RFID - Page 1
Forum section: electronics / projects

author: jbushnel
author_rank: Contributor
timestamp: January 15, 2015, 01:17:40 pm
content: I was going to buy an RFID antenna but I am a little confused how they work. They only have one BNC plug so the antenna gets the power AND transmits data through this?My application is a little different. I don't need to gather any data from an RFID tag, I just need to be able to produce a logic high once the RFID tag crosses the antenna.I guess I just want to know this... how do I provide power to the antenna so that, once the tag crosses, there will be the signal present? I think I will just use a schmitt trigger and a latch to get my logic high so I think I have that part down(this is just an example of the antenna)http://www.motorolasolutions.com/US-EN/Business+Product+and+Services/RFID/RFID+Reader+Antennas/AN710_US-EN

author: digsys
author_rank: Unknown
timestamp: January 15, 2015, 02:03:29 pm
content: Very few RFID antennas are powered. Usually they pulse LF 143KHz (MF, UHF) up to 10 per sec, which powers up the ID tag, which thentransmits the code over the top of that waveform. I've used that antenna type before, the connector is just for convenience.

author: jbushnel
author_rank: Contributor
timestamp: January 15, 2015, 02:16:50 pm
content: I was looking at a UHF antenna. It sounds like I will need to just buy a reader because building the circuit seems too tough.Is there anything cost effective that will read the antenna? I don't need all of the bells and whistles, I just wanted to something to simply read a UHF antenna and give me the output.

author: Marco
author_rank: Super Contributor
timestamp: January 15, 2015, 02:23:55 pm
content: Quote from: jbushnel on January 15, 2015, 01:17:40 pmAt what range?

author: jbushnel
author_rank: Contributor
timestamp: January 15, 2015, 02:30:47 pm
content: Quote from: Marco on January 15, 2015, 02:23:55 pmWell I was going to have the tag 5-10 feet from the antenna and use the UHF range.

author: Mr Simpleton
author_rank: Unknown
timestamp: January 15, 2015, 02:35:10 pm
content: UHF RFID is pretty impressive!Tags (is this what you mean by antenna?) are usually passive and receive their power from carrier emitted from the reader, and tags needs roughly -15 dBm to start to operate.This is usually the limiting factor when looking at what ranges you can detect a tag.The system uses back scatter principle, i.e. reader emits carrier, and the tag modulates the signal by modifying the antenna refection constant. And the response, usually AM modulated can be received."Cheap" readers often uses one antenna for simultaneous transmit and receive, and the received signal is only off from carrier by the modulated frequency, often 80 kHz. This makes the reader pretty complex... but luckily they are now heavily integrated and the RFIC designer has done the hard work for you.All this means that the reader is not cheap, usually costs $150-200 on e-bay from China. Look for Cottonwood, which is a blue print rip-off from Austriamicrowave demo-kit. Others like Impinj have more integrated readers, but cost 10x more. As for hooking up an antennas, it seems they do use reverse polarity TNC... hard to find, and forces you to buy their antenna.The frequency is 860-960MHz and dependant on where you are located, US uses 900 and EU 868 MHz. The maximum radiated power is 4 W ERP in US and 2 W in EU... and specifying ERP means you cannot improve the range by using hi-gain antennas.The protocol is complex and would be dauting to build your own reader from scratch...These documents are a good starter.. Never mind the Mickey Mouse, they *are* goodhttps://wireless.vt.edu/symposium/2009/2009Tutorials/Fundamentals%20of%20Backscatter_Part%201_Griffin.pdfhttps://wireless.vt.edu/symposium/2009/2009Tutorials/Fundamentals%20of%20Backscatter_Part%202_Griffin.pdf

author: jbushnel
author_rank: Contributor
timestamp: January 15, 2015, 02:40:10 pm
content: Quote from: Mr Simpleton on January 15, 2015, 02:35:10 pmThanks for all of the information! I will definitely look into it more!So I have decided to buy the Impinj threshold antenna (I can manage that price) But what else will I need with that? That is the antenna, will I need a reader like the one attached? I am just confused with how the set up works. Please bear with me, I have a lot of questions!

author: Mr Simpleton
author_rank: Unknown
timestamp: January 15, 2015, 02:43:46 pm
content: The antenna is the cheap part of the system!What you need is a reader, which is really a transmitter/receiver that operates in duplex. I.e. transmit a carrier when receiving the backscatter signal from the tag.Not sure what you want to do, but buying an Impinj antenna just for fooling around seems like waste to me! The antenna can be pretty much any resonant thing hooked up to the reader.

author: jbushnel
author_rank: Contributor
timestamp: January 15, 2015, 02:55:24 pm
content: Quote from: Mr Simpleton on January 15, 2015, 02:43:46 pmWell it is actually for a school project and it needs to work for me to graduate... haha like I said, I don't need anything fancy. I just wanted to buy an UHF antenna, and once the RFID tag crosses the antenna, then a logic high will be produced. I don't need any network capability or anything. Do you think there is anything out there that will suit my need? Or even a less expensive reader?

author: Mr Simpleton
author_rank: Unknown
timestamp: January 15, 2015, 03:03:24 pm
content: Hmm.... the technology behind passive RFID tag detection is ipressive and a bit complex...But if you do not need the long range I would look at e-bay and buy a cheap reader, build my own antenna if it is not integrated.As I said the reader antenna is the simplest component in the system. A resonant dipole is good enough. It seems you can find short range readers for $100 or so.One thing with the cheap readers is you need to supply code to interface and get data out..Now the tags are mute unless you do send the proper request command, it is not a simple proximity detect!

author: jbushnel
author_rank: Contributor
timestamp: January 15, 2015, 04:08:38 pm
content: So I am looking into standalone readers and it seems that a lot of them say their maximum distance... Doesn't the read distance depend on the antenna and not the reader?If it depends on both, does the read distance of the reader limit the distance of the antenna?

author: Mr Simpleton
author_rank: Unknown
timestamp: January 15, 2015, 04:49:34 pm
content: As I wrote in my first reply... The tag is passive and get its energy from the reader. Tag needs roughly -15dB to start operate. Back-scatter signal from tag is usually not the limiting factor in range.Standard says US can max emit 4 W ERP and EU 2W ERP, and how you reach the Effective Radiated Power is up to you, high gain antenna and less transmit power or max transmitter power and less effective antenna.Usually when talking about reader one refere to reader and antenna as a system...I'd say the most impact on range is how the reader is implemented give that it do uses max power.Some simple math: Radiated 33 dBm and you need -15dBm at tag, i.e. can tolerate 48 dB path loss. Why not try to see what pathloss you have between two resonant dipoles on 900 MHz. Just some wire soldered to a BNC connector. Zero cost... lots of fun

author: jbushnel
author_rank: Contributor
timestamp: January 15, 2015, 06:13:33 pm
content: Ok I think I have mostly everything I need. A couple more questions though... Will any tag work with an antenna and reader as long the bandwidth is the same?Lastly, is there just an arbitrary number stored on the RFID? or does it come "blank" and there needs to be something written on it?

author: Mr Simpleton
author_rank: Unknown
timestamp: January 16, 2015, 08:42:05 am
content: The tag can operate at any frequency between 860 and 960'ish MHz, it is simply a folded dipole and a logic chip glued to this. So the antenna will reflect (back scatter) any frequency that is radiated towards the tag. Given it receives enough energy to "wake" the logic chip it will try to respond to a proper request, but stay mute if it does not "understand" so simply sending a carrier or gibberish code will not create a tag response.The tags are bought preprogrammed, and some even have extra memory for user data.