swra046a

I trying to construct a loop antenna but I dont know how to match the 50 ohms cable line to the input impedance of the loop?

Is there any way I can fed the loop?

Tnx.

Hi,

You have to specify whether the loop antenna's circumference (of one turn) is around one wavelength or much smaller than one wavelength?

In case the circumference is around one wavelength, the one turn resonant loop (either a square with quarter wave sides or a full circle) has got about 110-130 Ohm real input impedance at its resonant frequency. How to match this symmetric output (or input in case of a transmitter loop) to 50 Ohm (unbalanced) line is here:
http://www.ycars.org/EFRA/Module%20C/AntMatch.htm

In case the circumference of your loop is much smaller than one wavelength, the input impedance is reactive (inductive) with very small real Ohmic value and this latter continuously increases as the frequency increases till the one wavelength value is reached. So the problem is matching a very small real Ohmic value with a high value inductive reactance in series with it to a 50 Ohm assymetric line at the receive or transmit frequency. For Bluetooth frequency for instance see this tabelle on input real and inductive impedances in case of one or more turn circular small loops:
http://www.eece.ksu.edu/vlsi/bluetoo...jkXLvalues.htm

Links for different practical small loop antennas with matching shown are here:

http://www.ti.com/rfid/docs/manuals/...naCookbook.pdf
http://www.mwrf.com/Globals/PlanetEE/Content/18055.pdf

And here is a useful link on small loops when the circumference of the loop is about 1/4 wavelength, how the input impedance changes at the different locations alongside the loop: http://members.aol.com/benprom/index.htm/theory.html

rgds, unkarc

Hi Unkarc
I want to design 433MHz loop antenna.
How to design it?
where to find design equations?
I want to use FR4 substrate.

A Chipcon/TI application note on 433 MHz SRD antennas:http://www.ti.com/litv/pdf/swra088
The application note comes to the conclusion, that a bend monopole gives better performance than a loop antenna. A tuned loop, as the above quoted G0CWT Magic loop, that isn't a pure magnetic loop to my opinion, may be another interesting option.

Hi,

Usually EM antenna softwares are used for such designs like Sonnet or Ensemble from Ansoft etc because you can easily test some "what if" questions when you change some parameters.

If you prefer calculation, here are some notes, starting with probably the best:

http://www.nordicsemi.com/files/Prod...tesallnRFn.pdf

http://www.maxim-ic.com/appnotes.cfm...ber/3401/ln/en

http://focus.ti.com/lit/an/swra046a/swra046a.pdf

Further good info on printed loops:

http://www.maxim-ic.com/appnotes.cfm?an_pk=3621
http://www.maxim-ic.com/appnotes.cfm/an_pk/3622

See these too:

http://www.tech-systems-labs.com/pdf-files/antennas.pdf
http://www.nordicsemi.com/files/Prod...otesnRF401.pdf

rgds, unkarc

PS by the way, FvM's finding that Chipcon/TI application note is a good catch, a printed loop antenna can be poorer performer than some other types.

Hi FvM & unkarc
Thank you very much.
These articles are amazing.
I will study these articles.
Chipcon's note is really good.