[amsat-bb] Re: RH and LH CP simutaniously

[Edward Cole]

Joe,

Makes a good point about this as appropriate as a receive-only system.

To answer Rod:  Pin diode antenna switching has 
been done by radio astronomers for years in what 
is called a Dicke Receiver.  They switch at audio 
rates between the antenna and a noise reference 
source to track minute gain variations in the receiver.

The basic problem with your approach is that half 
of the time you will be in the wrong 
polarization, leading to a 3-dB overall reduction 
in sensitivity.  The approach I outlined with 
dual receivers eliminates this.  But functionally 
a pin diode can be used for switching and 
controlled by a square wave at any desired rate 
up to the limits of the pin diodes response.

For a transmission system that will adjust to 
polarization changes.  The simplest is merely a 
coax switch to chose the best polarity at the 
time.  A more advanced system would use 
programmable phase shifters to accomplish exactly 
what polarization is desired (if you have a gov't budget, that is).

My bottom-line question to all these schemes is 
how you are going to determine what the best 
polarization to chose?  This is not a simple 
exercise in orbital mechanics and 
solid-geometry.  The Ionosphere adds Faraday 
rotation at random.  If you can predict Faraday 
then you probably will make a fortune in weather predictions, too! ;-)

73, Ed - KL7UW

At 11:49 AM 1/4/2009, RFI-EMI-GUY wrote:
>In theory, it would sort of work for receive. 
>But don't try to TX through the array as it 
>would create interference to other users on 
>adjacent frequencies on TX (NOT GOOD).
>
>Now as far as the RX application;
>If you think about it those adjacent frequency 
>users would also mix with the switching 
>frequency and become interferers to your desired 
>RX frequency. Interesting idea if it could be made to work.
>
>However if you have a second receiver, no reason 
>you could not have a diversity RX setup to select best polarization.
>
>     Hi all,
>     Has anyone ever considered the possibilty of using PIN diodes and a free
>     running square wave oscillator to electronically switch between RHCP
>     and LHCP?
>     The switching frequency would have to be high enough so as to not
>     introduce
>     a tone to the signal.
>
>     Just a thought I have had for awhile.
>
>     RoD
>     KD0XX
>
>--
>Joe Leikhim K4SAT
>"The RFI-EMI-GUY"©
>
>"Use only Genuine Interocitor Parts" Tom Servo  ;-P
>
>
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