[amsat-bb] CP vs. LP and all that implies
dan at post.com
Thu Jan 22 10:29:13 UTC 2015
Best info on-line I found here:
Note the losses of having the wrong circular polarisation from a
polarised source are very high. In the real world the antenna may not
be perfectly polarised on one end, so difference may not be as great.
One very useful/recommended setup for circular is school contacts on 2
metres with the space station. It is recommended to be able to switch
polarisation in the middle of a school contact as sometimes the signal
is not direct but reflected off a solar panel. In this case you may
need to suddenly switch polarisation.
I have a wimo 2m 6 element X circular antenna with the switching kit.
The antenna is extremely light and designed to be mounted at the very
rear, to prevent the boom interfering in the signal. I was not that
impressed by the elements strength, guessing a bird might be able to
bend them by landing on them. The driven element is made from a few
parts screwed together. I worry corrosion would quickly make it
unreliable. Being limited to 6 elements and quite short a longer
linear model may work nearly as good.
The 70cm model I have heard is much better in that the driven element
is stamped rather than assembled so less joins to corrode. Also being
smaller the elements are stronger.
The switch box is designed to be mounted on the mast near the antenna
meaning only one coax cable per band needs to be ran out, but then you
need DC multicore to switch polarisations. You can select V, H, LHCP,
RHCP. One downside is that they do not give the option of a control
box, so you need to build one by hand. You need to ensure you do not
transmit while switching.
All in all it is quite expensive, complicated and messy. Unless you
had an inferior setup, and found it lacking I probably would not
bother. You only lose 3dB using linear when the source is true
circular, so probably not really worth it unless you have maxed out on
antenna gain/ receive pre-amps. There are additional losses with
switches connectors etc.
I have never seen/used the M2 setup so I cannot comment on this. They
definitely seem to have more elements as a possibility.
At least with the switching option you can compare the signal on
different polarisations to see if it does help in the real world. You
could always try it on one band, then apply it to both bands if you
get a big difference.
What do others think?
More information about the AMSAT-BB