Differential PCB loop antenna tuning

Thread Starter

kellogs

Joined Aug 13, 2017
6
Hi,

The antenna in question is the one from here - a 40 x 25 mm square loop antenna for the 434 MHz range. It is described beginning with page 8.
My concerns are with tuning this already lossy antenna. The document says that a Cp1 capacitance of 1.17 pF is required to adjust its center frequency and I can only find 1.1 pF with a 0.05 pF tolerance ceramic capacitor that is big enough to hand solder and not too costly. That means i would have to somehow make up for the remaining [0.02 .. 0.12] pF, and within no more than about +/- 0.01 pF in order to maintain the antenna somewhat effective at the desired frequencies of operation (see "Figure 13. Loop Antenna Simulated Mag(Zin) vs. CP1 " on page 21 for how dramatic the shift in center frequency is for only 2% tolerance on Cp1).

In this situation, building up on their proposed solution to mitigate Cp1 on page 27, I have come up with the following design:PCB_loop_differential_antena_tuning.png

A bank of four (not three) C_tune capacitors of values 0.01, 0.02, 0.04 and 0.08 pF having thick traces that I could cut off if need be. I am assuming here that I will be able to accurately measure Cp1. The tune caps bank will be enough to make any value of the [0.02 - 0.12] range in increments of 0.01 pF. However, I don't know if this will work at all. My concerns are:

1. the introduced inductance; by both the vias and maybe by the thick traces themselves ? http://chemandy.com/calculators/flat-wire-inductor-calculator.htm shows some 5 - 10 nH per length of thick trace, but I don't know if this applies to differential antennas. I don't even know what path the return currents follow in this case (there is no ground beneath the loop traced antenna)

2. capacitance introduced by both vias + upper right trace superposition and by the coplanar parallel thick traces among themselves.

3. the copper introduced inside the loop might have a considerable effect on the radiation pattern.

I'd gladly take in comments for the points above and not only!

Thank you
 
Top