[TowerTalk] 40 meter vertical to 80 meters.

[John Tait]

Lee Buller wrote:
>  
>   Ladies and Gents,,,
>    
>   Last year, I build a homebrew 1/4 wave verticle and mounted it on the ground with 16 radials underneath it.  I am in the process of adding 16 more radials.  The antenna is made out of aluminum tubing starting with and 1" 1/4 and going down to 3/4".  The length is 32 feet and with 16 radials under it...it resonates at 7.100...at least it is flat there...and the MFJ 259 says it has imdedance of 55 ohms...R=53-55, X=3-6, Theta is 10 degress,  Works good too.
>    
>   What I want to figure out is how to had a "top-hat" to the antenna to bring it down to 80 meters.  I know that the MFJ brand of verticals uses a top hat loading scheme and I wonder how you calculate or figure out the capacitance one would need to build something like this?  Or, is it trial and error?
>    
>   In addition, the top of the antenna is fairly small tubing (I am afraid I will bend it with to much weight or load) and I need to come down on the antenna about 8 feet to install the top hat.  I guess one would have to use trial and error to find the right combination of "spokes and Lenghts" to make the capacitance hat.  Looking at the Handbook, it does not really have a "formula" for such things.
>    
>   Anyone have any insite?  Give me a starting point?
>   
I'd try about 12m of wire. Connect the centre of the wire to the top of 
your vertical, and run the two 6m lengths in opposite directions, as 
horizontal to the ground as possible. That's probably a bit too long, 
but you can shorten them until you hit resonance where you want it.. I 
assume you want to permanently change it to 80m?  If you want to run 
both bands, then you could mount a 40m trap between the top of your 
vertical, and the capacity hat wire...Like the "Battle Creek Special".
     Good luck with your project
          John EI7BA