QUESTION

Question

Hello,

I have this circuit matching.

The antenna is ANT3505B002TWPENS and the matching recommended is:

but, in my design I put the matching components with a PI configuration because I think is the best choice to match the 2 frecuencies (900 and 1800).

In first time we match 900Mhz with series C and parallel L (because this configuration have low influence at 1800 Mhz):

In second time we match 1800Mhz with parallel C and series L (because the this configuration have low influence at 900 Mhz):

And I remember that we can move across Smith´s chart with the next image:

Ok. We begin with our matching. In first time we calibrate our VNA (I have miniVNA Tiny).

I calibrate the short with 0 Ohm resistance at L8 and open L6:

I calibrate open with L6 open and L8 open:

And I calibrate 50 Ohm with a 50 Ohm resistor at L8 and L6 open:

Ok. Now we measure how is my circuit (shorting L6 and opening L8 and L9) and we have:

Point [1] is 900 Mhz and point [3] is 1800 Mhz.

My first impression is very bad because the 2 freceuncies are far of the center.

From now on I will put the exact value of the component to correct match although I solder and desolder components hundred of times to find the correct.

Now We begin with 900Mhz:

 

 

We have the 900Mhz frecuency at center, now we are going to match the 1800 Mhz.

Remember we can only use:

Because another configuration move the 900 Mhz frecuency and unmatch it.

And now I need to add an L in series to move the 1800Mhz at center of the circle, but, I don´t have any space to solder it with PI configuration.

I need to change my circuit matching configuration by this?

 

Why this data of the correct configuration by different frecuencies don´t tell at the datasheets of the antennas? I feel stupid because I design a complete circuit and now I discover that all is wrong.

More people make this mistake? With a PI configuration we can obtain a good matching, but, never a perfect matching?

Now I´m going to redesign the circuit and I need your advice J

Best regards (sorry about my english).

Answer 1

Hello Azicuetano,

I would like to mention first that this antenna is not one that we manufacture. It is actually made by Taoglas. 

If you look at their implementation of their return to ground you will see that it has a long trace (adding shunt inductance before it makes the ground connection). Since the trace is also hovering over the ground plane one the inner layers as it traverses to the grounding point the trace also acts as a shunt capacitor. Both of these two shunt components puts the two bands you are interested in at different starting points with your implementation of the return than that of the return on Taoglas' evaluation board. Your return is very short and has no components laid out to match with in order to get to the same starting point as the Taoglas' recommended matching topology and values. 

This is poor insight on their end to assume that their customers could add the same length trace for their device's return as Taoglas does and they are not offering the proper guidance to add the extra components needed to bring the bands into proper VSWR to work well.

I am sorry you have to redesign you matching topology but I commend you for your tuning efforts. You really seem like you know how to do one of the harder things in antennas which is being able to properly match your antenna.

Best regards,

Geoff Schulteis