another question to impedance matching

[doneirik]

Hi again,

I´m still working on 13.56 MHz inductively coupled RFID and have another question to impedance matching.

The equipment I have consists of an antenna (matched to 50 ohm) fed by a 50 ohm coax cable and a tranceiver (also 50 ohm).

Now I want to design new antennas to connect via the 50 ohm cable to the tranceiver, and I´m not 100% sure how.
Especially, do I need to match the antenna circuit to 50 ohm, as my coax cable is about 30cm long?

Since the free space wavelength is about 23m, wavelength/4 is 5.75m. Even with a velocity factor of 0.1, I think I could use a 0.575m long cable without having reflections?

any suggestions?
I´d also be interested if somebody has some links as to how to design 13.56 MHz rfid antennas from scratch.


regards
doneirik

 

[BrianR]

It isn't clear to me if you are talking about paralleling multiple 50 ohm antennas on a single 50 ohm line however as long as you terminate a 50 ohm line with a 50 ohm load then the length has no bearing on the impedance seen by the transmitter, it will be 50 ohms.

You normally want a 50 ohm load on the TX as this guarantees maximum power out and minimum spurious emissions.

If you load the coax with other than 50 ohms, (eg paralleling 2x 50 ohm antennas = 25 ohms) then the coax will act as a transformer giving some other impedance to the transmitter, the actual value depends on line length.