Newsgroups:
From: gary@ke4zv.atl.ga.us (Gary Coffman)
Subject: Re: SAREX/Mir Antenna
Reply-To: gary@ke4zv.atl.ga.us (Gary Coffman)
Organization: Destructive Testing Systems
Date: Thu, 11 May 1995 16:44:07 GMT
In article <3oqkga$cta@detroit.freenet.org> rburgan@detroit.freenet.org (Roland B. Burgan) writes:
>
>True, but much higer powers are need at 30 degrees or less,
>because of distance. And, you are attempting to compete with
>those hams for whom the satelitte is at a higher angle. At what
>point is it considered to be using excessive power? A pointable
>beam system should then be considered if working the sats at all
>over-horizon conditions is necessary. My reply was aimed at those
>just getting into the mode.
You need from 6 to 10 dB more power when the satellites are near
the horizon as opposed to when they are overhead. Unsurprisingly,
that's about the gain difference for a 1/4-wave vertical in the
horizontal and near vertical directions.
You're considered using excessive power when the signal you deliver
to the satellite is substantially stronger than that of other
users. What ERP you have to run to deliver that signal level to the
satellite is a function of your path loss of the moment. Using
a low gain antenna and a big amplifier, or a high gain antenna
and a little amplifier make no difference to the signal level
you put at the satellite. ERP is ERP however you develop it.
I advocate the vertical if you aren't going to use steerable
beams because its pattern most closely matches the changes in
ERP requirement for the LEO satellites from near horizon to overhead
of the various *simple* antennas. The turnstyle has the opposite
pattern characteristic to what is desired. While it will allow
slightly less power be used to generate the required ERP for
the short overhead segment, it requires so much more power for
the bulk of the pass that it's not a good choice. If a more
complex antenna is acceptable, a Lindenblad or eggbeater may
be a better choice. Both produce patterns that resemble a
flattened hemisphere, and don't have the null directly overhead
that the simple vertical has. (That null is very small in solid
angle, however.)
Gary
--
Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | we break it. | emory!kd4nc!ke4zv!gary
534 Shannon Way | Guaranteed! | gary@ke4zv.atl.ga.us
Lawrenceville, GA 30244 | |
Newsgroups:
From: gary@ke4zv.atl.ga.us (Gary Coffman)
Subject: Re: SAREX/Mir Antenna
Reply-To: gary@ke4zv.atl.ga.us (Gary Coffman)
Organization: Destructive Testing Systems
Date: Fri, 12 May 1995 01:31:54 GMT
In article <3orjqa$bgc@news.nyc.pipeline.com> dsantoro@pipeline.com (Dave Santoro) writes:
>So am I right in deducing from this conversation that ther is no ONE
>antenna that will provide reasonble performance over the entire pass?
No. The Lindenblad has a good horizon to horizon pattern. It's a bit
tricky to construct properly. I had the phasing backwards on my first
attempt. The eggbeater styles are also reputed to work well from horizon
to horizon, but I have no direct experience with them.
>I am now using a homemade J-Pole up on the roof, ant the signal
>practically disappears when the bird goes above 33 degrees.
The J-pole is, of course, a 1/2-wave vertical fed with a stub
match (the J). It has a much more pronounced overhead null than
a 1/4-wave vertical. That's how it gets its gain over a 1/4-wave,
by squishing the pattern down toward the horizon. So it's not too
surprsing that you have difficulty with the direct overhead portion
of the pass.
>What about using both with a coax switch?
That's another option. You can use a gain vertical for the portions
of the pass near the horizon, and a turnstyle for the overhead
portion. But remember you want a mast mounted preamp, so you need
remote switching. When you have to resort to that sort of manual
intervention, however, you're getting close to the complexity of
using a tracking antenna and might as well consider steerable beams.
Even modest beams, 3 to 5 elements, will offer considerable improvement
over any omni-directional antenna, and pointing accuracy constraints
are loose. The 3 dB beamwidth is on the order of 30 degrees for a
short beam, so you can move it in rather coarse steps.
I've used a simple 4 element beam, elevated with a rope. I fixed
the azimuth before the pass by rotating the mast by hand, and just
pulled the beam through the arc with the rope during the pass. It's
simple and cheap, if a bit Rube Goldberg. But even the most modest
TV rotator can handle such a beam if you prefer a less manual method
of rotation.
Gary
--
Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | we break it. | emory!kd4nc!ke4zv!gary
534 Shannon Way | Guaranteed! | gary@ke4zv.atl.ga.us
Lawrenceville, GA 30244 | |