RFID transmit and receive power levels

[Higgler]

What kind of power levels are involved in the use of RFID for the 900 Mhz band. I found a few websites and searched this website without luck, info available excludes the power levels.

An alternate question is what isolation or coupling levels in dB are typical between the RFID transmit and receive antennas. We are evaluating RFID placement and a simple antenna coupling measurement can let me know if it's feasible.
References appreciated too.
Thanks,

kch

 

[VEBill]

I'm reading between the lines that you're only interested in unpowered or passive RFID tags, and that you're not including any consideration of the larger battery-powered RFID tags.

I don't understand why you would be overly worried about the T/R antenna isolation.

It seems obvious that you will often have stronger reflections from the environment, and various items within the environment, than you would from ANY passive or field-powered RFID tag (if for no other reason than the tags are typically so small that they're very power starved).

Unintentional reflectors in the environment would typically be much larger than an intentionally small tag, and such unintentional reflections would often be much stronger than the RFID's 'reply' signal (by orders of magnitude).

Thus, even if you achieved perfect T/R antenna isolation, the perfect isolation would be ruined the first time anything entered the environment (unless it were something made with the very latest stealth technology ;-) ).

Of course, any practical RFID system would require a certain amount of antenna isloation for practical reasons (don't swamp the receiver), but the system design should not depend on a high degree of isolation unless you're planning to aim the system into the empty sky and float the RFID tags across the active area suspended on thin string.

So, aren't these system designed to operate in a manner such that the 'reply' is different than the emitted carrier? Some use frequency doublers; others modulate the field. In other words, the receiver must discriminate the 'reply' signal from the carrier by relying on techniques other than T/R antenna isolation.

Putting it another way:

Would your proposed RFID system work when the tag is attached to a cardboard box containing a product? ...where the product in the cardboard box just happens to be a large (~1m) three-sided corner reflector - open end facing your reader.

I believe that most commercial systems would probably pass this test because they do not rely on a high degree of antenna isolation. They can pull out the reply even buried under numerous reflections, some even Doppler shifted randomly as shoppers and shoplifters pass through the active area.

Apologies if I misunderstood the context of the question.

 

[Higgler]

A bit of a misunderstanding I believe.

When the reader antenna transmits power to a passive RFID tag, what levels of power are transmitted for the 900 Mhz series hardware and what power levels are typically received by the passive tag. (I realize levels vary alot, so maybe what's the minimum tag received power level typically used).

Hence, as an antenna engineer, I could design two antennas (reader and TAG), then measure S21 on an analyzer between reader and RFID tag and tell the customer that his desired system would work at distance X,Y,Z.... our tag will actually be a full sized antenna and maybe it'll be the same one as the reader antenna.

Maybe that'll clear it up.
Thanks,
kch

 

[VEBill]

I see. As usual I found the alternate interpretation.

When you asked, "what isolation or coupling levels in dB are typical between the RFID transmit and receive antennas...", you meant END-TO-END **THRU THE TAG**.

 

[Higgler]

True,
I'm used to measuring coupling thru two antennas, and since I've always tried to minimize that, the word isolation is in my brain. Coupling may be a word better suited for this application.

kch

 

[VEBill]

I assume that some types of RFID tags (especially the sort with id numbers built-in), would require a certain amount of field strength to 'light up', and then they would emit a more-or-less constant reply energy.

At levels too low, the end-to-end coupling loss would be infinite dB (P out, nothing back).

At the minimum level, the coupling would suddenly be X (perhaps that's the number you seek).

At higher levels, the coupling would tend higher (more P out, largely to no effect).

In other words, if you tried to express the end-to-end coupling loss, you'd find that it would be difficult to express it accurately as a single figure.

Disclaimer: no hands-on in this area (TOoMA).