Axial ratio

[gkuma]

Hi,
Does anyone know where I can find a dipole antenna in the range of 400MHz?

I plan on using this as a reference antenna to measure the axial ratio of a CP antenna. Can anyone help me on a good way to do this? Also if a different reference antenna I should be using.

Thanks!

 

[VEBill]

If you need a calibrated antenna, then look to the companies selling EMI measuring equipment. But you'll pay through the nose. The good news is that 400MHz is usually in the range of such antennas.

Of course, if it's a simple dipole, you could make one. Cross-polarization rejection can be a function of details such as the wiring of the feedpoint. And to calibrate it yourself (if required), look up the three antenna method. Off hand, I don't know how much calibration you'd need to measure cross-polarization. Probably not much.

Stand by for others' input.

 

[gkuma]

I was thinking of using something like this:

http://www.tessco.com/products/displayProductInfo.do?sku=85825&eventPage=1

Would that work? I searched EMI stuff and they are around $300.

I am measuring the axial ratio for a circularly polarized antenna. Is it the same way for a cross polarized antenna?

 

[VEBill]

$300 is dirt cheap for a calibrated EMI antenna. The kits I've seen cost about the same as a good used car.

I wouldn't recommend that particular one (your Tessco link). First of all, it's a monopole (not a dipole). Second, it requires a ground plane. So it's not a good choice for probing polarization.

I've been thinking about the required accuracy of your test antenna. Let's say you build Qty 2 dipoles using, for example, coat hanger. Let's assume you build them carefully and do neat work with the feed point. You should be able to achieve about a -20dB null when cross-polarized. -20dB means they're only picking up 1% of the power when cross polarized.

Now your CP Antenna Under Test: If it's any good, it should have an axial ratio something close to 1:1 (or 0dB). Maybe it's a poor CP antenna and it has an axial ratio of 3dB. This 3dB delta should be easy to measure to a reasonable degree of accuracy using a home brew test antenna.

By the way, you'll need to be very careful about where you perform any of these tests. In a lab and you may see enough reflections to show up as you move things around. If so, go outside and aim the source up. Or hire an EMI chamber, but outdoors is free.

By the way, what's the application?

 

[gkuma]

Ya I found the emco 3121 kit thats used for $1500 but I wont ever go lower than 400MHz. I will actually be doing the test outdoors first and then in an anechoic chamber. The application is for an antenna communicating with a sattelite and gps system.

It needs to be completely accurate. I do have a conical log spiral antenna which I may try to test with. The antenna should be very close to a 1:1 ratio as per simulation.

I have done the complete test indoors and then outdoors with a different reference antenna and realized how much a room and reflections disrupt my measurements.

Can I use any linear sense antenna to test with? More specifically what kinds besides a LPDA.

Thanks for the help

 

[VEBill]

"completely accurate"

No such thing. You had better determine what the actual accuracy requirement is, and make sure your measurement technique is at least six, and preferably ten, times more accurate.

"should be very close to a 1:1 ratio"

It's easy to prove it's close. And increasingly difficult to prove exactly how close.

Easiest of all to prove ONLY that it's better than it needs to be (PASS/FAIL against the specified requirement).

I doubt that the satellite system is overly concerned about the last fraction of a dB.

 

[gkuma]

Well I guess I am just doing a relative measurement between the antenna I am replacing and mine. Currently I need 6dB and my antenna simulates very close.

So can I use a home made dipole and achieve sufficient data to determine where my antenna is?

 

[VEBill]

Build two dipoles and use them to determine the cross-axis rejection between them. 20dB or better should be possible provided you don't have problems (e.g. reflections).

Then, it shouldn't be difficult to prove the ratio for the Antenna Under Test is (much) better than 6dB.

By the way, there is an 'antenna and propagation' forum on this website in case you have time to double-check. We don't encourage double-posting, so point to this thread if you choose to start another.