patch antenna polarization

Antennas question.....I know if i have a transmitting antenna that is circularly polarized, and a receiving antenna that is vertical polarized...i will get low link loss no mater how i rotate that circularly polarized patch antenna. The link will have a 3 dB loss higher than normal, since i am only receiving one of the two components of the transmitted signal.

But does it need to be "circular polarized" to do this? I mean, what if i had a "dual polarized" transmit antenna. One that sends one transmit component vertically polarized, and one component horizontally polarized, but without the 90 degree phase shift? Can i rotate that dual polarized antenna around its center point, and get uniform link loss....or will there be certain rotation angles where something odd happens and the signal has a dip in level?

biff44 said:

I mean, what if i had a "dual polarized" transmit antenna. One that sends one transmit component vertically polarized, and one component horizontally polarized, but without the 90 degree phase shift?

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You win nothing, compared to circular, because you also have 3dB losses in the splitter for driving two polarizations. Each polarization is 3dB down, compared to driving a linear polarization antenna at the full power.

Not sure what your "dual polarized" setup does. If you split the TX signal without phase shift to two linear polarized orthogonal antennas, you get 45° polarisation. The same as using one rotated antenna.

Sorry, i was afraid i was not being clear.


Let me try to ask this in as simplified a way as i can.
Lets say i transmit a signal, and send it to a 3 dB equal phase splitter. The two outputs of the splitter are fed to two individual quarter wave stubs. The two stubs are rotated 90 degrees to each other...so that one is horizontal polarized, and one is vertical polarized.

If i then have a single quarter wave receiving stub....obviously if it is oriented horizontally, it picks up the transmitted power, minus the path loss, and minus 3 dB for the splitter loss.
If the receive antenna is oriented vertically, same deal....

But what if the receive antenna is oriented at a 45 degree angle to either of the two transmit antennas? will the received power be transmit power, minus path loss, minus 3db for splitter, minus another 3 db for not being oriented in line with either of the two transmit antennas, but PLUS the fact that i am recieving half of each polarization...which gives me back 3 dB?

I know a circular polarization transmit antenna will give me the path loss plus 3 dB loss for only receiving half of the transmitted power, but in the above diagram, is the path loss the same as a circular polarized transmit antenna? Or is there some sort of weight vector cancellation thing going on?

I guess what i am asking...should i go to the trouble of making an actual circular polarized antenna, or will a DUAL polarized transmit antenna work just as well?

biff44 said:

Sorry, i was afraid i was not being clear.

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You were clear, but perhaps didn't understand the answers.

biff44 said:

Let me try to ask this in as simplified a way as i can.
Lets say i transmit a signal, and send it to a 3 dB equal phase splitter. The two outputs of the splitter are fed to two individual quarter wave stubs. The two stubs are rotated 90 degrees to each other...so that one is horizontal polarized, and one is vertical polarized.

Click to expand...

Sure, but as FvM explained, this "half power vertical plus half power horizontal" is equivalent to a single antenna with linear polarization that is oriented at 45°.

biff44 said:

But what if the receive antenna is oriented at a 45 degree angle to either of the two transmit antennas? will the received power be transmit power, minus path loss, minus 3db for splitter, minus another 3 db for not being oriented in line with either of the two transmit antennas, but PLUS the fact that i am recieving half of each polarization...which gives me back 3 dB?

Click to expand...

You have a linear polarization again, just rotated by 45°. If the receive antenna is at the same 45° orientation, you receive full power. If the receive antenna is at 90° from that, your receive power is zero. Just like any other linear polarization: both components from the two antennas are in phase (timewise) and combine constructive or destructive.

That's what is different for the circular polarisation: the two components are ortogonal in space and phase (time). They never interfere, so with a linear receive antenna they don't add but also they don't cancel.

biff44 said:

I guess what i am asking...should i go to the trouble of making an actual circular polarized antenna, or will a DUAL polarized transmit antenna work just as well?

Click to expand...

The dual circular antenna would effectively be linear polarization again.