Bart Bailey wrote:
In ps.com
posted on 14 Nov 2006 04:37:18 -0800, wrote:
Begin
A vertical HF antenna can be built with, dah, a vertical electrical 1/4
wave over
either very conductive, think salt water marsh, or an array of raidals.
The more the
better up to around 190 or so, Contrary to popular ham myth 16 radials
is barely
enough. 32 is better with 64 and moer starting to get there.
I have a Butternut HF2V with 0 (zero) radials, but a fairly good ground,
no salt marsh, just two stakes three feet apart driven into the ground
in an inner city neighborhood in San Diego, and I typically propagate on
80m better than the guy who gave it to me that uses the same model, but
with an ideal radial pattern. We run similar power levels so am thinking
I might be getting some parasitic excitation of very close power lines
that he doesn't have, nor the accompanying noise either. Those power
lines are a 138kv tie line, a 12kv distribution line and a 4kv
distribution line, all less than 100 feet from the antenna. Another
possibility is that since everything in the shack is very well bonded
and grounded, that at RF maybe I'm getting some counterpoise effect on
the entire city wide grid, now that's a long wire!
--
Bart
When it comes to radiation patterns it is hard to calculate what is
really gonig on ,
and even harder to make meaningful measurements. Ground reflection,
amoung
many other things, makes a lot difference. Whne I was much yougner, and
I
expected to be able to make meaningfull measurements, I wasted several
weekends
by trying to measure the real pattern from a 20M dipole at about 30'
elevation and
a 20M vertical on a large grounded metal roof. I nver got results that
made any sense.
Your observation about power lines is correct. "Nearby" wires can
produce
pattern distortion beyond the abiltiy to calculate. One nice thing
about 6M
and up is the ease with which you can make readings that knid of relate
to
reality.
My 1/4 has a 6dB lobe toward the front because I drive a hatch back
Civic
and ther is moer surface metal toward the front. Of course the lobe
isn't
exactly straight ahead, but canted about 15 degrees to the right for an
unknow reason. The NEC modeling program prediction and measurements
in an open field agree to a impressive degree. And by using the remote
S-meter
in a local repeater and having it repeat the level as I drive a tight
circle confirmed
the theory and my measurements. But I still can't explain the lobe's
offset. The
antenna is centered to within a few mm. I thoguht maybe wiper blades so
I removed
them, then I thought BCB radio antenna so I removed it. No change.
Sadly at HF reality and theory have almost nothing in common. The nice
peaks
and nulls of a 10MHz horizontal dipole are seldom as distinct as the
graphs
shown in text books.
If it works then it is good regradless of what theory might say. Some
of my
first DX HAM contacts were on really bad, not effeceint, random
antennas,
When I started using "correct" antennas, dipoles for the most part, my
ratio
of heard and worked versus heard and not worked increased dramaticaly.
But
I still have a wide range match box with my QRP rig just in case I
can't errect
a real antenna. I once worked Scotland runing 5W CW on 80M into a
random,
~~100', wire wraped along a rail fence.
Terry