When deciding is Circular worth it, look at your local terrain. Reflections are the opposite polarity. If there are large buildings or mountains that create reflections as the satellite travels there will be a large amount of QSB from the add/subtract of the signal and the reflections at the antenna. Circular to circular reduces the reflections by more than 20dB, and also reduces rotational fading from the satellite spin. If you are out on a prairie or 20 miles out to sea you most likely will not need circular polarization.
Art, KC6UQH
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Angus Sent: Friday, September 19, 2008 1:19 PM To: amsat-bb@amsat.org Subject: [amsat-bb] Re: Polarity questions
By having circular polarisation that matches the satellite (RHCP or LHCP but
the same polarity) you have very little QSB fading during the pass of the satellite. Using my own system I normally start a pass on Horizontal as the satellite is at AOS and then when it rises switch to the correct circular polarity and as the satellite gets near to LOS I am back on horizontal polarity with good quality signals through the entire pass. try listening to a NOAA weather satellite on 137mhz on a fixed polarity antenna i.e V or H and for some of the pass the signal is very good but as the signal changes polarity the signal drops down into the noise and through
the satellite pass you will get these peaks and troughs of signals. This is why if possible it is best to use circular or better still have the ability to switch polarity during the pass. regards Gus ----- Original Message ----- From: "pe0sat" pe0sat@vgnet.nl To: amsat-bb@amsat.org Sent: Friday, September 19, 2008 7:02 PM Subject: [amsat-bb] Re: Polarity questions
On Fri, September 19, 2008 16:58, Ryan Butler wrote:
Tim Tapio wrote:
Hi Tim,
How much signal reduction is there as the result of having the wrong circular polarization?
The difference between RHCP and LHCP is 20 dB
The difference between Horizontal and Vertical Linear is 20 dB
The difference between (RHCP or LHCP) and (Horizontal or Vertical) is 3 dB
If I understand right, you only get 3db extra when using CW/CCW but I have also read somewhere that you also lose 3db when running CW/CCW so why use Circulair polarisation and not just Horizontal or Vertical pol.
Ryan, NF0T
73's Jan / PE0SAT
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