Topband: Second inverted L

[Jim Brown]

On 10/2/2020 10:32 AM, Peter Krulewitch wrote:
> Want to add a second wire inverted L off my tower, tower height 90 feet.

That can be quite productive. You need to begin with a simple model in 
NEC that includes both wires, the tower, the aluminum on the tower 
that's connected to the tower, and whatever counterpoise you plan to 
use. It should also include any other towers or vertical wires (like 
coax feeding dipoles). N6LF has published his studies showing that even 
fairly short towers can influence the patterns of nearby verticals.

I have two wires sloping from just below the rotator on the 120 ft tower 
that holds a small 3-el straight SteppIR and a 30 ft long 6M Yagi. The 
wires are insulated from the tower, and each is fed, one at a time, from 
the base against four elevated radials (four for each antenna). One 
slopes to the east, the other to the west. The tower is grounded, and 
serves as a passive reflector, and there are ten on-ground radials 
connected to it to make it more efficient as a reflector. The result is 
about 2 dB of gain in the direction of the wire that I feed. Each is fed 
by it's own run of 50 ohm coax.

On the advice of retired ARRL Antenna Book editor N6BV and another very 
experienced topbander NI6T, I carefully used NEC to study interaction 
between the tower, those sloping verticals, and my first topband 
antenna, a 100 ft Tee vertical. That study showed me that the tower 
acted as a reflector for the Tee, giving me about 2 dB to the south, and 
that if I shorted the base of the Tee, it moved the patterns of the two 
sloping wires 30-40 degrees north -- one to JA, the other to EU.

On the air experience confirms the modeling -- as I switch between the 
three antennas, I can hear signals from those directions getting louder.

73, Jim K9YC