Date: Mon, 12 Jan 2015 09:58:03 -0800
From: Jim Brown <jim@audiosystemsgroup.com>
To: amps@contesting.com
Subject: Re: [Amps] The genius of ham radio
Understanding HOW antennas work allows us to achieve a better result
faster. Sure, we could build a dipole, operate it at various heights in
increments of 5 ft, and use a drone with instruments attached take a lot
of measured data to see it's directional pattern, both vertical and
horizontal. Bring a very fat wallet to this process. OR, build a model
of that antenna in NEC and have it compute the 3D pattern at various
heights in increments of 5 ft. I've done that in a day or so. I now
KNOW, in dB, the value of 10 ft of additional height on 80, 40, and 20M.
That work, BTW, is on my website.
http://k9yc.com/VertOrHorizontal-Slides.pdf
73, Jim K9YC
## This pdf is very well done..and a real eye opener. It also assumes you
are in the middle of a wheat field in Kansas. Your typ back yard in suburbia
is not like that at all. You are crowded by other homes, 40 ft tall utility
poles,
metal fences, metal cars, aluminum window frames, miles of house wiring,
utility drop wires etc. Stucco homes have miles of well grounded chicken mesh
beneath their surface. Toss in street lights and also freestanding 30 ft tall
aluminum street lamps.
## Your backyard ground mounted 40m full size vertical, even with lots
of ground mounted radials does not fare too well. Its radiating into all this
junk, rendering it semi useless at best. Inverted L verts on 160 + 80m
are in the same boat. How are you going to get radials laid out 360 degs,
with a home and garage in the way ? Trying to elevate radials on 160 +80m
in a typ small backyard is unobtanium. 30 ft high dipoles and yagis on the
upper 20-10m bands still does not put the ant into the clear. You are still
10’ below the utility lines, and still radiating into your neighbours homes.
## Dipoles that dont rotate.... you just lost 14 db off the sides. A rotary
dipole is a huge
improvement over a fixed dipole. When you increase the height from 30 ft to
50+ feet, the difference is almost staggering. The ant is now in the clear.
## trying to shunt feed a tower on a city lot, with less than optimum length
radials..and
only across 180 degs usually amounts to an exercise in futility. Trying to
get any type of RX ant
away from the TX ant is another exercise in frustration. A quarter wave
sloper or loaded
loaded quarter wave sloper at least puts the feedpoint at the top of the tower,
with less
reliance on radials.
## Even a 50 ft tower is only clearing the HV utility lines by 10 feet.
Trying to guy a tower on a
small city lot is usually dangerous at best..and makes for any install or
removal of ants very
difficult. What does work on a city lot is the tallest, freestanding tower
you can install.
Then you have something to work with. Not everyone is blessed with tall trees,
suitably located
on their property. The pdf is superb though. In the clear and ...higher is
better. 70-75 feet
is a good compromise between a 50 ft tower and a 100 ft tower...when on 20m.
For folks with
crank up towers, dial in the correct height for whatever band you are on,
factoring everything else in.
Jim VE7RF
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