From what I understand about antennas, they have a certain impedance. At the antenna's resonant frequency, the capacitive and inductive reactances cancel and the reactive part of the antenna impedance is 0 ohms. However, there is also something called the radiation resistance that still exists at resonance (for a dipole, it is commonly 71 ohms). How can this be? At resonance, doesn't the feedpoint of the antenna look like a short circuit due to resonance? When the antenna is matched to a transmission line, do you use the reactance or the radiation resistance?
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At resonance the imaginary part is zero, yes. Why is it surprising that there's still a real part?
Imagine a series (or parallel) RLC circuit. At the resonant frequency, the L and C impedance are equal and opposite. But you still have the R.
Another thing - the resistance that's left is not just radiation resistance, some of it is loss resistance - real resistive loss in the wires, etc.
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\$\begingroup\$ So then why does the radiation resistance depend on the antenna type, and not just resistive losses in the coax/wire? \$\endgroup\$– crocboyCommented Aug 3, 2015 at 18:45
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\$\begingroup\$ It's just the physics of the antenna. One explanation on a dipole is quite simple. A dipole has a fundamental characteristic mode, like a guitar string. If you wiggle a guitar string in the middle, where it moves a lot, then you will have to move a long distance, but not push very hard. If you hold it near the end, to get the same volume you'll only need to push a small distance, but with a lot more force. It's the same (but opposite) with a dipole. Feed it in the middle, 70 ohms. Feed it near the end, much higher impedance. \$\endgroup\$– tomnexusCommented Aug 3, 2015 at 18:55
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1\$\begingroup\$ If the equivalent circuit of the antenna was purely C & L, then it would never (permanently) remove any energy from the circuit. Since energy is conserved, this would mean that it couldn't radiate any energy to be detected by a receiving antenna, and it wouldn't be very useful. \$\endgroup\$ Commented Aug 4, 2015 at 1:53
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2\$\begingroup\$ It's worth remembering that if radiation resistance went to 0 at resonance, the antenna wouldn't make much of an antenna! \$\endgroup\$– hobbsCommented Apr 5, 2016 at 22:02