I've heard of a magic "resonant frequency" that antennas are designed for, what happens if you transmit a frequency through an antenna that does not have the same resonant frequency?
- Part of your output power will be ultimately wasted as heat in the antenna, feedline, matching elements, transmitter etc. (less range)
- Energy will "bounce back" from the antenna to your transceiver. May potentially damage it.
- Energy "bounced" from the antenna may cause intermodulation distortion in your transmitter. The transmitter will become a poor mixer (the device may not be compliant/legal, may transmit undesired frequencies).
- When receiving (if the same antenna is used for RX/TX) you will "hear" other frequencies better than the one you are using for communication (worse sensitivity, higher noise level).
What happens if you send a broadband signal through a high pass filter? It gets attenuated. Same thing for an antenna more or less.
Here is a plot of what the impedance of a resonant antenna looks like. If you transmit signals in the capacative or inductive region, the current going into the antenna will 'see' more impedance and the signal will either reflect back to the receiver or get attenuated.
So that may be a good thing if you want that signal on that particular frequency to not get transmitted. Or a bad thing if you have a signal that needs to be transmitted with an antenna with the wrong impedance.
Source: radio electronics
Ideally, your antenna system (consisting of the feed line, the antenna, other components in line) will all present the same impedance at a given frequency; resonance is one way to potentially achieve this, but NOT a requirement.
The typical impedance values are 52 or 75 ohms in an unbalanced system (using coaxial feed line to the antenna) but with a balanced system may be 300-400 ohms...I've seen others. In a perfect world, you'd want the TX output design impedance to match the characteristic impedance of the feed-line which would also match the feedpoint impedance at the antenna.
System losses stay low under these conditions.
In general, a properly designed antenna will present this feedpoint impedance (say 52 or 75 ohms) at or near resonance and depending on the type will vary as you move to either side of resonance. As you move from resonance, inductive and capacitive reactance will exist and change the feedpoint impedance. (for example, a 1/2 wave dipole antenna presents about 73 ohms, Real, at resonance. This is handy. If you shorten it, it looks capacitive, if you lengthen it, it appears inductive.)
So if you use antenna designed for a given frequency on another frequency, and don't change anything in the feedline or other components, you'll likely end up with an impedance mismatch between the feedpoint of the antenna and the feedline, which will also result in a mismatch between the transmitter and the feedline (transformed according to the length of the feedline).
Depending on the degree of the mismatch and the type of feed-line, this condition will result in increased system loss -- meaning increased heat dissipated by the antenna system components (like the feedline), thus taking away from electromagnetic energy actually radiated.
Does this mean that an antenna resonant on one frequency can't be used on another? Nope. It just means that in order to maintain, good efficiency the complex impedance that differs from the nominal feedline Z needs to be transformed via an antenna tuner or a matching network to the feedline Z to ultimately present the transmitter with a load it is designed to work with. This is commonly done in order to use a non-resonant antenna efficiently and effectively.
Resonance is handy, but not necessary if you employ proper matching at the right place in the system.