Ok, I have a question for an antenna expert. I'm a UCF student
designing a UHF RFID system for a new kind of robot positioning system,
and I have two choices to make:

1. Circular vs. Linear polarized antenna for the RFID reader
2. Halfwave dipole vs. Fullwave loop antenna for the tags

The reader antenna transmits a signal that electrically couples the tag
antenna, inducing a current that the tag uses to transmit its ID number
back to the reader. It's a long range application, so it's important
that I maximize the power transfer to the tag. Here's my current
understanding of the matter, let me know if I'm wrong:

A L-pol reader antenna with a dipole tag antenna will get great power
transfer, but there will be lots of null zones if the tag is oriented
improperly.

A C-pol reader antenna with a dipole tag antenna will get great
orientation indifference, but power transfer to the tag will suffer.

A L-pol reader antenna with a loop tag antenna will also get great
orientation indifference, but sub-optimal power transfer.

A C-pol reader antenna with a loop tag antenna will get both great
orientation indifference and maximum power transfer.

So I believe I want to do the last one, but I wanted to get some input
from people with antenna experience, in case I have no idea what I'm
doing, which is entirely plausible.
Thanks!
Nathaniel

On 17 Jun 2006 11:33:49 -0700, wrote:

Ok, I have a question for an antenna expert. I'm a UCF student
designing a UHF RFID system for a new kind of robot positioning system,
and I have two choices to make:

1. Circular vs. Linear polarized antenna for the RFID reader
2. Halfwave dipole vs. Fullwave loop antenna for the tags

Hi Nathaniel,

Both Loops and Dipoles are dipoles. This is not about geometry so
much as it is about phase driving two separate, but close antennas.

I doubt it any RFID tags are polarized, but I could be wrong. If so,
then you will need a more complex antenna for the reader. Either way,
you should check with the vendor of the tags. They have every
motivation to make sure you know antenna requirements.

73's
Richard Clark, KB7QHC

I read this paper myself:
http://www.ti.com/rfid/docs/manuals/...naCookbook.pdf

--
Christen Fihl
http://oz1aab.Fihl.net/

Thanks for your help Richard.
I think you're right about the tag antennas not being polarized, but I
know the big antenna mounted on the reader can be. These are the one's
I'm considering:
http://rfidusa.com/superstore/index.... 5d20420be597e


Richard Clark wrote:
On 17 Jun 2006 11:33:49 -0700, wrote:

Ok, I have a question for an antenna expert. I'm a UCF student
designing a UHF RFID system for a new kind of robot positioning system,
and I have two choices to make:

1. Circular vs. Linear polarized antenna for the RFID reader
2. Halfwave dipole vs. Fullwave loop antenna for the tags

Hi Nathaniel,

Both Loops and Dipoles are dipoles. This is not about geometry so
much as it is about phase driving two separate, but close antennas.

I doubt it any RFID tags are polarized, but I could be wrong. If so,
then you will need a more complex antenna for the reader. Either way,
you should check with the vendor of the tags. They have every
motivation to make sure you know antenna requirements.

73's
Richard Clark, KB7QHC

On 17 Jun 2006 21:43:40 -0700, wrote:

Thanks for your help Richard.
I think you're right about the tag antennas not being polarized, but I
know the big antenna mounted on the reader can be. These are the one's
I'm considering:
http://rfidusa.com/superstore/index.... 5d20420be597e

Hi Nathaniel,

Just build two dipole antennas and try one first. One may do the
trick. The circular polarized (or more likely cross-polarized)
antennas are mostly to anticipate a random orientation of the tag. If
you can put all the tags in the same orientation, you only need one
(the favored) polarization. Figuring out which is relatively trivial.
If you need circular/cross polarized, it isn't much more getting there
than cabling.

73's
Richard Clark, KB7QHC