[TowerTalk] Loading tower on 80 and 160 meters

[Gary Schafer]

Trying to make a vertical for 80 meters and 160.

I have an LM354 crank up tower with a full size 40 meter rotatable dipole
(about 67 feet in length) on it. The tower is about 54 feet tall with a 10
foot mast on top.
The dipole is mounted at 2 feet above the tower which puts it at about 56
feet above ground.

I put a drop down wire on a rod that extends about 2 feet out from the top
of the tower. The rod is mounted at the top of the tower at 54 feet.

The tower seems to be resonant at around 9.2 MHz with a grid dip meter or an
mfj 259 hooked to the wire and the bottom of the tower.

With a series capacitor on the wire I can get a dip in SWR at around 2.3
MHz. Going lower or higher in frequency will raise the SWR significantly
Also tried an omega match but still can't get it to the range wanted.

I want to tune this thing in the 80 meter band first and second on 160.

Can someone model this and tell me if my gamma wire is too long?

I found an article that approximates electrical length of a 54 foot tower
with a triband antenna to be around 100 feet.
This calls for a gamma wire length of around 30 feet in the article and a
series capacitor around 100 pf for 80 meters.

I don't know how much different the top loading of the 67 foot 40 meter
element will be compared to a triband beam.

With the crank up tower I can put the gamma rod at the top, 54 feet or on
the middle section at 37 feet or on the bottom section at 20 feet. Those are
the only options.

Any help appreciated.

Thanks
Gary  K4FMX