How to detect RHCP and LHCP

How to detect RHCP and LHCP waves

Hi,

How can I detect the polarization (right or left hand) of CP wave with a short dipole antenna (receiver) with a power detector (a diode detector)?
I have smt in my mind but not sure....

I know short dipole will have a linear polarization. so it will be x^ or y^ .... (vertical or horizontal pol.)

E_wave= Ex ± j Ey

and V= - ∫ E_wave • dl

So according to the output voltage - or + I can differentiate the incoming waves...

any suggestions?...

thanks

If you have only power detector, the best way to detect polarization is using circularly polarized antennas. Polarization is detected according to received power levels of the antennas. You may use lhcp-rhcp helical antennas, you can make them on your own.

At first I was going to say it could not be done with a simple dipole receiver. But on thinking, if you AM modulate the RF signal, and then transmit it with the cp antenna...you should be able to rotate the dipole antenna while detecting the envelope and watching it on an oscilloscope. You should be able to tell from the detected envelope as you rotate, if one orientation is leading or laging the other, and thus determine RH or LH polarization.

hi
if you set your antenna in 45 u able to achieve slant polarization which able to detect LHCP and RHCP with -3db gian

thanks guys a lot...

biff44 said:

At first I was going to say it could not be done with a simple dipole receiver. But on thinking, if you AM modulate the RF signal, and then transmit it with the cp antenna...you should be able to rotate the dipole antenna while detecting the envelope and watching it on an oscilloscope. You should be able to tell from the detected envelope as you rotate, if one orientation is leading or laging the other, and thus determine RH or LH polarization.

Click to expand...

Biff44, this helps, I think. AM modulation is required to compare phase difference btw signals (leading or lagging). But I could not get one point. Do I have to rotate this small dipole very fast to able to measure phase difference?

But also to distinguish them I have to send both RHCP and LHCP, first one then other one? What if I only receive one signal (RHCP or LHCP) and dont know which one is transmitting? I mean I dont have any control on transmitted wave, just receiving it. Can I figure out just turning my antenna CW or counter-CW?

thanks all...

Rotate very slowly. i.e. put the dipole antenna in a holder and set it horizontal. Then look at the scope trace (ch 1 is the analog modulating signa, ch 2 is the detected modulation from the dipole antenna) and see the phase shift. Then go back to the dipole and rotate is perhaps 30 degrees. Go back and look at the scope.

Not 100% sure if this will work, but the two components for RH or LC CP are delayed by +90 or -90 degrees (forget which is which), and the modulation envelope will get phase shifted just like the carriers.

Hi,

I don't think that the proposal of Biff44 works. The carrier as well as the sidebands of the signal in one linear polarization are at 90º out of phase (lead or lag) with respect to the components in the other orthogonal polarization. Them, the envelopes are the same (let's think in the phasors).

As an alternative to the use of CP antennas, I would use two identical dipoles in the same plane, crossed at 90 degrees, feeding a 90º hybrid with equal length lines. Only one of the output ports will have output, according to LH or RH polarization.
Alternatively, the two antennas can be radially shifted one quarter wavelength in space and their ouputs combined into a combiner (180 degrees hybrid, Wilkinson combiner, etc.).

Regards

Z

zihnisinir has only power detector! I think my suggestion is most reliable and practical method if he succeed to make antennas properly.
We cannot make use of oscilloscope directly in this application due to low power level of received signal. I think it is not reach even-60 dbm with a dipole antenna unless he is next to the btransmitter.

Other methods requiring phase comparison etc. dont work unless zihnisinir has a good coherent receiver.

zorro said:

As an alternative to the use of CP antennas, I would use two identical dipoles in the same plane, crossed at 90 degrees, feeding a 90º hybrid with equal length lines. Only one of the output ports will have output, according to LH or RH polarization.

Click to expand...

This is an antenna having both ports: one for LHCP ant the other for RHCP.

In any case, both Circular Polarizations must be measured (with antennas of not very differet gain) and their value compared. This is because the axial ratio of the transmission as well as the antennas will not be perfect. So you need: two similar CP antennas, or two dipoles with combination.
Regards

Z

well, just because you want to do something does not mean it can be done simply. You are going to need either a pair of receive antennas (one RHCP, the other LHCP), or some sort of dual pol antenna (vertical and horizontal) with two receive channels and some processing circuit.

A dipole is not going to be able to do such a complex task. I assumed you were doing a simple lab experiment.

Yes, that is not a simple task and we can use only two receiver antennas, circular antennas or a dual pol antenna, as you mentioned. I couldnt find a solution neither.

But one of my friends suggested a very funny idea. He told me to bend one part of the dipole such that it will become like a dual pol antenna (sort of). :grin:
It is cheating but an interesting and intelligent idea, I think...

Thanks to everyone....