Patch ant with both Left and Right Circular polarization?
Patch ant with both Left and Right Circular polarization?
(OP)
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





RE: Patch ant with both Left and Right Circular polarization?
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.
RE: Patch ant with both Left and Right Circular polarization?
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
RE: Patch ant with both Left and Right Circular polarization?
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
RE: Patch ant with both Left and Right Circular polarization?
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
RE: Patch ant with both Left and Right Circular polarization?
"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.
RE: Patch ant with both Left and Right Circular polarization?
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
RE: Patch ant with both Left and Right Circular polarization?
- 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
RE: Patch ant with both Left and Right Circular polarization?
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.
RE: Patch ant with both Left and Right Circular polarization?
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
RE: Patch ant with both Left and Right Circular polarization?
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.
RE: Patch ant with both Left and Right Circular polarization?
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