[amsat-bb] Antenna Setup

[Daniel Cussen]

>ll your comments / feedback here, just one additional
> question from me,
> In real world / every day use - what difference does using X-Pol (Cross
> Polarized) antennas versus
> a straight beam mounted vertically ? - How much of a difference does it
> make to the received
> signals ?

This is the best site I have seen with good explanations:

http://sv1bsx.50webs.com/antenna-pol/polarization.html

Many/most satellites are not true polarised sources, rather just omni
antennas on one side. Often they are not controlled so they spin and
twist. As a result they can appear to change from vertical to
horizontal to LHCP to RHCP.

If you have circular, and the satellite is linear, the worst case
(according to the chart) is 3dB. while if you are vertical at the
satellite appears horizontal the worst case it 30dB. So in normal use,
you can change from 30dB fading to 3dB fading.

Some polarisation switches have the option to remotely change from
RHCP to LHCP to vertical and horizontal. Thus you can compare them all
such as wimos unit:
http://www.wimo.com/download/1808xx.pdf

Sometimes even true circular polarised sources, can reflect off
something, (e.g. the solar panels on the ISS) and suddenly a RHCP can
become LHCP.

So I think the answer is if you see sudden fading in the middle of a
pass, it may be due to difference of polarisation, but sometimes it is
dues to the antenna pattern of the transmit antenna being away from
the Earth side of the satellite. The only way to know for sure is to
switch polarisations quickly and compare.

I would consider it if buying a new beam and if you have elevation
control. Circular is also useful for ground contacts as DX stations
tend to use Horizontal Beams, and mobile and base tend to be vertical
omni. With RHCP you only have a 3dB loss to either user at the other
end.