[Newbie] Circular polarization from two linear antennas ?

[Koen]

Hello,

       would you have any thoughts to share on the diy feasability of the scheme exhibited here :

            https://www.taoglas.com/datasheets/MAT.12A.pdf

       If I understood it somehow, coupling two linear polarized antennas with a 90-degree phase shift could yield an RHCP antenna for GNSS signals ?

       Is this something I should try ? Is it a waste of time ? I haven't found any other example on the web so I'm wondering about the usefulness of this technique.

       I don't have the volume for an rhcp patch antenna but could easily place two 100MHz-bandwidth linear antennas on the edges of the board (by which I'm trying to mean : not narrow-bandwidth ceramic chip antennas like the example above).

Thank you very much,
Koen

[1design]

Yes, that is one of the ways it can be done, fit an OMT and a 90 deg. shift to a feed horn and you have a circular polarization. The hybrid also works

[Koen]

Assuming I currently have a good-enough® GNSS receiver with a linearly polarized antenna on the PCB's north edge. Could I add the same antenna on the west edge, add this convenient little coupler (Anaren/Xinger C1517J5003AHF), match the RF path lengths and expect it to work somewhat correctly ? Or is it much trickier ?

Thank you !

[hagster]

Nothing wrong with this method of making circular polorisation. Its done a lot at UHF(search for turnstyle antenna for examples).

The benifit of having RHCP is that you get 3dB better performance than an LP antenna. I do note that the Taoglass antenna is only about 50% efficient so they have effectivly thrown that 3dB away in losses.

[Koen]

Thank you all for the answers so far. I will try it and thought about it all day.

Currently, I have an antenna on the North edge for GNSS rx. And another different antenna on the West edge for an ISM tx/rx around 868MHz.

Now, could I replace these by two identical dual-band ISM+GNSS linear antennas, link them to the coupler and use its RHCP path for GNSS and its LHCP path for ISM ? Would these indeed be orthogonal and naturally quite isolated from each other ?

The GNSS path contains a SAW already.
The objects are handheld and nothing guarantees their orientation so circular polarization on the ISM link could be better. Line-of-sight is guaranteed.

Thank you very much ! This is exciting.

[Koen]

Sorry to *up* this but I would definetely appreciate your opinion on the idea to use the RHCP path for GNSS and the LHCP path for ISM (TX included). Thank you !

[hagster]

The Hybrid Coupler used only covers 1150 to 1630 MHz, so it would probably not be circularly polarised in the ISM band. Hence you may not get much isolation between the 2 ports on the ISM band.

The SAW filter will give you about 40 to 50dB isolation for out of band signals. In band phase noise from you ISM Tx will just sail through.

[Koen]

Hello, thank you for your answer !

The coupler I'm looking at now would be Anaren C0727J5003AHF announced for 0.7-2.7GHz.

The current GNSS SAW is NJR NJG1159 :
    55dBc typ. @f=704 to 915MHz, relative to 1575MHz

I can move to NJR NJG1157 :
    85dBc typ. @f=704 to 915MHz, relative to 1575MHz

I can add a band pass filter on ISM TX.

Any input on how to do best are welcome. Thank you !
This is mostly for learning and the pleasure of seeing things improve.

[hagster]

It's certainly an 'adventurous' design. I have not seen it tried before.

Remember the figures quotes for the Hybrid and SAW filters are for ideal 50ohm inputs/outputs. The GNSS and ISM modules will only be 50 ohms in their respective bands. Unless you have absorptive filters the out of band impedance will certainly mean the impdence is not 50ohm. Also in the real world you never achieve perfect circular polarisation, so there will be some loss in isolation because of that.

Attached is a quick test using the free QUCS software. You can download 'touchstone' (4 port *.s4p files) from Aneran(and other manufacturers). This will let you stick 'real' components into the model. You should be able to add your SAW filters(2 port *.s2p files) and see what the isolation between your input ports is going to be.