Is this RFID antenna design possible?

[40hrCaptain]

Hey everyone,

Looking for someone in the know regarding RFID reader antenna design. Part of a project I'm working on requires a custom RFID transmitter antenna. If possible, I'd like to design the antenna as an integral part of a steel cylinder. I know there are on-metal tags, but is it possible to design an on-metal reader antenna? Please see the very rough diagram I've attached - the red represents the antenna location. It would basically need to be mounted on the face of the steel cylinder with some sort of epoxy. Read range requirements are very low, less than 1cm. The antenna would likely need to be 10-12 inches long.

 

[Jeff Rosenbury]

I've done some microwave antenna design, but not RFID work.

The answer is, it depends.

Antenna design is both an art and a science. There are some requirements your diagram doesn't cover.

The big thing though is the lack of insulation between the steel and the conductor. The spacing of the (non-existent) gap is critical as is the material of the insulator. You will need to provide for that. The feed is also important and typically matches the transmission line's impedance (usually 50Ω).

Still, the basic shape looks fair. (Antennas that conform to curves are always a bit weak, but it should/might be good for 1 cm.)

I'm not sure how the steel will affect the signal. I've never worked with high permeability ground planes. (Does the rod need to be steel?) It might just lower the image current amplitude which would have little effect. Or it might significantly change the electrical size. (For higher power applications core saturation would be a problem, but not for your application I think.)

It seems a simple design. Perhaps fabricate one and hook it to a VNA? Tweak it until it works.

Often that's cheaper than spending hours running software (assuming you have a VNA).

 

[40hrCaptain]

I have a very tight profile constraint of a few mm from the surface of the cylinder - do you think it would be reasonable to design an insulated antenna that protrudes less than 3mm from the cylinder face?

Can you explain feed and why it typically matches impedance?

As far as shape, can I expect this design to create a magnetic field ~perpendicular to the cylinder face at all points within the antenna plane? Is there a limit to how far I can wrap the antenna around the cylinder? Say 180, 270, or even 359 degrees? (the idea being, it would allow me to read tags from all sides of the cylinder)

Thanks a ton Jeff

 

[Jeff Rosenbury]

I think it can be done.

Expect to spend some time and money.

Matching the transmission line impedance allows the transmission line to be something like a length of coax instead of a fixed geometry, which I doubt you want.

 

[tech99]

I think it can be done.

Expect to spend some time and money.

Matching the transmission line impedance allows the transmission line to be something like a length of coax instead of a fixed geometry, which I doubt you want.

Why don't you use a slot along the length of the cylinder, forming an Alford Slot?

 

[40hrCaptain]

From the looks of it, an Alfred slot would significantly degrade the structural integrity of the cylinder.

 

[Jeff Rosenbury]

From the looks of it, an Alfred slot would significantly degrade the structural integrity of the cylinder.

You might consider etching out a few more mils for separation from the ground plane.

Also consider an array of half/quarter wavelength dipoles rather than the dipole slot you have shown. That might give you better 360º coverage.

Another option might be a helical antenna running around the rod, though that would take some original design work with the iron rod in the middle.

Another option might be to use the rod as an antenna if you are lucky on the length. You might be able to wrap some grounded conductor tape around some of it to adjust its electrical length. By clever use of grounding tape, you could make all sorts of slots, or whatever.

There are lots of designs that might work. Finding one that does will take some time though.

Good luck.

 

[tech99]

You might consider etching out a few more mils for separation from the ground plane.

Also consider an array of half/quarter wavelength dipoles rather than the dipole slot you have shown. That might give you better 360º coverage.

Another option might be a helical antenna running around the rod, though that would take some original design work with the iron rod in the middle.

Another option might be to use the rod as an antenna if you are lucky on the length. You might be able to wrap some grounded conductor tape around some of it to adjust its electrical length. By clever use of grounding tape, you could make all sorts of slots, or whatever.

There are lots of designs that might work. Finding one that does will take some time though.

Good luck.

I think the original proposed solution will have a problem because the height is so small and the radiation resistance will be miniscule. This creates problems with efficiency, matching and bandwidth.