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