Electronic – Antenna: Impedance matching vs. tuning


I always try to tune my pcb antennas in length, so they operate in resonance, by cutting the antenna with a scalpel and then matching the tuned antenna to the transmitter.

But recently an RF engineer told me, that you could sometimes get the same radiated power, by matching the impedance without trimming the antenna to resonance.

I have measured an untuned IFA to have impedance of Zin = 7.7+j90 @ 865MHz.

Should I try to tune this antenna to resonance, or will it (in practice) work just fine to match the untuned antenna to my transmitter ?

Best Answer

I have measured an untuned IFA to have impedance of Zin = 7.7+j90 @ 865MHz

The 7.7 ohms "real resistance" measured represents both the radiation resistance AND the antenna losses so, if you can adjust the antenna length to give you a higher value of overall resistance (and therefore a higher figure for radiation resistance) you will get better antenna efficiency and more easily match your output driver (50 ohms maybe) to your electrical radiation resistance.

You should also find (that with a simple dipole or quarter wave monopole) that the reactive component becomes smaller and therefore you more easily can dump power into the antenna. Here's what a monopole impedance looks like: -

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At a quarter wavelength (optimum for a monopole) the reactive impedance becomes zero ohms and the resistive part (for an ideal monopole) becomes about 37 ohms. If the antenna is significantly shortened the real resistance rapidly approaches zero so, if there are real antenna losses (as always) any power pushed into the antenna is mainly wasted on heating the antenna and a significantly smaller percentage of power gets radiated.