Here are some facts instead of theories about verticals. I have built two
verticals, the first one 70 feet tall, no top loading. It covered 40,80 and
160
with base selected L networks. The values are given in Ham Radio May 1984.
It worked very well on all three bands, for example used to qso 3Y0PI on 160m
ssb.
When 160m became a higher priority I wondered if I could get up a taller
vertical?
My lot size for placing guys limited the choice to 90 feet. This time I
added 4 top loading wires each 42 feet long as part of the top guys. I use
this
antenna on both 160 and 80 and it works equally well on both bands and is
simple mechanically with no traps etc. I erect it with the N6RK lever arm
method.
More results: 205 countries on 160, and a 75m qso with VK0IR from the black
hole on that expedition.
Matching is done with L networks. For 160m an input shunt ceramic doorknob C
of 1000 pf. For the low part of 160m a series C of about 2500 pf, and about
1300 pf for the high part of the band. On 80m, a series 15 uh coil and a 75
pf C shunt across the antenna. I tune the rotary coil with a screwdriver motor
between 80 cw and 75 ssb. Two seperate networks could have been used.
The input impedance on 160m is about 37 +j40 ohms. With all my notes and
papers and handbooks I couldn't find the 80m impedance, but using the ON4UN L
network computer program it calculates out to about 200 -j600 ohms.
Ok, are there any magical heights? The 70 foot vertical worked well, and the
90 foot one better. The shorter ones result in more earthworm power but
still get out well.
There may be reasons for certain heights. I found from Smith chart work that
the 70 foot was optimum for 80m as a single series C was all the network
needed to cover most of that band. I don't know the answer for 100 feet -
maybe
the input impedance would be something hard to match, and I would expect the
performance would not be much greater than for the 90 footer. The hoisting
effort and the cost of good guy material would be greater.
73 Bob W7LR in MT
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