PCB Loop Antenna

[col_implant]

Hi All,
Im using a PCB loop antenna for near-field communications in the 433MHz ISM band. The antenna is simply a PCB track (35micron x 2mm, 12mm diameter loop) with a short transmission line (track) to it. I have no ground plane extending beyond the transmitter, just the loop itself.

I'm attempting to do some theoretical analysis of the antenna, to see if our results match up with the theory. I have been through a world of literature at this stage and the only theory of PCB loop antennas I have found relates to either microstrip (track above ground plane) or stripline (track between two ground planes). I have also found something on wire loop but this seams unapplicable too.

All i really want is some theoretical values for transmission line Z0 and propagation coefficients, and some impedance value for the loop. Sorry if there is something obvious im missing... im a bit of a nube to RF theory

 

[RadioRon]

Quixotron has been working in a similar field, so perhaps he can post with some help. In the meantime, can you describe exactly how you are feeding power from the transmitter to the loop? You mention a trace but you do not mention the length of the trace, how it connects to the loop, and how the transmitter attachs to this trace. It would be useful to know these things in order to help predict the transmission line characteristics.

If you want a theoretical background on a pcb loop, then go ahead and study the electrically small wire loop as it will be quite similar. The impedance of the loop will depend partly on how you are feeding it. Can you describe the geometry of the copper traces at the feedpoint (or better still, provide a picture of some sort).

 

[arhi]

col_implant, are you trying to make pcb antena like this one:
RFid Reader Extra Development Board | mikroElektronika

It does not have a ground plane, just one big spiral on one side, via in the middle and spiral continuing on the other side of the PCB, I just got this board few days ago so I can take some pictures if that can help. I cannot help with the theory as I'm illiterate when it comes to analog elco..

btw, you might find pretty good info here:
https://www.electro-tech-online.com/custompdfs/2008/05/AN2731.pdf

some useful info here:
Antenna & Propagation engineering - RFID Patch antenna

Also attached a very useful document about designing pcb antenas, found it few days ago while trying to figure out how this pcb antena on rfid reader works

 

[arhi]

for some reason I was unable to upload the attachment so find the PDF about "pcb antena design" here: **broken link removed**

hope this helps (not that I understand much from that document, lot of math and examples)

 

[col_implant]

Cheers for that, not really what I had in mind though.

The antenna we are using is quite similiar to the one used on this eval board.. https://www.electro-tech-online.com/custompdfs/2008/05/0004734_evb72015_rev007.pdf

 

[arhi]

Cheers for that, not really what I had in mind though.

The antenna we are using is quite similiar to the one used on this eval board.. https://www.electro-tech-online.com/custompdfs/2008/05/0004734_evb72015_rev007-1.pdf

That is "simple loop antenna" ... I found couple "calculators" online to help you design pcb loop antenna .. just google it, I believe you will find it easy

 

[RadioRon]

Cheers for that, not really what I had in mind though.

The antenna we are using is quite similiar to the one used on this eval board.. https://www.electro-tech-online.com/custompdfs/2008/05/0004734_evb72015_rev007-2.pdf

A good starting point to estimate the radiation resistance is to use an expression for the radiation resistance of a small loop (from Balanis) where Rr=20pi^2 x (Circumference/wavelength)^4. If your loop is approximately 80mm in circumference, and your frequency is 433 Mhz then Rr=0.035 ohms. Small loops typically have such low radiation resistances delivering efficiencies of well below 1%, so it is reasonable to guesstimate that your ohmic losses might be about 200 times your radiation resistance, or 7 ohms. This will dominate the real part of your feedpoint impedance. Since this is just an educated guess, the reality may be somewhere in the range of 2 to 10 ohms. The reactance will simply be the inductance of your loop of wire. At least, if the conductor were a thin wire this would all be close enough. That it is a pcb trace will alter the inductance, and will also alter the ohmic losses as well. But in which direction? I'm not sure without some research, but I don't think the changes will be large. In any case, your loop input impedance is quite low. The antenna is a balanced structure and should be fed appropriately.

The feed structure in the link example is a very short parallel transmission line fed through an LC network meant to change the unbalanced output of the IC to a balanced line and to effect impedance matching to match up with the loop. The transmission line is very short so you can probably ignore the effect its length might have on the angle of the impedance. The unbalanced to balanced aspect of the matching network is important and should not be ignored.