315 MHz CP antenna?

[biff44]

Anyone have any suggestions for designing a dual polarization antenna for a receiver. I have a small key fob with a loop antenna, but the user can hold it in any orientation. Obviously, when he holds it so it is cross polarized with my monopole antenna on the receiver, she'sa no work! I was thinking if there were some sort of dual polarization antenna on the receiver front end, it would not really care what way the key fob was held (although there might be a 3dB hit in maximum received signal). I saw a TI patent (5,592,182) for a dual crossed loop design that was supposed to be CP. Anyone know anything about this, or another cool way to do it.

So in summary, I have a single polarizaton (loop) antenna in a key fob that I want to receive with a dual polarization antenna on the reciever so that it would be relatively insensitive to user operation.

 

[MacGyverS2000]

I'm wondering why you're having difficulty... car alarms (and just about every other car remote add-on these days) have no problem working from many yards away with siple key fobs. I can hold mine at just about every angle imaginable and get enough signal through to pop the alarm from 30' away.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[biff44]

Because I am looking for 800 foot range.

 

[VEBill]

Backing up a bit, are you getting your desired 800-foot range when the keyfob is held with the optimum orientation?

 

[biff44]

Yes, with a monopole at the receiver, I get 800 foot range when the keyfob is held for vertical polarization. I now want a more complex antenna at the receiver so I can hold the keyfob in other physical orientations and get ~similar performance.

 

[MacGyverS2000]

Out of curiosity, what kind of sensitivity are you currently getting with the monopole? What's your transmitter strength (still within FCC 15 rules?)?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

A Quadrafiler Helical Antenna (QHA) is nice, but a bit bulky for your application. Perhaps two monopoles at right angle phased accordingly.

Personally, I'd look at getting a better alarm (?) system. There are two-way systems advertised with 3000 foot ranges.

Or park closer.

 

[biff44]

What do you mean by "phased properly" I tried two monopoles at right angle, and the formed a really nice null at certain azimuths. They were combined in 0 degrees phase.

 

[VEBill]

To make CP out of two linear antennas, they need to be a right angles to each other AND phased at 90 degrees (either in the feed harness, or in the relative positioning along the 'direction of fire'). If you Google crossed dipoles and circular polarization I'm sure you'll find the info.

Note that you'll still have a -3dB penalty as compared to the optimally-aligned linear, when receiving a linear wave. So you'd need 1100+ foot peak range to start (neglecting feed harness losses).

Disclaimer: I don't think anyone has ever found happiness in a car alarm. Once you have the perfect car alarm, someone will probably steal it.

 

[biff44]

Thanks! I was searching for crossed loops when I should have been searching for crossed dipoles. It looks like a drooping crossed dipole is the thing for me, I just have to figure out how to fit it into the package.

 

[biff44]

BTW, any ideas on how to do the 90 degree quadarature hybrid? I do not see one on the common sites (minicircuits, etc). I did find a lumped element one here, but it looks like a lot of parts (expensive chip inductors):

http://www.microwaves101.com/encyclopedia/Branchline_couplers.cfm

And yes this is Part 15.

 

[VEBill]

The complete phasing harness can be made from nothing but cables. You need to take into account the velocity factor of the cable for the lambda/4 delay section. You can also use cables to match impedances.

Hint: 315 MHz isn't far off the UHF Military SatCom band and they also use CP. If you Google around, you can find designs for Crossed Yagi antennas designed to receive MilSatCom signals and some of those designs will include details for the phasing harness. However, your antenna's elements might have a different feedpoint Z than the Yagis.

 

[biff44]

The receiver, as you might guess, needs to be omnidirectionsl. So a Yagi is not in the cards.

 

[VEBill]

The hint was referring to a probable source of phasing harness design information. Just beware the impedance matching part.

 

[biff44]

Oh, I get it now!

 

[Higgler]

VE1BLL had the answer earlier, the QHA is simple to build and is omni and you don't need to make a hybrid to get phasing between two antennas. Look at Seavey antennas for dimensions and ideas and scale things.
If you can possibly do directional, helical is the way to go. We are doing a set of helicals with a switch to get omni, that would probably double your range.

kch

 

[VEBill]

Another advantage of the QHA is that no one in their right mind will mess with your car if it has some huge contraption of an antenna mounted on the roof. ;-)