On Fri, 25 Apr 2014 09:10:34 -0700 (PDT), Wassim
wrote:
1) We are told that vertical antennas over salt water are
highly effective.
Very effective. We have a local AM broadcast station with two
antennas (KSCO/KOMY) located in the middle of a brackish lagoon. With
10KW/5KW day/night power, they're stronger than most other stations at
the same range.
Why?
With a vertical, the ground acts as a counterpoise which makes up the
other half of a vertical dipole. You want this counterpoise to act as
an RF conductor. Dirt is an absorber. Salt water is a conductor.
Incidentally, modeling such an antenna is covered in the current issue
of QST, May, 2014 Pg 50 "Modeling a Ground Mounted Monopole Can be
Tricky".
What would an ideal antenna to take
advantage of this look like?
A better question would be how to avoid the grounding problem. A half
wave vertical is a good way. No counterpoise (ground) is required. I
don't quite agree with this page:
http://www.qsl.net/v73ns/vert.html
but it does give you the general effect of using something other than
a 1/4 wave monopole.
I'm not sure I can answer you question on an ideal antenna without
knowing the frequency of operation. The trick will be getting the
best ground, without going overboard and creating an un-necessarily
large grounding system.
2) We are told that a yagi should be mounted as high up as possible.
Is this really true?
True. Again, the answer depends on the frequency of operation. At
VHF frequencies, the effects of the ground is minimal. On the lower
HF frequencies, antenna height has a big effect on the antenna
impedance, pattern, and takeoff angle.
Why?
At VHF, because the earth is not flat and the RF horizon is further
away as you get higher. There are calculators available to give you
the approximate range at various altitudes (at both ends).
http://www.qsl.net/w4sat/horizon.htm
For HF, because the ground is a dandy RF absorber. Getting away from
the absorbent ground sends more RF in the desired direction.
What are the physical/electrical facts behind this phenomenon?
Too much to cover here. Plug your mythical antenna into a modeling
program and see what happens. That should be sufficient to illustrate
the effects of altitude and different grounding. Incidentally, I did
a rough model of a 1/2 wave 300 MHz vertical dipole at various heights
above ground. Note the changes in takeoff angle:
http://802.11junk.com/jeffl/antennas/vertical-dipole/slides/animated-v-dipole.html
3) When a yagi is mounted high above ground, it still performs
better if the ground is pretty conductive. Why?
Again, the effect is different at HF and VHF/UHF. If you're too close
to the ground or buildings, you're going to make a mess of the antenna
pattern, takeoff angle, and antenna impedance. You're also going to
have some of the RF produced absorbed by the ground. At VHF/UHF, it's
fairly easy to get a few wavelengths away from the ground to reduce
the effects. At HF, not so easy.
Another part of the puzzle is that a conductive ground reflects the
signal, while a not so conductive ground absorbs it. The reflected
signal may not be going exactly in the direction you want, making the
ground system part of the antenna, but at least it's not going into
heating the ground or building. With luck, it may be what the
receiving station is hearing.
Thanks for helping me out.
Sure, but next time, please include some numbers.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558