Satellite Antennas Part 4 by GM4IHJ
In part 3, natural Faraday rotation of the signal plane of polarisation as
it comes through the ionosphere was discussed, and a pair of antennas ,
one horizontally polarised and one vertically polarised was suggested as a
simple remedy if the antennas were connected to the receiver via a 2 in ,1
out switch. This allows the operator to manually select the best
polarisation match at any time. The problem is that satellite operating
often demands an operator with at least 4 sets of hands - to tune the
receiver, turn the antenna, operate the mike , keep the log and key the
tracking computer. So having automatic polarisation adjustment can reduce
the operator load.
Circular polarisation is the preferred method for doing this. The signal
can be transmitted automatically on an antenna shaped like a corkscrew to
give it left or right hand spin. This "HELICAL" antenna configuration
causes the signal to rotate its polarisation at its transmit frequency. Eg
a 145.980 MHz downlink rotates at 145,980,000 times per second , ensuring
that minor changes due to Faraday have no effect, provided you receive it
on a similarly shaped corkscrew with the same direction of spin. Opposite
spin, causes a large loss of signal strength, so it should be avoided.
The helical antenna seems therefore to be a perfect solution. But there
are problems. It is difficult = more expensive to manufacture. It is
awkward to mount , and most important, it is difficult to match to
ordinary feedline coax cable. You cannot simply connect it to coax. For
these reasons , almost no Antenna manufacturers sell inexpensive helical
antennas for amateur band frequencies. It has therefore become common
Amateur radio practice to achieve circular polarisation by using carefully
measured phasing lines to couple up two or more dipoles or yagis so that
the individual units fire signals which add in phase in a circulating
pattern. The simplest form of this type of antenna uses two yagis set on a
common shaft with their elements at 90 degrees to one another. Then
depending on the physical spacing of the two sets of elements along the
shaft, the antenna feed it split to feed one yagi direct whilst feeding
the other the correct phase separation to imitate rotation as the signal
emenating from the first yagi passes through the second yagi and is
supplemented by this second signal at the correct phase curvature.
Be careful measuring the element spacings and cutting the feeder lengths.
Some antenna manufacturers sell phasing harnesses with fitting
instructions but if you have not done this before, enlist the aid of an
amateur radio operator who has a succesful antenna up and running.
Users of fixed polarisation station antennas need not worry too much . If
a satellite downlink comes from a circularly polarised antenna on the
satellite, you do not lose all that much by receiving it on a fixed
polarisation antenna. So while circular polarisation may be desirable for
working Oscar 13, it is not essential. A simple antenna will still work
but a badly constructed or reverse polarised helical or double XY yagi ,
will be a disaster. That said , circular polarisation on small amateur
satellites is often far from perfect for reasons beyond the builders
control. More about that in part 5