Matching Multi-band Antennas
Cellphones and laptops need multi-band antennas. Practically every cellular device must support at least two bands and most must support more. WiFi devices often need to support the 2.4Ghz and 5Ghz bands. So, there’s great demand for multi-band antennas today.
A multi-band antenna must have good radiation characteristics at all of the relevant bands, that’s the first challenge. I’ll talk about that another time. The second problem is to impedance match the antenna to the front-end. Often a lot of performance is left on the table because designers don’t understand multi-band matching. I was taught the right approach by Devis Iellici.
One approach is to design a matching circuit appropriate for one particular band. For example, suppose the unmatched antenna has worse performance in one band. The designer could make a circuit for that band and simply see what happens to the other bands. This is a bad plan since the effect on the other bands is most often detrimental.
A better method is to design the matching circuit with all bands in mind. For any narrow band there are several simple circuits that can improve the match, its best to find them all. I measure the Smith chart of the unmatched antenna and find the worst performing band using a radiation measurement such as efficiency. I then use Smith chart software to find all reasonably good matching circuits that can be made with three elements. I then pick the circuit that has the best performance in the other bands, that can be estimated using the Smith chart software too.
For example, suppose we have a quad-band PIFA antenna for cellular. It must cover 824Mhz-960Mhz and 1710Mhz-1990Mhz. Suppose the lower band has the worst performance. I would then find every three element LC circuit that matches the low band reasonably using Smith chart software. I’d then try each one out in the high band and see which performs best there. Then I’d implement it, test it and tune further using trial and error.
For this to work best the designer must take into account the parasitics of the components. Sometimes that can be done using the Smith chart software or by using a full microwave circuit analysis package.