On Oct 10, 8:47 am, micky wrote:

I thought RFIDs, like toll road cards, could only be read when the
reader was 10 or 20 feet from the chip. That there was no battery or
transmitter in the card and it worked by resonating with the
transmitter in the reader.


Partially correct. In the case of the TIRIS system developed
by Texas Instruments in the U.S. , used in
California and many other states, the card contains a lithium
battery. The RCS area of the card to the reader beam changes by
changing the electrical configuration of a piece of metal on
the card. The tag itself emits no radiation, but rather changes
it's ability to reflect the 915 Mhz illumination signal, much like
you would signal to someone with a mirror by reflecting
sunlight. The Reader has a coherent receiver which compares
the incident energy to the modulated reflection.
Battery life is about 5 years.
You can see the specifications by googling "California
Title 12 toll tag specifications" .

RFID systems operate at different frequencies for
different purposes. For animals, the TIRIS system, developed
in Germany, uses 134 khz. It is a small coil of wire and a
computer chip in a small capsule, about aspirin size. The
Reader is a BIG pulse of a magnetic field which the coil uses
to make a voltage pulse, "kicking" the computer chip to
send out data at a low frequency.

Some systems are made to work at 10 Mhz, but I am
not familiar with them.

In the case of the 915 Mhz toll tags (TIRIS), the power
level from the transmitter is about one watt, and an
antenna gain of around 12 db, giving a max incident
power level of around 16 watts, for a couple of milliseconds.
It is WAY WAY WAY below government specified general
requirement for safety at that freqency.. The expected field
strengths at the tag position are around 1/2 V/m (from memory) ,
spelled out in the California Title 12 specification.
In other words, standing directly under the Reader on a tollway,
the effect is miniscule compared to, for instance, a cell phone.