[amsat-bb] Re: 70cm Yagi and AO-51

Simone T terrando at gmail.com
Tue Jan 1 09:51:42 PST 2008

Hi Andrew, All,

First of all, Happy New Year!

A circular polarization is made of two orthogonal linear polarizations. This
is why two crossed yagis (each one transmitting and receiving only linear
polarization) realizes a circular polarized antenna. RHCP ad LHCP results by
phase-shifting of + or - 90 degrees one of the two signals.
If these two signals have exactly the same amplitude, and the phase shift is
exactly 90 degrees we have an ideal circular polarized signal, this means
that the so called "polarization vector" rotates on a perfect circumference.
Of course this in reality is not possible, and our polarization many times
can be like an ellipsis (or a potato HI).

So speaking practically, if your antenna has the wrong circular polarization
(RHCP instead of LHCP) in theory you should not receive nothing! In reality,
as the signal polarization has not a perfect circular shape, the actual
attenuation can be as high as plenty of tenth of dB...

On the other side, if you use a linear polarized antenna to receive a
circular polarized signal, you will loose one half of the power of the
signal (no matters if right or left handed), as the circular polarization is
made in equal parts by two linear polarized signals. So this means -3dB (At
least, in theory...).

Last but not least, if you use a circular polarized antenna to listen to a
linear polarized signal, you won't see any difference than using a linear
polarized antenna. In this case you can have an advantage, as the signal
polarization can even rotate during its travel, so you'll not have
additional losses.

Hope this helps.

Happy 2008 to All.


Simone - IW1FYV

2008/1/1, Andrew Rich <vk4tec at people.net.au>:
> If I have this correct.
> 1. LHCP recieved on a RHCP can knock your signal down by some 20dB
> 2. LHCP matched with LHCP will result in a good signal.
> So how does a linear antenna equate when receiving a circular signal ?
> And the other way around, a circular receiving a linear signal ?
> ----------------------------------------------------------------------------
> Andrew Rich VK4TEC
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