915Mhz Circular Patch RFID antenna 1

[TonyPhilipsson]

What could I do to improve the performance on
the PCB antenna?

It's 7 and 1/4" across at the widest part.
What are the right measurements for this type of design?

The center screw is grounded to bottom plate
with 1/2" metal-spacer.

Narrow 910-920 MHz bandwidth is OK.
Uses 1watt amp to provide 30dbi


Now we are getting 5feet reading range from
tags that are pretty small.

Would like to get 6-7feet and also improve the reliability
to read all the tags fast and not miss any in a stack of up to 400units in under a second.

 

[VEBill]

There are one or more patch antenna experts hanging out on this forum, but I'm not one of them. Perhaps I can start the proceedings with a few questions that might be useful.

"dBi" normally means dB of antenna gain relative to an isotropic radiator. I've not seen it used as you've phrased it.

Have you optimized the antenna's gain pattern for the application? In other words, how are the tags distributed as compares to consideration of the overall antenna gain pattern?

If the tags are scattered in all possible elevations and all possible azimuths (anywhere on "the sphere") then you'll need a very low gain antenna that provides complete coverage. Or the operator would have to be willing to manually 'paint' the coverage area.

On the other hand, if the tags are all co-located in a small clump, then you could use a high gain, highly directional antenna, and this would provide maximum standoff range.

Once the antenna gain pattern is optimized for the coverage area then there's nothing left to improve with a passive antenna design. At that point you could use more power, or multiple antennas with automatic switching. There's also loss to double check. Your loss budget at 900 MHz should be a fraction of one dB.

The good news is that the difference from 5 foot range to 6 or 7 foot range is only about a couple of dB. Unless you've already perfected everything, you should be able to improve things by that much.

 

[Higgler]

What is the physical layout of the reader and the tags so we can get a picture of what your hardware setup looks like? Can you post or describe the layout?
1st thought, change from a patch to a horn antenna.

Best high tech solution for highest power density; if 7 feet or 84 " is your optimum max distance, and you have simply one reader antenna talking to RFid's. Optimum antenna aperture size should be (0.4 D^2/Lambda squareish), the 0.4 factor peaks the power density in the near field for antenna size = sqrt(84"*(11.803/0.915)/0.4)=50" square approximately. That's pretty large, but if you need to optimize power density in free space at 84", that's needed.

For a 50" square antenna array, that's 36 antennas spaced 10" apart in a grid pattern, 6x6 array, all in phase. I'd use 32 antennas since 1x8 power dividers are $138 at MiniCircuits and 1x16's are $295. 32 connectors = $250 (Pasternack 2 hole sma extended dielectric warehouse clearance), 34 cables <= $1000. Plus ground plane and sheet metal to make the patch antennas.

Feed antenna with reflector might be simpler, or a simple parallel plate setup is maybe an option to stop your energy from spreading out in 3 dimensions and only spread out in 2 dimensions.

 

[TonyPhilipsson]

The RFID reader is a M10 from Skyetek

http://www.skyetek.com/ProductsServices/EmbeddedRFIDReaders/SkyeModuleM10/tabid/533/Default.aspx

Placing it within a few inches of antenna,
so coax pigtail is very short.

The tags are these, and they do get very good readings compared to other tags these size.

http://www.frontech.fujitsu.com/en/services/rfid/a511a611/

On my top post, the attachment shows the
layout of the antenna we are using now.

 

[Higgler]

Module says 30 dBm max output power, so if it's 28 dBm, the tag wants a 6 dbi antenna, you'll need 8 dBi if the output power is a little weak. VSWR 1.5:1 or lower for best performance it says.
detection ranges on the tag;, min/max.
2 watt eirp gives 130 cm range, = 51 inches = 4 feet ish.
4 watt eirp gives 200 cm typical = 79 inches = 6.5 feet ish typical.

You want to get at the very top of their range. They suggest 6 dBi antenna, you need a 10 dBi antenna or higher. Higher the better.

for $54, buy this 10 dBi antenna from Arc Wireless ARC-PA0910B01 Arc 902-928MHZ 10dBi Flat Panel, NF
and see how it goes.

http://www.wlanmall.com/panel-antenna-p-1057.html

don't worry about the 1.7:1 VSWR spec. That's almost always at edge of band, besides 1.5 and 1.7 are not much different.
See if one works, if it's close and you need a little more range, use either 2 identical antennas(13 dBi) or 4 identical (16 dBi), they're only 13 inches square and a couple inches thick and combine them with a combiner from minicircuits for $90 or $140, with some cables and adapters. If you're not familiar with the term "phased array", you'll need some more help.
so about $54 to $200 to $500 in parts for 10,13,16 dBi worth of gain respectively. One of those should work.

 

[TonyPhilipsson]

Got the antenna today.

Biggest problem is that it's polarized.
The tags needs to be held in one direction to work.

As the tags are placed random in one big bin,
this would not work.

Getting two antennas maybe is an option,
But M10 does not support dual antenna natively.

 

[VEBill]

Circular polarization is one partial solution, but it comes with a -3dB loss.

 

[Higgler]

A few options;
buy another antenna, combine it or switch it, or buy a circularly polarized antenna.

