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nuckolls.bob(at)aeroelect
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 Posted: Fri Jan 28, 2011 3:07 pm Post subject: Comm Antenna & SWR: More Information |
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At 03:36 PM 1/28/2011, you wrote:
| Quote: | --> AeroElectric-List message posted by: "Noel Loveys" <noelloveys(at)yahoo.ca>
b ??? The VOR antenna whiskers are horizontally polarized
dipole with major lobes fore and aft with nulls off to the
sides. Further, dipole antennas require no ground plane.
VOR is not a transmitting antenna. The VOR transmitters are on the ground
and are also horizontally polarized. The receive lobes of the antennae are
part insignificant as aircraft do not approach the VOR sideways. |
When you said:
"The important thing with com antennas is good omnidirectional ( all directions ) coverage. A horizontally polarized antenna in a tailcone, outside of having too small a ground plane, will have a dead area in front of the plane and another one behind it. I can’t see how this can be considered good for airplanes heading into or away from a control zone."
I didn't realize you were talking and an HF Comm trailing
wire. As soon as you mentioned horizontally polarized
antenna on the tailcone I thought you WERE talking about
a VOR antenna which IS commonly mounted on the tail and
IS horizontally polarized.
A further confusion point was your mention of a control
zone (always short range communications). I'm unaware of
any control zone services offered on any frequency where
an HF trailing wire would be the antenna of choice. The
most popular HF antenna for small airplanes produced at
Cessna was a wire that ran from the top of cabin to the
tip of the vertical fin and then out to one wing-tip.
Radiation pattern looked like a dying flower with badly
chewed petals. Here's a drawing from a kit I wrote for
a U17 HF installation way back when . . .
http://tinyurl.com/4eokmk8
| Quote: | b:No such "null" exists fore and aft.
Experience as well as radio theory tells me is sure does exist on the tip of
horizontally polarized signals. |
Agreed, we weren't talking about the same antenna . . .
| Quote: | Then why do large Aircraft often have two com antennae? One on top,
Sometimes in the vert stab and one below. The reason for the two antennae
is because of shading of the signal to stations directly below the plane
(upper antenna) or shading of a other aircraft more or less directly above
the plane (Belly position antenna) |
Because they have two radios and it's industry
practice to have one antenna for each radio and
to locate them as far as practical from each
other. "Shading" of a potential station right
over or below any given antenna is an exceedingly
rare event that lasts only seconds at the speeds
these airplanes fly.
Just got off the phone with a high-school buddy who's
been an ATP since about 1965. With 18K hours in
everything from DC-3's with dual ADF to Boeings
with cockpits full of glass, he tells me he's
never seen any difference in performance between
top and belly mounted antennas. He's a ham too
and speaks/understands radioeze . . .
| Quote: | | For some reason Bob I think we are off the same page again. |
Correct. HF antennas on light aircraft are
EXCEEDINGLY inefficient, e-field antennas
and unless 'bent', they'll have strong nulls
off the ends irrespective of polarization. I thought
we were talking about aviation antennas likely to
be installed on airplanes flown by members of this List.
Bob . . . [quote][b]
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kuffel(at)cyberport.net
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| Posted: Sat Jan 29, 2011 9:01 am Post subject: Comm Antenna & SWR: More Information |
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Noel said:
<< The new units are programmed to a specific aircraft.
The first problem I see with this is as soon as a signal is
received it can be identified as a single seat something or
the other and not all the resources that could be put in the
field will be used. This is selective response and I for
one am against it. >>
Different response is news to me. "Don't know how they do
it Outside" but up in Alaska my direct experience was the
effort was the same for one or multiple people down. Now
when two congressmen go down, including the House Majority
Leader, then you get extra resources (can you spell SR-71?).
Tom Kuffel
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