Topband: Using a three or fou rwire cage feed will help

[Herbert Schoenbohm]

GM4FAM Wrote:


> Greetings!
>
> I have just put all materials together for a 1/4 wave vertical on 160m
> which will be going up early next year.
>
> It consists of a 18m grounded tower + 9m of aluminium tubing + a 12m
> Spiderpole (complete with 2mm uninsulated wire).
>
>
> a) Gamma matching the tower (with top pole assembly) or
> b) Running a separate insulated wire down from the base of the Spiderpole
> directly to the coax at the base.
>
> If b) what is the minimum separation distance for the downwire from the
> tower itself?
>
> Hope someone can help!
>
> Chris,
>   

 From what you described I would strongly suggested constructing a three 
or four wire cage around the grounded pole both for efficiency and ease 
of matching.  If finding the 50 ohm sweet point is not easy to model or 
moving the tap up and down you can tape the cage to the pole about 65 
per cent of the overall height and use an LCL or CLC to match the coax.  
Some report that an Omega match serves to move the tap up and down 
without climbing and finding the tap by trial and error.  This may not 
be convenient with a pole so you could get close by modeling or by 
accepting the impedance that the 60 percent tap produces and then match 
it at the bottom.  Your match will most likely require canceling the 
reactance of the cage and stepping up the impedance to match what you 
have.  With the cage the B/W should be better plus you get some 
advantage of isolating other cables taped to the mast along with some 
lightning protection over any attempt to insulate the pole at ground.


Good Luck,

Herb Schoenbohm, KV4FZ
> _______________________________________________
> 160 meters is a serious band, it should be treated with respect. - TF4M
>