# [amsat-bb] Re: Elk Antenna ?

Robert Bruninga bruninga at usna.edu
Thu Jun 4 05:58:10 PDT 2009

```> I want... a dual band 144/440 antenna on my 85' tower.
> Can I get away with a fixed elevation of about 30
> degrees and horizontal polarization?

Three facts say that 30 degrees is the wrong angle.

1. The amount of time that LEO satellites are above 30 degrees
is less than 20% of all view times.

2. You don’t need gain when satellites are above 30 degrees
because they are much closer and 6 to 10 dB STRONGER.

3) You DO need gain at the horizon where they are 6 to 10 dB
WEAKER.

So putting the beam at 30 degrees is counterproductive to LEO
satellite reception, you will be giving up 3 dB where you need
it most!.

The answer is to elevate it to 15 degrees.

1) You still have all the gain on the horizon where you need it
2) You don’t loose any gain from 0 to 30 degrees where you need
it 80% of the time
3) you lose a dB or so above 40 deg where you DON’T need it at
all
Etc.

I don’t know where the 30 degree angle came from, but it is
pervasive through the literature, but is just plain wrong.  The
only time to use 30 degrees is if your antenna is so low that it
cnanot see anything below 15 degrees anyway, and you are already
missing out on 70% of all satellite view times.  So if 15 deg
and below is ocmpletely blocked, then OK, set it to 30, but it
wouild be better to raise the antenna, and see the horizon where
satellites spend most of their time.

See the scale drawing on www.aprs.org/rotator1.html

Oh, and it is better to be vertical because some satellites are
magnetically stabilized and in the northern hemisphere, they
will be more vertical than horizontal. See the plots on

http://www.aprs.org/attitudes.html

> I recognize that this is a compromise, but I'd like to avoid
> the additional complexity of dealing with an azimuth rotor.

Elevation rotator is absolutely not required for any LEO
satellite.  And since we do not have any working MEO or HEO or
GEO satellites, then it is the simple way to go.

Hope that helps.
Bob, WB4APR

>
> Any thoughts appreciated.
>
> 73 de harry, W6DXO
>
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