted L you will get the
message...if you don't believe me.
73
Herb Schoenbohm, KV4FZ
John Clark wrote:
> Looking to build an inverted L can anyone point me toward a good set of
> plans. Or plans for something that will work
> have way decent.
>
> I plan on hanging it on my tower.
>
> A 75' Rohn 25 with a Force 12 C-3 on top I have 3 acres of land to work with
> for radials and the like.
>
> Thanks
> John N5ER
>
>
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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