Patch ant with both Left and Right Circular polarization?

[groundhog1]

Is it possible to do a patch antenna with both Left and Right hand circular polarization?

Say you had a square, probe fed circular patch antenna. You put the probe feed somewhere on the diagonal to match and create the circular polarization. What if you put another probe on the other diagonal?

Would you get isolation between the feeds or would they interfere with one another?

Thanks,
gh

 

[VEBill]

Depending on how the two CP antennas were phased, you'd probably end up with linear polarization. Just as circular polarization can be made using two crossed linear antennas (properly phased), the reverse is also true.

What you can do is use two crossed linear antennas with phasing networks to provide separate LHCP and RHCP outputs. I don't know if you could implement this all in one patch structure. Others might know - standby.

 

[groundhog1]

Thanks VE1BLL for the tip.
I mapped out the fields on a patch and as you suggested, and it looks like you get linear polarization (or total cancelation depending on how things are phased).
groundhog

 

[Higgler]

Yes, you can get dual circuilar from one square patch. Locate the feeds accurately for dual linear polarization(not on the diagonal) and purchase/design a good 90 degree hybrid, you should be able to get 25+ dB isolation from the two ports.
If you have room, use two different antennas, it's simpler. Space them apart by a wavelength and you'd get 20+ dB isolation.

kch

 

[groundhog1]

Higgler,

Are you saying that it can be done simultaneously or are you saying you can pick one or the other in a switching action?

I mapped out the currents on the patch and fields in space, and they seem to cancel in the simultaneous case.

But then I talked to a friend at work with a much bigger brain than mine, and he said that it still may be possible to do it.

So now I'm confused again.

groundhog

 

[nanren888]

Are you transmitting or receiving?
"Simultaneously" yes.
Combine after reception.
If you have a fed for horizontal linear & one for vertical linear, then circular is only a phase-shift away. So with two phase shifters, you have both "simultaneously".

Play it backwards for transmit I guess.

 

[Higgler]

Yes, you connect the two antenna ports to a 90 degree hybrid (which has four ports, antenna 1, antenna 2, Right hand output, left hand output), common stuff.

You can also make a 3 port 90 degree hybrid, just a simple 1x2 power divider and two cables to your two antenna ports, just make the two cables 90 degrees phase length difference at the frequency of operation. You only get one of the circular polarizations that way though. It's a bit simpler since 1x2 power dividers are more common than 90 degree hybrids.

Kevin

 

[jsoumagne]

i have a slightly different issue. I need an antenna that can receive both an LHCP and an RHCP signal coming from a geo satellite and come out from the antenna with one cable.I need to get a decent min gain (1 or 2 dBic over 25-55 degrees elevations). I was thinking to two solutions :
- two separate antennas (one for LHCP & one for RHCP) with their active LNA and a combiner post LNA: the issue with this is that noise power is doubled and my G/T is taking a 3 dB hit
- one linearly polarized antenna, but it is difficult to find a satisfactory concept: a colinear stacked dipole would do but at our frequency, it is too big (17 cm). a wire patch planar is a bit shy from a gain perspective.
I think that we need a conical beam to meet our specs.

Any idea is welcome

 

[VEBill]

Background info: 'Stacked' LNBFs for satellite TV are fairly common. Not quite as common as the more common switched voltage (13v/18v) polarity selection, but not uncommon. A 'Stacked' LNBF receives two polarities, either LHCP + RHCP for DBS, or vertical + horizontal for FSS. The two polarities are both down-converted at the same time to separate 500 MHz bands ('stacked' in the frequency domain), typically starting at 950MHz. They were invented so that many satellite TV receivers can easily share one satellite TV antenna (for apartments) and use only one cable and simple, cheap amplified splitters. The downside is that the distribution system needs to be 2.2 GHz compatible (instead of just 1.5 GHz). This is why most satellite TV RG-6 cable is marked "2.2GHz compatible".


If you're starting from scratch, you can use two crossed linear antennas and appropriately phased combiners to generate both LHCP and RHCP from one system.

All the above assumes that you want to keep to two signals separate. If you don't, then just use a linear antenna.

If you're in the Aerospace business, don't forget that your platform probably moves around quite a bit. Usually you'd need more than hemisphere coverage even for a large aircraft to account for pitch and roll. Smaller platforms might need spherical coverage (0 dBi). Apologies in advance if you know this already. Lots of system designers seem to forget that airplanes tend to manoeuver.

There are companies that sell active antenna combiners for UHF (300MHz) milsatcom (eg. QDC-100) if your application calls for it.

The optimum antenna technology depends heavily on frequency (that you haven't mentioned) and application. Crossed dipoles are common at the low end, steered Luneburg lenses can be used at the high end. Between are a thousand other choices.

 

[jsoumagne]

Thanks VE1BLL.

In fact our frequency is 1.5 GHz and we need to design a car antenna. We need ideally 2 dBic over 25-55 degrees elevation and a G/T of -19 or -19.5 dB/K clear sky (30 K sky temp),over 25-55 degrees elevation whatever the azimut, one cable output with the two polarized signals present. The two signals are not cofrequency (no frequency reuse).

Jerome

 

[VEBill]

(I guess we've hijacked a thread - oh well...)

Maybe a patch? There are a few people (not me) on here that are experts on the subject of patches.

My instincts tell me that there wouldn't be any advantage to detecting two CPs and then combining into one cable, versus simply using a single linear antenna (even with the obvious -3dB penalty of using a linear antenna for CP). Worthy of further input from others to confirm.

It shouldn't be difficult to achieve +2dBic gain if the effective beamwidth is confined over such a relatively narrow field (as compared to the 0dBi sphere).

Another option might be to aim for a wider elevation pattern, slightly lower gain, and use a remote ultra-LNA right at the antenna to compensate. The wider pattern might make the overall system more reliable than the extra dB of gain.

 

[Higgler]

My coworker has just completed a design for a 1.2-1.6 GHz dual right/left circular pole antenna with shaped antenna gain pattern. Pretty high tx power rating on it too. It's for GPS/Inmarsat/Iridium. It's a novel shaped single patch antenna. It's also grounded center conductor using a tube to enable any higher frequency patch to sit atop this lower frequency patch. Grounded center conductor helps minimize static buildup and protects your transceiver. Gain is +0.5 to +4.5 dBic from 25 to 55 degrees, no hybrid loss added yet. Peak gain is at Zenith and rolls off to the horizon smoothly. What's the ground plane size and shape? Sounds like we'd need to make a Zenith antenna pattern dip to achieve your higher gains at 25-55 degrees, probably feasible depending on your size requirements.

What's your thickness requirements, This is about 2" tall and 4" diameter with no added ground plane and dome shaped.

khiggins at toyon dot com for detailed questions