Why do we want high Radiation Resistance in an antenna?

[shedrick94]

I just read that if we have an antenna, then if the radiation resistance in the antenna is small, then the antenna is an inefficient antenna?

This seems somehow counter intuitive to me. Could anyone help explain?

 

[sophiecentaur]

I just read that if we have an antenna, then if the radiation resistance in the antenna is small, then the antenna is an inefficient antenna?

This seems somehow counter intuitive to me. Could anyone help explain?

It's because when you try to match the source to an antenna with a very low radiation resistance, you find that the resistance of any practical matching network (despite your trying to transform to a low value) gives you a source resistance of several Ohms feeding a Radiation Resistance of a small fraction of an Ohm (perhaps) and that means a lot of wasted energy. In the case of a very short monopole and especially a long or medium frequency monopole, the Ground Network ('mat') resistance will be appreciable in comparison.
It's a bit like trying to heat up a block of copper by passing current through it from a normal power supply - it's like a short circuit and all the heat goes in the power supply and the wires.

 

[phyzguy]

The goal of the antenna is to radiate electromagnetic energy. Any electric circuit has resistance because conductors are not perfect. So the circuit basically has these unintended (we say "parasitic") resistances in series with the resistance of the antenna. If we then pass a current through this combined circuit, some power gets dissipated in the parasitic resistances, and some in the antenna. The ratio of the power dissipated in the antenna to the total power dissipated is the efficiency of the circuit. We make this efficiency higher by making the radiation resistance of the antenna greater than the parasitic resistance, ideally much greater.