[TowerTalk] Accuracy of models

[Jim Brown]

One of the more interesting pieces of work I've seen that demonstrates
the complexity of what is happening in a ground system was published by
three hams in NC in the Feb 1983 issue of QST. It is downloadable from
the ARRL website as a pdf.  

Like most hams, I don't have anything approaching an ideal QTH for MF
lor low HF verticals. Although the mountaintop I showed a link to last
week has possibilities, getting radials in presents a challenge because
the antenna has a dirt access road surrounding it on three sides. That
doesn't mean we won't try. :) 

My practical rule of thumb is to get as much copper (or other
conductivity) in the ground system as possible, and concentrate on what
is happening close to the base of the tower. If I can put in longer
(and resonant) radials, I will, but if I can't, or if I can do only a
few long/resonant ones, I'll add as many shorter ones as practical. And
I won't spend any time analyzing or handwringing about the perfection
of the design. In other words, I see it as a brute force issue -- I can
never get "enough," so I'll simply work for all I can get.  I recently
picked up another 6 dB or so on 160 by adding bonds from a wrought iron
fence to my ground system.  (I figured that out by getting comparative
signal reports within the ground wave range, and it has been confirmed
by improved skywave signal reports). 

One further thought for our original questioner re: what I think may be
a mobile installation. I recently put a Hamstick on my S80 Volvo using
a Lakeview license plate mount. The mount "floats" from the trunk lid,
so I had to bond around it. A quick ohmeter check showed that the trunk
also 'floats" from the rest of the car body, so I also bonded around
the hinges. Each of those actions reduced the resistance (and increased
the VSWR), but the resulting installation gets out pretty well. So the
message is, in any mobile installation, don't take anything for granted
re: the electrical integrity of the vehicle's body, especially at RF.  

Jim Brown  K9YC