13.56Mhz RFID Copper Tape Antenna Question

I have constructed a copper tape antenna (2' x 4') using the TI HF Cookbook and Design Notes as a guide. The problem I am is that I measured the inductance of the antenna to be 0.77uH using a MFJ 259b. Using that inductance the capacitance should be 177pF. I have a variable 80-180pF capacitor to tune the frequency. The problem I have using the MFJ 259b i measure the resonance frequency to be around 27-28Mhz. Varying the capacitor does noting to the frequency. What could be wrong that nothing will change the frequency? Thanks.

Hai are you using any simulation tools for this design?

How are you measuring "the resonant frequency". It is sometimes difficult to see it. If you have a big mismatch, the "resonance" can be broad or of little height, and you might not see it. I have access to a fancy network analyzer, and sometimes I hook up an antenna to it and don't see any resonance at all (sometimes I see many different resonances and need to figure out which one is the real/desired one!). It is not so easy to test sometimes.

Sometimes I swap methods. I might start off by looking at VSWR, but if I can't see anything I hook up a broadband antenna to the 2nd port and look for a hump in the transmission power and tune for it instead.

What does your circuit look like. You might want to change the topology. For instance, you might be loading the coil too much with an external source for it to resonate. Try having the copper element and the variable capacitor form a closed loop, and lightly couple into the loop with small capacitors.

No, but i built basically the same exact antenna as specified in the Hf Cookbook so I was assuming the dimensions of the corners and T Match bar were going to be correct.

I am measuring the frequency using the MFJ 259b in resonance mode. I do realize that the bandwidth is going to be very small if done correctly but I am getting no spike at anywhere around 13.56 and when i plug it into my reader it is not picking up any tags. The bandwidth does seem to be quite large at the 27-30Mhz it says is the frequency currently.

Right now I have a 1/2" gap on one side of the loop that is closed using a 10Kohm resistor and 8-180pF cap. Then the other side has the T-match bar. I did try moving the T-match bar and that made is even worse, so I'd say where that is at is probably pretty much optimal.

One thing to be wary of when analyzing an antenna at these frequencies (low) is that just about everything in the room is within the near-field region of the antenna. You should take care to point the main beam of the antenna into a far-away corner of the room, so that your equipement isn't messed up by reflected energy coming back into the antenna (can really mess up a return loss measurement). Additional, if you can avoid aiming the main beam at metal objects, that will help, too.