[amsat-bb] Re: Polarity questions
aa2tx at comcast.net
Sun Sep 21 04:46:58 PDT 2008
At 02:29 AM 9/21/2008, Nate Duehr wrote:
>I do have a question for the polarity gurus, however...
>When *transmitting* circular vs. linear polarity, are the apparent
>losses the reverse of the numbers being used for receiving? I am
>inclined to think that has to be true.
>How much more power has to be put into a circularly-polarized antenna
>versus a linearly polarized one of the same gain, to have a distant
>station receive the same signal if they're linearly polarized?
This is a perfectly good question, Try this:
Let's say you have a horizontal RX antenna down range.
At the TX side, you have a horizontal and a vertical antenna
and they are quadrature phased so they make circular polarization.
Now your tx can put out 100 watts. The 100 watts gets split equally
between the horizontal and vertical antennas so you are putting 50
watts into the horizontal and 50 watts into the vertical.
The RX antenna down range doesn't know you are sending circular!
Since it is horizontal, it only responds to the horizontal signal.
It doesn't even see the vertical signal and it couldn't care less
about the phasing. It sees the 50 watts you put into the horizontal
antenna. The 50 watts you put into the vertical antenna is just wasted.
Now imagine you disconnect the vertical TX antenna and put all
100 watts into just the horizontal antenna. Now the RX antenna down
range sees the 100 watts you put into the horizontal antenna versus
the 50 watts you did before so you are up 3dB.
So, circular to linear, you wasted half your power and you lose 3dB.
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