Hi, all,
I am new to RF, but (un)fortunately i am handling a transferred
project on RFID. Some of the rf concepts i am not not so sure, and
seek for assitance here.
Let me give a bit more on the original designs first. They are tags
that talk to each other unlike the passive tag. This design is already
working with a quarter wave-length antenna. My task is to modify it
to a smaller (shorter) one.
The design is like :
antenna----matching circuit----transceiver----mcu

Now comes the questions:
1. when we talk about the load impedance, where should it start
counting? From transceiver down left or from the matching circuit down
left?
2. the original matching circuit is working for the quarter wave
length antenna. Do i need to adjust the value (C and L) if i replace
them with other forms of antennas(e.g chip antenna, loop antenna)?
3. How am i going to measure the load impedance? What equipment is
needed?

thanks. Any advice is favorable.
Sigma

Montoya (Sigma),

You did not give any indication of the frequency you use, and commercial
RFID tags range from somewhere around 125 kHz to over 5 Ghz. Makes just
a little bit of difference.

This can't be part of a legitimate commercial project or else the
company would already be bankrupt for lack of adequate staffing.
Assuming it is a student project the recommendation would be to contact
your professors and fellow students to learn more about RF.

For some specific ideas check out some of the voluminous documentation
available from RFID manufacturers. For example, Texas Instruments has a
lot of RFID documents available for downloading, including a 47 page
technical note on antennas for RFID. The high level site address is
www.ti.com/rfid

73,
Gene
W4SZ


Montoya wrote:
Hi, all,
I am new to RF, but (un)fortunately i am handling a transferred
project on RFID. Some of the rf concepts i am not not so sure, and
seek for assitance here.
Let me give a bit more on the original designs first. They are tags
that talk to each other unlike the passive tag. This design is already
working with a quarter wave-length antenna. My task is to modify it
to a smaller (shorter) one.
The design is like :
antenna----matching circuit----transceiver----mcu

Now comes the questions:
1. when we talk about the load impedance, where should it start
counting? From transceiver down left or from the matching circuit down
left?
2. the original matching circuit is working for the quarter wave
length antenna. Do i need to adjust the value (C and L) if i replace
them with other forms of antennas(e.g chip antenna, loop antenna)?
3. How am i going to measure the load impedance? What equipment is
needed?

thanks. Any advice is favorable.
Sigma

Sigma wrote:
"This design os already working with a quarter wavelength antenna. My
task is to modify it to a smaller (shorter) one."

An antenna shorter than resonant length uses a lossy loadong ciil(a) in
most cases to neutralize its capacitive reactance. The antenna system
must be resonant so that reactance does not limit energy into the
antenna.

A 1/4-wave of wire is only 1/2 of an antenna. The other 1/2 of the
antenna must be supplied by another 1/4-wavelength of wire or by an
image-maker such as a large conducting sheet or a ground plane. The
image maker is larger than a lone 1/4-wavelength of wire.

A 1/2-wavelength of wire is resonant and it is a complete antenna. A
1/4-wavelength is not.

A shorter than resonant-length of wire is less directional than a
full-length antenna, but not by much.This means there is less "gain" in
it at its best direction. Its real problem is its fast rising capacitive
reactance as it is shortened from resonance. Another problem is its
dropping radiation resistance which causes its loss to heat resistance
to take a larger share of its total energy.

There is a way to slightly reduce the length of a resonant dipole, and
that is to make it fat instead of thin. This is usually costly,
inadequate, and inconvenient. The length of a 1/2-wavelength conductor
is only 94% of a 1/2-wavelength in free-space if its diameeter is 1/1000
of a wavelength. See Fig 8 on page 2-5 of the 19th edition of the ARRL
Antenna Book.

Another way to reduce the length of a resonant dipole is to attach large
conductive areas to its ends as capacitive hats for loading. This is
almost lossless as compared with coils.

The antenna may not be the problem if you are working in the near field.
You may have an advantage in the much higher energy very close to the
antenna. The antenna`s induction fields are much larger than its
radiation field up close. See page 865 of Terman`s 1055 edition.

You might not use a salient antenna at all. You could use a metal
1/2-wavelength slot. See Chapter 9 of Kraus` 1950 edition of "Antennas".

A practical solution may be a 1/4-wavelength, end to end, folded dipole.
This consists of a resonant 1/2-wavelength of wire. It takes only 1/2
the space of a !/2-wavelength open-ended dipole. Its gain is only 0.5 dB
less than the 1/2-wavelength dipole. Its problem is a feedpoint
resistance of 6000 ohms which may be hard to match. See page 509 of TV
and Other Receiving Antennas" by Arnold B. Bailey.

Best regards, Richard H. Harrison, KB5WZI

Thanks, Harrison and Gene,
Actually this product is going to be deployed in about three months
time in one of our public sectors. And it really gives me and my boss
nightmare every night. For small company like us, there is no choice
but to grab everything we could to deliver.
The original prototype is well designed and tested. the frequency is
433 m. Therefore quarter wave length would be about 17.2 cm, which
would be too long for a commercial handheld product.
Our task is to re-package it and produce. And that is why i need to
modify the antenna. I tested quite a few antennas including loop,
semiloop, modified dipole and microstrip antenna as given in
http://www.rfm.com/corp/appdata/antenna.pdf
I am using copper fold to make the antenna plane and the groundplane.
However, the results is far from satisfactory.
For the above mentioned antennas, i did not changed the matching
network given in the transferred design which caters for 17.2 cm
antenna. Do i need to do so? Thanks.
Sigma montoya

Sigma

Yageo sells a $2.00 ceramic antenna for circuit board mount for 433 that
you probably already know about. Contact me if you want any info about how
to get that antenna.

Jerry


"Montoya" wrote in message
om...
Thanks, Harrison and Gene,
Actually this product is going to be deployed in about three months
time in one of our public sectors. And it really gives me and my boss
nightmare every night. For small company like us, there is no choice
but to grab everything we could to deliver.
The original prototype is well designed and tested. the frequency is
433 m. Therefore quarter wave length would be about 17.2 cm, which
would be too long for a commercial handheld product.
Our task is to re-package it and produce. And that is why i need to
modify the antenna. I tested quite a few antennas including loop,
semiloop, modified dipole and microstrip antenna as given in
http://www.rfm.com/corp/appdata/antenna.pdf
I am using copper fold to make the antenna plane and the groundplane.
However, the results is far from satisfactory.
For the above mentioned antennas, i did not changed the matching
network given in the transferred design which caters for 17.2 cm
antenna. Do i need to do so? Thanks.
Sigma montoya

Thanks, Jerry.
How can make a 50 ohm semi-loop antenna as stated in the link
http://www.rfm.com/corp/appdata/antenna.pdf

"Montoya" wrote in message
om...
Thanks, Jerry.
How can make a 50 ohm semi-loop antenna as stated in the link
http://www.rfm.com/corp/appdata/antenna.pdf

Sigma

Although I am confidant that I could design and build an antenna from the
above sited antenna concept, I have way too litle knowledge of your
restrictions. If you did want to get some help with this antenna maybe you
could write a more difinitive description of your needs and restrictions for
the really good antenna guys who read this antenna news group.
If your "specs" are realistic, I'd bet money that even I could design a
satisfactory "semi-loop", and I might be one of the least prepared antenna
designers who frequent this site.

Jerry