>> Transmit and receive aerial systems *MUST* be resonant for efficient
>> power transfer and anything like sensible range.

> As for range I need ~10" max, I'm looking to use this for
> identification & timing of small go-karts.

> For the TX, I assume this means making a resonant tank with the coil
> inductor and a matching capacitance?  I can measure various wound cores
> with my Atlas LCR (so long as L is >1uH) and calculate the C to test
> this out.  If I am on the wrong track here please say.

Now you've explained the application it allows some fine tuning :-).
You don't need efficient power transfer.

I'd aim for max sensible tx loop.
Air cored should be fine.
Various Microchip app notes cover  RFID aerials ICs and applications.
Some of these would be of  use to you.

Large rx loop helps minimise kart position effects. Resonance will still he=
lp.

TX and RX "tanks" can be series or parallel resonant.

Try these:

_________

Microchip microID=AE 13.56 MHz RFID
System Design Guide
208 page PDF
http://ww1.microchip.com/downloads/en/devicedoc/21299e.pdf

* CONTAINS*

AN680
Passive RFID Basics

MCRF355/360
13.56 MHz Passive RFID Device with Anti-Collision Feature

MCRF450/451/452/455
13.56 MHz Read/Write Passive RFID Device

AN752
CRC Algorithm for MCRF45X Read/Write Device

AN707
MCRF 355/360 Application Note: Mode of

AN710
Antenna Circuit Design for RFID Applications

MCRF355/360 Reader Reference Design

AN759
Interface Control Document for the
13.56 MHz Anti-Collision Interrogator

AN760
PICmicro=AE Microcontroller Firmware Flowchart
of DV103006 Demo Reader for MCRF3XX and MCRF4XX Devices


___________

Microchip microID=AE 125 kHz RFID
System Design Guide
PDF 210pages
http://ww1.microchip.com/downloads/en/devicedoc/51115f.pdf

__________

Microchip AN710 Antenna Circuit Design
Intro page:  http://www.microchip.com/stellent/idcplg?IdcService=3DSS_GET_P=
AGE&nodeId=3D1824&appnote=3Den011776
AN710:  http://ww1.microchip.com/downloads/en/appnotes/00710c.pdf
Passive RFID tags utilize an induced antenna coil voltage for
operation. This induced AC voltage is rectified to provide a voltage
source for the device. As the DC voltage reaches a certain level, the
device starts operating. By providing an energizing RF signal, a
reader can communicate with a remotely located device that has no
external power source such as a battery. Since the energizing and
communication between the reader and tag is accomplished through
antenna coils, it is important that the device must be equipped with a
proper antenna circuit for successful RFID applications.

_________

Microchip AN678 REVIEW OF A BASIC THEORY FOR
 ANTENNA COIL DESIGN
http://ww1.microchip.com/downloads/en/appnotes/00678b.pdf


>
> For the RX I am more confused.  Professional systems simply use a
> length of wire buried in the track with a resistor at one end and a
> black box at the other (which connects to a coax feed line).  I had not
> considered whether this was "active" (like RFID) or completely passive.
> Regardless the maximum length of the detection wire is around 20m, which
> doesn't seem like a highly tuned/coupled system!

You can resonate most loops at sub MHz frequencies.
May not be resonant in this application BUT that will increase signal level=
s.


> My solution last year was a completely untuned inductor that I simply
> saturated from a 9v battery.  The resulting mess of a spike (and ringing
> afterward) was picked up by a passive loop and the gaps between these
> measured to identify the transponder.  I am sure I can improve this
> year, if only to stop polluting the local radio spectrum!

Look at TX coil drive in various RFID app notes.  A simple oscillator
with the TX loop as it's oscilltor tank cct may well suit.


 Russell

>

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