An antenna is said to be resonant if its input impedance is entirely real, i.e. Zin = R + j*0.
In this case the voltage and current are in phase at the antenna's terminals. This property makes the impedance matching of an antenna to a transmission line and receiver easier, as the imaginary part of the impedance does not need tuned out.
Dear sir
The input impedance cannot be entirely real in practical situation. The antenna has to radiate input power efficiently to the free space for this it was open.
A practical antenna has an impedance that is a function of frequency. We do define resonance as the frequencies where the imaginary part is zero. Most practical antennas will resonate at multiple frequences, for example a 40-meter dipole is resonant near 7 MHz but it is also resonated at 21 MHz.
Having resonance does not necessarily mean that the antenna is useful for transmitting at the frequency. It also depends on the antenna beam power pattern and the ability ot impedance match the transmitter to the antenna.
In most practical situations, the transmitting system is made up of the transmitter, feed line (coax), impedance matching network, and the antenna. The matching network can be located at the antenna (best) or the transmitter (next best). If the frequency of operation changes, the matching network must be adjusted accordingly.
I don't know what you mean by an "open" dipole.
The power to an antenna is the transmitted current squared times the impedance of the antenna so there is a real part and an imaginary part., I^2*(R+jX). Since both I and R vary with frequency, the desirable condition is to maximize the I^2*R product regardless of the value of the imaginary power. In other words, maximum power out of the antenna is not always at resonance because of practical considerations in the transmitter and matching network.
Good luck.
The Resonance means imaginary part of impedence at the frequency is zero and resistive part of antenna at feed point and feed system is same then maximum power transfers from source to antenna and NO POWER reflects. This provides situation to have possibility of maximum radiation provided ideally NO LOSS in antenna.
For maximum radiation this dipole is ideal provided it is to be used only for single frequency
if from "open" you mean at the center, then it is because if it is continuous , then the current will be in same direction. For radiation we should have E and H field. For E field +ve and -ve charge is required. so if the antenna is discontinuous from middle then there will be two polarity along the bisected arm of the antenna and E field can be generated.
But to get an E field the antenna must be terminated at the end . But the question is at what distance? So, say, if for half wave dipole we terminate the antenna at lambda/2, then consider the current pattern will be from +ve end to -ve gradually making a half wave peaking at the center. If there is one whole cycle, then the loop will be completed and E field can propagate.this is when "open" means at the end.
The meaning of ' OPEN' is in a physical term, than let me tell you it is electrically not open. The elements are terminated to space which has an impedance (Intrinsic). Thus it not open
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