Topband: Inverted L

[Herb Schoenbohm]

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
>
>
>
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