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In RF front-ends the diagram shows an antenna followed by a band-pass filter.

I am also working on a project involving a micro-strip patch antenna followed by a dual-band band-pass microwave filter.

The antenna resonates at a particular frequency and the received energy is transferred to a filter which further passes only required frequencies which seems to be redundant.

Why not just design an antenna with the required frequency response to avoid designing a separate filter after the antenna?

Thank you.

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The antenna resonates at a particular frequency

Not true.

A whip (monopole) antenna (for example) produces an optimum output level for frequencies that are close to one-quarter wavelength. It will also produce a decent output level for frequencies surrounding that optimum frequency and the "spread" of those frequencies from "centre" can be considerable and be much wider than the nominal transmit channel spacing for the band that you are interested in.

It will also produce decent amplitude at frequencies corresponding to three-quarter of the antenna length and also at multiples of half a wavelength above and beyond. Take a look at the impedance of a typical monopole to understand this: -

enter image description here

You should be able to see that at 0.25\$\lambda\$ the reactive impedance falls to zero (tuned) and ditto at 0.75\$\lambda\$.

See also my answer here for the impedance as frequency rises and note that: -

  • The length of the antenna (irrespective of aligning to \$\lambda\$) dictates how big the amplitude is i.e. longer antennas produce a bigger signal naturally
  • The main characteristic change with length is the projected impedance
  • That projected impedance may or may not significantly interact with your actual circuit interface to produce a wealth of unwanted "good reception" areas versus wavelength.

So, given the nature of most radio bands and the extent of interferers, you usually need tight control by using electronic/electrical filters.

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Your assumption seems to be that filtering only one time is enough.

That might be true but it might also not be true, it depends factors like:

  • How much filtering outside your band of interest is needed? If there are strong signals present at frequencies you do not want, you might want to suppress these as much as possible before they end up (with too much power) in your receiver.

  • What is the quality factor of your antenna, it might not filter as well as a dedicated filter. In some situations you might even want an antenna that is quite broadband, so receives much more than you want and then filter out the band of your interest with a filter. Why would you want a broadband antenna? It might be that the antenna is influenced by its surroundings. For example in a mobile phone, there's the "hand effect" which means the antenna is de-tuned when a (human) hand is nearby, holding the phone. You would still want the antenna to receive the signals so it should not be de-tuned too much by the presence of a hand.

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