[TowerTalk] Fwd: Rooftop tower as Vertical Antenna

[Jim Brown]

On 12/19/2018 5:39 AM, Kelly Taylor wrote:
> It seems to me the trick will be electrically isolating that which you want as an antenna from any conductors (ground wire, rotator cable, coax shield, etc.) leading down from the system.

It's a mistake to think of the rooftop tower as the antenna -- the 
grounding wire is also part of the antenna, and it can be a useful part 
if the base is the feedpoint and some sort of counterpoise is added. 
This makes the vertical portion of the antenna longer, which makes it a 
better antenna for the lower bands. Since the ground is for lightning, 
it seems to me that shunt feeding might be the way to go. Remember that 
the loss in a short piece of RG8 is pretty low on 80 and 160M, even with 
high SWR, so a tuner in the shack should work as well as one at the 
base. Also, the antennas on the tower that are electrically connected to 
the tower will serve as top loading, making it longer.

> Tuned radials for the band(s) of interest would be a start,
I would simply get as much copper or other mesh down as possible, or a 
K2AV folded counterpoise.   See http://k9yc.com/160MPacificon.pdf for a 
lot of ideas about radial and counterpoise systems. While it's written 
specifically for 160M, all of the concepts apply equally to 80M if you 
divide lengths by 2.
> as would chokes on the coax and rotator cable.

Yes, very important. They should be at the base of the tower.

73, Jim K9YC