Question about wire antenna design for 915HMz RFID EEPROM chip

[Triode]

So I'm doing a quick proof of concept demo for work that uses these RFID chips. I'd like to make more of the demo units but I am out of the sticker tag antennas I use and I can't get more in time.

I have the SL3S4021 model with two frontends. For my demo I want them to read from as far away as possible.

I know firmware and some electronic design but RF is a big field that I'm not an expert in, I'm mostly guessing and checking here other than basic RF knowledge. I can see this uses differential. Since it's 915MHz I've tried just a wire loop antenna, 32.8 cm, and a 1/4 lambda one of 8.2 cm. They work pretty well. They need to be inside a case so that's making things more difficult.

It's probably worth noting that this responds via backscatter modulation and both receive and transmit are needed.

Can anyone give me tips on how I should design this? It's going to be crammed into a small space, I have about 20 mm wide x 100 mm long space.

I'm not sure of the influence of:

• Thickness of wire (I have lots of magnet wire)
• full wave quarter wave, half wave?
• Loop or split? I understand the amp is differential. If I do split it, it needs to be very symmetrical right?
• If I do a loop what shape effects are there? If it doubles back over itself does that make it worse, or is it better than being shorter?
• I'm open to trying other types of antenna as well. I have copper sheet. I don't have time to make flex PCBs or anything.

Stuff I've found so far:

• A wire loop works pretty close to as well as the cellular 6 antennas I had before I ran out, those are split bowtie antennas 100mm long. I get 80% of the range with a oval shaped wire loop in open air. But when I smash it into the case I have to fit it in most shaper perform far worse
• Separating the antenna with a 4mm piece of foam just to keep it off metal and wires helps a lot, it gets about 60% of the in-case range that the stickers do.
• I haven't tried a dipole antenna

Full data sheet

https://www.mouser.com/datasheet/2/302/SL3S4011_4021-1127687.pdf

 

[rjenkinsgb]

A quarter wave or dipole antenna typically needs to be 5% shorter than calculated. around 78mm, due to capacitive effects.
If not in open air, it will ned to be shorter still as there will be greater capacitive loading.

A quarter wave cannot work in isolation, it must be used with a ground plane.
Or use two quarter waves on balanced signals or signal and ground to make a half wave dipole style setup.

This app note shows RFN grounded. Grounded rather than floating that would be suitable for the for the other half of the dipole, as a kind of counterpoise. The two halves need to be approximately in line with the feed points close together.

https://www.nxp.com/docs/en/application-note/AN11237.pdf

Presumably the two separate RF systems & antenna connection pairs are to allow for antennas in different planes, to reduce directional effects?
The device data sheet is near enough useless when it comes to antenna connections...

You could also use helical antennas in place of straight quarter wave antennas to reduce the length.

Interesting oddity - the wavelength of 915 MHz is 0.32768.... metres; that looks familiar; 2^15, anyone?

This gives full details of a commercial one, it looks like you could duplicate it with something like 0.5mm wire on a 5mm former? The number of turns, spacing and length to the connection point are important.

https://linxtechnologies.com/wp/wp-content/uploads/ant-915-vheth-ds.pdf

Helicals are less affected by proximity and are rather less directional than straight quarter wave antennas. They are lower gain but that's often compensated for because they are less critical in other ways.

 

[Nigel Goodwin]

A quarter wave cannot work in isolation, it must be used with a ground plane.

You mean like every walkie talkie style transceiver ever made, or all portable VHF radios

But it's partially true - while it does work, it's less efficient - years ago I used to stand my 2m Kenwood TR2200 on my car roof if I was near the car, this then acts as a ground plane and provides considerable improvement. But generally, it works 'fairly' well anyway.

Basically a quarter-wave whip IS a dipole, and gives the same results - the second half of the dipole is effectively created by the 'reflection' in the ground plane.

 

[rjenkinsgb]

You mean like every walkie talkie style transceiver ever made, or all portable VHF radios

The body of the radio acts as the ground / counterpoise, it's a very different situation to a tiny IC connected to a quarter wave many times its size!

I was simplifying based on the context of the question.

 

[rjenkinsgb]

Basically a quarter-wave whip IS a dipole, and gives the same results - the second half of the dipole is effectively created by the 'reflection' in the ground plane.

Yep, that's a description I used for teaching at the local radio club in the 80s - though the radiation angle is high with a ground plane rather than flat with a vertical dipole.

 

[Triode]

Thanks for the helpful comments and info both of you.

Doing some experimentation, with some of this info helping me make educated guesses I found that the best design I could make was a pair of 8 cm wires (just under 1/4 wave) on a short 50 ohm coaxial cable pointed straight away from each other, like a basic ham dipole but smaller. It may be better without the cable but there was no way I could place the antennas, since the chip they connect to couldn't fit just anywhere.

Then I had another variable to test. I could either put them straight, but about 1 cm of each end would be up against metal in the enclosure, or I could bend them into a W shape and fit them inside so only plastic covered them. Even out of the case the range on the W didn't seem to be any worse, and obviously inside the case it was much better than being up against metal.

Then I laminated it on to the wall of the case with glue and brushed it down, and backed it with some foam to get it as far from the wires inside as I could. This seems to work well. It actually out performs the manufactured sticker antennas I had of roughly the same size. With my handheld reader the hand made dipoles read from about 3 meters away, the stickers read to about 2.5m, and for a speed comparison at a distance of 2 meters for both the dipole read 980 times per minute, with the sticker getting about 640. The difference was even larger in the tool (1.9m vs 0.9m for max range, 250 vs 60 reads/min at 1 m for speed). I'm guessing because the sticker couldn't be reformatted and part of it always lands where it's on top of metal is why the difference was even larger.