Topband: tower effecting antenna

[Herb Schoenbohm]

Alan,


An 80 foot vertical, assuming that you connect a wire at the top of the 
20 foot fiberglass which is connected to the 60 foot metal tower top, 
and also assuming that you have a fair to good ground system, can be a 
good 160 meter vertical for DX communications.  If you can put some 
10-15 foot  spiders at the top of the pole even better. 

To raise the feed point impedance and even improve the bandwidth 
somewhat, I would recommend you drop a 3 or 4 wire cage from the top of 
the 60 foot tower and then tap the tower to the cage  for the 50 ohm 
sweet point so the bottom of the cage will exhibit close to 50 ohms.  
There will be some inductive reactance that can be remove with a 
variable capacitor (and the replaced with a fixed mica value capable of 
handling the current, depending on your power levels).

Contrary to some opinion...the tower need not be self resonant at 1.8 
Mhz.  The key is a good ground and a good match to what ever is there 
vertically.  Even top loading enhancement with some short 20-30 foot 
guys ...and then insert insulators before making the rest of the drop to 
ground guy points, while keeping the angle of declivity not less than 45 
degrees should help a bit.  But modeling the spiders at the top may show 
that they may be good enough for you need.

Herb Schoenbohm, KV4FZ




Alan Wells wrote:
> I have a 60feet tower with a 20 feet  glass firbe pole in the top, on topband i have a 80 feet wire going vertical to the top of tower  spaced 3 feet from the tower, and 2 loading wires about 60 feet coming off the top. i am sure the tower is having some effect on the sytem, as anybody had the same problem.
>
> thinking of using the tower as part of the antenna putting a 20 feet aluminimun pole in the top, then using 4 loading wires from the top , the tower is grounded, about 18 feet up the tower there is 2 lugs i can put a gamma arm out and tune from the shack, i know the gamma arm is usually right at the top, if i put enough top loading on would this tune at the 18 feet point.
>
>                                                                           many thanks
>                                                                           g4erz
> _______________________________________________
> 160 meters is a serious band, it should be treated with respect. - TF4M
>
>