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TELE-satellite International — The World‘s Largest Digital TV Trade Magazine

— 06-07-08/2012

— www.TELE-satellite.com

Polar Mount Antennas

Motorised Antennas for

Satellite Reception

Adolf Oberhuber

If you consult the relevant literature you will find that

no or – at most – insufficient background information

is available about how this type of motorised polar

mounted antennas actually works. The same is true for

the required formulas and their derivation. The following

illustrations are meant to provide answers to all questions

in association with a polar mount. This also includes,

inter alia, various size determinations for positions as well

as calculations for the satellite orbit based on a specific

example. Altogether this report is a collection of data and

calculations which, the author believes, have hitherto been

scattered across an endless number of publications and

accounts. For the sake of completeness and reference,

technical terms and their origin will be explained in a

separate addendum at the end.

In general, if a satellite antenna is to

be rotated with a motor you would need

two motors – one for the rotating move-

ment (azimuth) to the selected satel-

lite and the other one for adjusting the

dish to account for the differing heights

above the ground (elevation). Mounting

the rotation axis in parallel to the rota-

tions axis of the earth, i.e. in a pole-to-

pole direction, alleviates this problem so

that only one motor is required. How-

ever, this can only be achieved with a

more complicated and extremely precise

installation and alignment of the rotation

axis.

Dealing with the installation of satel-

lite receptions systems – both in theory

and practice – requires gaining an

understanding of sizes and proportions

relating to different sizes. As even the

relevant sizes and their derivations are

hard to come by, the following will estab-

lish all relevant data.

Before performing any calculations it

is paramount to recall the interdepend-

encies between the earth and satellites.

This may also provide a welcome oppor-

tunity to correct some common miscon-

ceptions.

The earth

Due to its rotation the diameter of the

earth at the equator is larger than the

distance between the north pole and the

south pole, which means the earth does

not have an exact spherical shape. How-

ever, the differences associated with this

fact can be neglected for our calcula-

tions, so that we accept the established

median radius of the earth of 6,731 km

as given.

Historically and depending on the

purpose, positions and directions are

Without exception, all calculations

and remarks are for the northern hemi-

sphere. If you live south of the equator

you need to interpret them accordingly,

such as antenna alignment to the north

instead of the south, inclination of the

rotation axis towards the south instead

of north and so on. For all calculations

you merely need a standard 10-digit

electronic calculator capable of per-

forming trigonometric operations. If you

intend to receive several satellites at

once you may choose one of the follow-

ing set-ups:

1. Several antennas:

Usually, such a set-up is only chosen for

commercial applications as it requires a

lot of space and also is more costly than