Nice report Scott!
Where you are on the mountain can create some astonishing results;
1) If you were somewhere on the side - below the peak - you probably had
significant low-angle gain away from the mountain. At the height above
the ground you had your antennas, the ground nearby acted like a
reflector in a beam. In this case the main lobe is not straight up, as
in a NVIS antenna on level earth, but since were on a slope it was
skewed down toward the horizon where it's better for working longer
distances. Of course the signal is much, much stronger *away* from the
mountain than through it ;-) Some dedicated mountain enthusiasts even
arrange inverted V dipoles held by a single support sticking out
sideways on a cliff face to enhance this effect, but it's very real even
on more reasonable slopes.
2) If you were on the very peak of the mountain, the mountain itself
acted like an antenna support. For the ground to act like normal level
ground, it needs to extend several wavelengths away from the antenna. At
our home QTHs on nearly level ground, that's a limitation. On a mountain
peak, that's a real advantage!
Objects many wavelengths away (like those other mountains in the
vicinity) aren't likely to cause any noticeable effect. That's where
Marconi and the experts of his time disagreed. Hertz had shown that a
simple piece of metal would stop electromagnetic waves. What people
(including Hertz) didn't realize at the time was that Hertz' waves were
in the VHF range, from what later engineers could determine. They are
stopped by a significant piece of metal. Marconi, after hearing his
spark signals over a small hill on his family's estate, was convinced
that Hertz was wrong. Marconi was right, since he was using very low
frequency signals in the range of a few hundred kilohertz.
I've seen the effect described in a way one can visualize as putting a
breakwater - a stone wall jutting above the waves - in a harbor
entrance. A small breakwater will stop the turbulent rapid (high
frequency) water movement, but longer, slower (lower frequency) waves
will wrap right around it and show up on the other side. The bigger
the breakwater, the longer wave (lower frequency) wave it takes to pass
around it: shorter (higher frequency) waves are more effectively
blocked.
Understanding that, and not realizing there were other propagation
mechanisms, such as refraction of radio waves by the ionosphere and even
the surface of the earth itself, is what led early engineers to believe
that lower frequencies - longer waves - were essential for long distance
communications. It was that misunderstanding that put us Hams on 200
meters and down. We were given the short waves in the mistaken belief
that they had no commercial value and no one, including us Hams, would
be able to get out of our back yards on such short wavelengths.
Of course, as soon as we Hams discovered how effective ionospheric
propagation was for long range short-wave communications, just as you
did working stations on the far side of those nearby mountains, they
took most of those useless short wave frequencies back :-)
Ron AC7AC
-Original Message-
Subject: [Elecraft] dipole antenna camping trip
Here is some antenna information that might supprise you!
I was planning a camping trip to the mountains and needed a dipole
antenna
for my K2/10. I would be camped in a heavly wooded area and space
between
trees would be small. So before leaving home I built a 20 meter dipole
because
it would be short and would be easier to get in between the trees. Then
I
decided
it would be nice to have 40 meters if I had the room, so I took two end
clamps
off of a battery jumper cable and clamped them to the end of the 20
meter
dipole, then added enough wire to the clamps to make it a 40 meter
dipole.
When I installed the antenna at 8000 feet alt I had enough room for the
40
meter addition. The center was 25 feet off the ground, one end was 20
feet
up
and the other 6 feet off the ground.
I found that with the 40 meter attachment with the battery cable clamps
the
antenna
was resonate on 40, 20, and 15 meters with an swr of .1 on all three
bands.
Even in the heavly forrested area I worked stations all over the US and
Canada.
Just to the north of me was a mountain range that went above the
timberline,
but I didn't seem to have any trouble getting over it,
and to the south was another that was about ten thousand feet high.
They didn't seem to effect my signals. I was running 10 watts the whole
time.
The bears even left me alone!
73
Scott N5SM
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