180 degree Hybrid Coupler for rfid application 1

[ElCap]

Hey guys,
im designing and creating a rfid system as a project with a few friends.
i have the task of designing the reader antenna.
the antenna part of this proj is being discussed further but the point of this post is to get some info on directional couplers.

ive been informed that we will need a 180 degree directional coupler between the reader and the antenna.

ive included a link to a pic of what im talking about to help you guys understand what im trying to say.

As i understand it, the antenna is transmitting and receiving the signals at the same time. and the received signal is going to be relatively weak compared to the transmitted signal, so its hard to seperate the signals

the info that i have been reading bout the 180 degree coupler seems to say that it will split up the incoming signal into two signals or combine two signals into 1.

What i need (as shown in pic) is be able to direct the transmitting signal to the antenna and direct the received signal to the receiver module.

Am i right in choosing a 180 degree coupler?
if yes could some explain it some more as i dont understand it fully?

Thanx
Rob

 

[VEBill]

http://www.minicircuits.com

I thought that some (most?) RFIDs tags emitted on a different frequency.

The coupler will only provide a certain isolation and external reflections will still arise in any case. In other words, your signal processor will have to further remove the carrier while extracting the data.

 

[pstuckey]

If the Tx and Rx frequency are the same then a circulator would probably be the best option. If the Rx signal is not the same freq as Tx, I would look at using a diplexer.

Peter

 

[ElCap]

the rfid systems that operate on different freqs are the ones that use inductive coupling as there means of "talking" to the tag and the return signal is double the freq of the transmitted.

but for 915mhz inductive coupling cannot be used and backscatter coupling has to be used.

 

[Higgler]

The 180 coupler isn't any better than a zero degree coupler, why would you use a 180? Doesn't make any sense to me and they're harder to make.

If you need isolation from Tx to Rx, make two antennas and space them apart and make them cross polarized (if linear polarization is OK).

kch

 

[ElCap]

well i found out that the coupler works in both directions (duh), which makes more sense.

our system is half duplex which i thought works with the transmitter and receiver operated at different times to each other, but in fact the transmitter is on all the time. it sends a carrier wave whens it not on, for the tag to add a signal to.

just setting the scene....

so can a directional coupler work both ways at the same time there's a transmitting signal going out and a received signal coming in?

 

[Higgler]

Any coupler works both directions. I still vote for a regular "isolated power splitter" = zero dB coupler with internal isolation resistor.

Normally, full duplex systems need a diplexer (high pass & low pass filter combo, 3 port device) to separate the transmit and receive energy. tx is usually a higher frequency than receive.

kch

 

[ElCap]

so am i thinking too complicated about this?

so just a normal coupler will work?

will the coupler be able to pick up the received signal and couple it to the Rx even thou its alot smaller than the transmitted signal?

 

[ElCap]

so how is power split up between the ports with a zero db coupler? i spose the transmitted power will go straight thru to the antenna, but will the received power get to the receiver?

 

[ElCap]

i havent actually heard of a 0db coupler.
and cant seem to find one thru googling either

 

[VEBill]

"zero dB coupler"

I wonder if Higgler meant 'zero degree'?

 

[Higgler]

Sorry for the zero dB word, that just means equal split 3 dB coupler, or zero dB difference between the two outputs.

Sorry,
kch

 

[ElCap]

yeh thats cool
Thanx everyone for your help with this!!
1 problem done, plenty to go!

 

[gayib]

Hi
I am looking for posibilities to self - make an UHF RFID reader because I wanted to test some applications.

Does someone know how can i get some clear clues. Not pure theory please! I need to construct it.

 

[bmnet]

A company called Alien have got some RFID readers, wouldn't it be better to use an already built one than trying to reinvent the wheel..

Ps. Instead of using two antennas, just try a circularly polarized one... Then tx using H-polarized and rx a V-polarized ID...

 

[K7FE]

The wilkinson combiner/splitter is a zero deg 3 dB device and I am sure that is what "Higgler" meant.