To avoid the 3 dB loss mentioned above, having two antennas with a switch that sequences between the V and H oriented antennas will give you the best overall range.

Alternately, to get circular polarization, two antennas next to each other, combined with a $25 combiner

http://www.minicircuits.com/case_styles/GW1052.pdf

and equal length rf cables would make an imperfect but useful circular polarization.
Orienting the antennas atop each other or side by side will make some difference. Put them on a lazy susan and rotate them for optimum result. This combiner is a 4 port and needs a 50 ohm load on the "iso" port.

A more expensive polarizer can be fitted over the present antenna, I don't expect you can find one online, i didn't in a quick peak.
their are low cost circularly polarized antennas out there, here's one but it's gain is lower than the one you just purchased.

http://lairdtech.thomasnet.com/item/rfid-antennas/s9028pcl-circularly-polarity-antenna/s9028pcl?

 

[TonyPhilipsson]

We already used the Laird you mention, it's OK but not better than Wireless Edge.

This is the antenna we are using now.

http://store.microcom.us/mt262006trhak.html

And is the PCB layout mention in the first post.

Would your two antenna system with a comb/switch
be superior over the that one?

If we make our own antenna with this PCB layout from first post using Roger6002 be superior?

A slight increase size of PCB is a good way
of getting higher dbi?

 

[Higgler]

The one you're using is +9 dBic, which is really +6 dBi for your RFiD linear antennas.
Compare that +6 dBi to the new one +10 dBi and you do better by 4 dB when everything aligns well, so you should be seeing a bit more range to some of your RFiD's.
So the switch with two of them will help the misaligned ones communicate. If you can do a test with an rfid going thru in one direction, then turn it sideways and half way between forward and sideways, you'll see if that's enough performance.

There is another way to get more energy at your RFiD's though. Can you describe the physical layout. i.e. antenna on a post, RFiD's on a conveyor belt 7 feet away, height difference of x feet? Or is the flatplate antenna up in the air and the rfid's coming in on a conveyor belt underneath it.

One thing that works great is to simulate a metal box, put one antenna on one end, your rfid's on the other end. Change your metal box to chain link so the rfid's can push thru it (assuming they are on packages), or stop the chain link before getting to the boxes underneath. The chain link will contain the RF energy and your communication distance will double.

it essentially makes a transmission line from your antenna to your rfid so energy does not spread out in three dimensions, but stays contained in the chain link (chicken wire would be optimum, but not practical). You can also put sidewalls only on one side to limit energy spread to two dimensions. Two antennas (V and H pole) with a switch always helps when your antenna rfid is single polarization.
Plastic with aluminum reynolds wrap would be good too to guide the energy as a quick test. Other options apply too.

 

[Higgler]

slight increase in antenna size won't work. buying more antennas and combining them with cables is the best way to optimize it.
Your optimum size antenna is approx. 50 inches square for a flat antenna.

If you know the definition of an ellipse and apply it to this problem. You use one small antenna in the focal point, make an ellipse (but only a small part of the ellipse, and have your rfid's at the other focal point). You could have one small antenna, bend a large sheetmetal or chicken wire ellipse around it, have it maybe only an 8 foot piece of the ellipse, then set dimensions to focus on your area that rfid's pass thru.

If you do this say for a loading dock, you'd picture a nice arch on top of your door, with a simple antenna (circularly polarized basic patch hanging underneath it. Set dimensions of the arch and patch to get a hotspot where the rfid's pass thru.
if you get your hot spot too narrow and miss some rfid's on the edges, just bend your arch outward towards having a flat sheet instead of an arch to widen the hotspot on the rfid passage way. Bet that'd work. Even getting close to an ellipse would work, bending the ellipse in one direction only might be enough with an antenna that's long and thin radiating at your focal point.

 

[TonyPhilipsson]

Do this splitter work?

http://www.streakwave.com/Itemdesc.asp?ic=17-605&eq=&Tp=

As I gone order the second antenna from this website anyway.

These RFID systems gone go in hotel storage areas.
So not one big expensive system, but multiple smaller
less expensive is the goal.

We will evently make our own pcb antennas with built-in
rfid reader based on this chip.

http://www.austriamicrosystems.com/eng/Products/RF-Products/RFID/AS3992

 

[Higgler]

That splitter will work but has a catch to it. You need to make the two cables different lengths by 90 degrees phase difference (or 0.25* (11.803/(0.915 gHz *sqrt(2.1)))= 2.2 inches physical length for standard teflon type cable). that can be a bit of a pain making delta length cables. The GW1052 minicircuits hybrid combiner is the style you want, they might have N connector options. With that one, you use same length rf cables to connect the antennas. Slightly simpler.



The most space efficient antenna for circular polarization is a helical antenna, use on most satellites, including all the GPS satellites, they are 10-14 dBi gain circular polarization too. Looks like a cork screw atop a pie plate. 13" long is 12 dBic, diam. of corkscrew is Flat is prettier though in a nice hotel. Here's a link to a helical antenna if you want to peek at one

http://www.amazon.com/Shure-HA-8089-PWS-Helical-Antenna/dp/B0016IQEEG