[Fwd: Re: Topband: toploading of shunt fed tower]

[Herb Schoenbohm]

Jim Hoge wrote:

> 
>I have a 70' crankup tower that I am considering shunt-feeding for 160m. The question is this: The tower supports a Tennadyne T-12 12 element log periodic antenna that is not electrically connected to the mast and tower. Should I install a relay .....
>

Jim,  I had a similar situation with a Force 12 antenna and I relied 
solely on the boom which was electrically connected to provide at least 
some valuable top loading. Yes the tower was short on 1.8 but with the 
proper cage wire shunt feed it works well. So this solution depends a 
lot on your boom length...or....

I have heard of a coil to boom approach which has high enough reactance 
not to bother the LP performance and act like an RF choke on the higher 
frequencies but low enough in inductance to "couple in" the elements to 
give some additional toploading.  Perhaps one of the other readers of 
this can give you the coil dimensions and let you know of their 
experiences with this approach.  I did not try this as the 70 foot with 
just the boom as a top load, plus a 15 foot stinger on the top works so 
well.  (Maybe you heard KP2ZZ during the  CQWW 160 Contest using this 
antenna.)

Another, and simpler approach is to connect a two wire "T" wire 
sufficiently long enough to drive the tower to resonance on 1.8, 
however, these wires will have some interaction with  the  T-12 LP.  
Also one could droop them down when operating on the higher frequencies 
and raise them up as horizontal as possible when operating on 160, as an 
alternative.

Relays would be my last choice as the possibility of failure from 
moisture and contact spacing issues are there since the voltage is very 
high at these points on the tower as it approaches a 1/4 wave.

Good Luck

Herb Schoenbohm, KV4FZ