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I don't know if it's right place to ask it, but I searched google and books the answer I get seems to be like resonant antenna have small bandwidth and have standing wave instead non resonant antennas have wider bandwidth and travelling waves. What I don't get that how should it affect my choice of choosing one over other for a particular situation, like what I lose when I choose one over other and what I gain.

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A non-electrically-resonant antenna is when the antenna is used at its optimum "natural" frequency. Example: a half-wave dipole where the incoming signal wavelength corresponds to twice the antenna length. In these circumstances, the dipole appears as a resistance fairly close to 74 ohms and, either side of its "natural" resonant frequency, it might look a little capacitive or a little inductive but, the main component of its impedance is resistance. This makes it difficult to directly electrically tune to gain any benefit and, because of this, it has a naturally "decent" bandwidth.

Another example is the quarter-wave monopole antenna used on FM radios to receive 88 to 108 MHz broadcast stations. These are non-electrically-resonant and have a decent bandwidth.

A good example of an electrically-resonant antenna is the old-fashioned crystal radio antenna. Typically it's a monopole but, because it's being used to receive transmissions in the 1 MHz or lower part of the spectrum, it's physical length is well-short of a quarter wavelength and, because of this, its dominant impedance is capacitive. The received signal voltage is also significantly smaller than a proper quarter-wave monopole but, because crystal radios use a tuned inductor, the arrangement of short-antenna (dominated by capacitive reactance) and the tuning inductor magnify the low signal voltage significantly. Together they are an electrically-resonant circuit. They can be "selective" enough to tune across lower the frequency bands and receive each transmission without much co-channel interference.

The decision to choose one over the other is usually down to accommodating the space required for the antenna. If there isn't much room to have a full-length "natural" antenna, then you will choose a "short" antenna and possibly implement a direct tuning regime to select the desired station.

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  • \$\begingroup\$ Sir I got what you emphasised but how do I compare it with a not resonant antenna . \$\endgroup\$ – SUNITA GUPTA Sep 27 '20 at 13:00
  • \$\begingroup\$ I covered both in my answer. First: non-electrically resonant and then below that the electrically-resonant antenna @SUNITAGUPTA \$\endgroup\$ – Andy aka Sep 27 '20 at 13:03
  • \$\begingroup\$ Sir I pardon to say but from "A good example..... " i.e. in third para didn't you said that how a not resonant antenna can also be made highly tuned. \$\endgroup\$ – SUNITA GUPTA Sep 27 '20 at 13:11
  • \$\begingroup\$ @SUNITAGUPTA - OK, I've added a phrase in brackets after "short-antenna" : (dominated by capacitive reactance) - does this now make more sense? Do you understand that a capacitor and inductor used together can make a highly resonant circuit? \$\endgroup\$ – Andy aka Sep 27 '20 at 13:18
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    \$\begingroup\$ These things are barely affected and remember that this is a question and answer site. If you now want to explore how an antenna might subtly change its characteristics based on loading impedance then ask a new question. I walk away from questions that evolve excessively because it wastes too much of my time. It's just a nudge to remember what this site is about - it's not a forum or talking shop. \$\endgroup\$ – Andy aka Sep 27 '20 at 14:29

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