What is the difference between tuning an antenna and matching an antenna

[BusyEng]

What is the difference between tuning an antenna and matching an antenna?

Assume that I have an antenna that is not exactly resonating at my desired frequency. Should I first try changing the antenna geometry to move the resonance frequency to my desired band and then find the right matching components for it or keep the antenna untouched and just find the matching components for the antenna as it is? In other words, does adding matching components actually helps tuning the antenna as well?

Thanks

 

[jossion]

what you told first is right. you have to resonate it at the desired frequency or near to that. then to match it for the desired bandwidth you can use lumped element or distributed element matching.

 

[zorro]

Hi, BusyEng

Tuning: adjusting the antenna so that the impedance is resistive at the frequency of interest
Matching: Make that the impedance is the complex conjugate of the feed or load impedance, or characteristic impedance of a transmission line (normally, but not always, resistive, usually 50 ohms)

You can get matching at a particular frequency without tuning first, but it is not good. Useful bandwidth in that case could be too narrow. There is a limit (Fano-Bode limit) to achievable bandwidth in those cases.

Regards

Z

 

[WimRFP]

Tuning is varying one component that changes the resonant or center frequency of a filter or antenna (with one resonating structure). The component can be: length of wire (cutting to increase the resonant frequency), variable capacitor, variable inductor, dielectric slider, etc.

Matching is varying one or more components to get a good match between the source (for example a transmitter) and a load (for example an antenna). Tuning via single component may not result in a good match.

Mostly matching requires varying two components. Some examples: increasing the length of a quarter wave radiating trace/wire [inductive behavior] together with a parallel capacitor (or decreasing the length [capacitive behavior] with adding a parallel inductance). An L-match (consisting of an inductor and a capacitor) is frequently used as matching circuit.

Tuning you can see as a one dimensional process, where matching is a two (or more) dimensional process.

If you have a quarter wave structure close to a ground plane, its resonance impedance is well below 50 Ohms (for example 5 Ohms). Therefore tuning is not sufficient to get a good match. You can use an L match (2 components) to convert for example 5+j0 Ohms to 50 Ohms.

Increasing the length of the structure somewhat can bring the admittance to (for example ) Y = 0.02 - j0.06 S. This equals 50 Ohms parallel with an inductance of j16.7 Ohms. Now you only have to add a parallel capacitor to counteract the -j0.06 S. So it is not always required to have resonance before matching. You may save a component by tuning somewhat above or below the resonance frequency.

In case of narrow band antennas in combination with production tolerances or unknown operational enviroment, one may provide the user with access to only one of the two matching components. The fixed (non-accessible) component is based on average operating conditions and the user only needs to "tune" the antenna. Think of adjusting a trimmer capacitor, adjusting some threaded wire to increase or decrease the length, or just cutting the whip.

I forgot to mention that the word "tuning" is also used to describe "matching". For example a single or double stub tuner is used to achieve a good match between source and load.

Though the word "single" in single stub tuner may suggest a one dimensional process, it is has two dimensions (length of stub and position of stub).