The K9YC modelling with EZNEC
<http://k9yc.com/VertOrHorizontal-Slides.pdf> is quite interesting.
Certainly has me thinking about vertical dipoles. The half-wave end-fed
looks like the perfect candidate for a simple vertical dipole.
Doug -- K0DXV
On 6/25/14, 11:55 PM, Jim Brown wrote:
On 6/25/2014 5:43 PM, Dauer, Edward wrote:
So, I've been selecting two of the tallest
candidates a couple of hundred feet apart and stringing a stout nylon
rope
between them. In the middle of the cord I attach the balun for the
Vees,
thereby allowing the legs to be in the clear, moveable from side to
side,
and tied to smaller (8') trees at their distal ends. In one
variation on
the theme I had a 40 meter dipole as the center section of the
supporting
rope, tied to the same balun as an 80 meter vee. In another I tried a
linear-loaded 80-meter Vee, about 45' on a leg; it loaded fine but
didn't
perform as well as the full length version.
If you can suspend a flat antenna between two tall trees, why would
you want an inverted vee, which is a less effective radiator?
Your two trees 200 ft apart could support a full size 80/40 fan and a
20/15/10 fan, in line with each other. A high 80/40 fan is a VERY good
antenna, and is easy to build.
My technique has evolved to starting with #8 bare copper from the big
box store, stretch it VERY slowly between a tree and a trailer hitch
until it breaks. Do this carefully where there's no one around to get
hurt. Now you have #10 hard drawn copper, which is pretty strong, and
pre-stretched. Use that for the longest dipole in each fan. Use #12 or
#14 THHN (house wire) for the other elements. I make spacers by
cutting 1/2-in PVC conduit into lengths of about 16 in for 3-wire
fans, and about 12 inches for 2-wire fans. 5-6 ft between spacers is a
good rule of thumb. Hold the spacers in place by soldering short
lengths of copper around the spacer to the bare copper of the long
element.
The higher your antenna is, the more robust your center insulator
should be. A high 80/40 dipole (80 ft or more) will be closer to 75
ohms than 50 ohms. A 20/15/10 fan will be close to 50 ohms. Use RG8 or
RG11 depending on the Z at resonance. Don't waste a dB or two with
small coax. My 110 ft 80/40 fans are fed with Belden 8213.
For weights, I fill 6 gallon water jugs with dry sand, and tie one to
one end of each span. The other end can be fixed. I have pulleys high
my trees. If you don't have a pulley and weight, your antenna WILL end
up on the ground, and it won't take a big storm for that to happen.
My HF antennas are all at the 110-120 ft level in a dense redwood
forest that towers 50-75 ft above them. They work. My "seat of the
pants" observation is that attenuation increases with frequency, and
is greatest with vertical polarization. 432 MHz is a waste of time, 2M
sort of works, and 6M works pretty well.
For an analysis of the value of height, study this. It supports the
statement earlier in this thread that a high dipole beats a low
tri-bander.
http://k9yc.com/VertOrHorizontal-Slides.pdf
When Fred observes that the ends of antennas are "hotter," he means
that this is voltage maxima and a current minima, so good insulation
is needed to whatever the antenna is attached. I once melted heavy
dacron rope that was tied directly to the end of said dipole (well,
twice, actually). The extra ingredient was that it was wet. Duh.
73, Jim K9YC
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