Jessie:
I expect that the only place you'll find that two antennas give you a
consistent advantage is when you try to work long haul DX on 80 and
160 (and maybe 40). The vertical gives you the low angle transmission
assuring that the other fellow will hear you, but most of the noise
is low angle, and you might not be able to hear him over the noise.
In that case a separate receive antenna (such as small loops and
travelling wave antennas) that has low efficiency but favors the
desired signal over noise will be an advantage on receive.
You are correct that no particular polarization of your antenna is an
advantage for receiving skywave signals. The actual polarization will
be a random sum of several different elliptically polarized
components. Those will be different for every signal and will evolve
over time for the same signal.
I have two straight 20 m dipoles, one horizontal and about 45 feet
off the ground, and the other vertical with the feedpoint 20 feet off
the ground. Both have served as excellent QRP DX antennas. Neither
consistently outperforms the other on DX; the better performance on
any given contact depends on propagation conditions. I've confirmed
this with both on the air tests and extensive VOACAP computations.
Clearly because of its high angle component, the horizontal is the
more consistent performer in domestic contests like the NA QSO Party.
Those deep nulls that you see in the theoretical radiation patterns
of dipoles reasonably high off the ground seldom actually occur. The
non-uniformity of the ground electrical properties and ground
elevation mean that the idealized conditions set up the wave
cancellations that produce the nulls do not actually occur. A
practical dipole at a realizable elevation over the ground will be
only slightly superior on its broadside compared to the endfire
direction.The theoretical null of my horizontal points right at
Europe, but I still make lots of European QRP contacts with the antenna.
If you really want an antenna that discriminates against signals from
undesired directions, you need you need a design like a Yagi, a
phased array, or a small loop.
If your interest is in reducing cross station interference between
colocated stations, as on a field day site, you should do some EZNEC
studies using the near field. (Cebik has some material on this.) The
near field patterns that cause interference with nearby equipment
look nothing like the far field patterns.
Also if you're looking for punch on field day, you should do some
VOACAP/VOAAREA angle of arrival calculations customized to your field
day QTH. A great many field day contacts are high angle.
73,
Steve
AA4AK
At 06:51 PM 1/16/2006, Jessie Oberreuter wrote:
On Thu, 12 Jan 2006, Stephen W. Kercel wrote:
[...] don't be too enchanted by the low angle pattern, it is also a
bit lossy. If you overlay your EZNEC vertical dipole elevation
pattern with the EZNEC broadside elevation pattern of a straight
dipole at say 5/8 wavelength elevation, you'll see that although
the vertical concentrates its energy at low angles, the actual dBi
values are not that much better than the horizontal.
Indeed, a low dipole generally has much better low angle
output than a vertical, but is a lousy dx antenna b/c it isn't a
selective listener. This has lead me to consider using a vertical
for listening, and a low dipole for trasmitting.
I've been lead to believe that polarization has little effect
on HF reception and propagation outside of the ground wave radius,
so next Field Day, I'm hoping to see if we can't use the above
arrangement to kill two birds with one stone when working dx. By
making t/r antenna switch boxes for all of the rigs, it should be
possible for us to reduce cross-station interference, and have more
low angle punch, by having both stations on a given band tx on
dipoles and rx on verticals.
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