Antenna common mode impedance
Antennas need to be well balanced.
Well balanced means appropriate baluns at appropriate places on the feeder and/or appropriate feeder length with appropriate termination.
The common mode impedance of the antenna defines what is appropriate.
Common mode current
When a feeder picks up a noise source, the current propagates as a common mode current.
On a coaxial cable, this current flows on the outside of the outer conductor. On an open line feeder, this current flows on both conductors in the same direction. For the common mode the feeder behaves like a single wire.
Differential mode current
The normal mode for carrying a signal over a feeder is the differential mode.
On a coaxial cable, the current flows inside in opposite directions. On an open line feeder, the current flows also in opposite directions. For the differential mode the feeder behaves like two wires.
Why am I telling this?
Try to imagine the antenna as being a transmission line. In the next figure the dipole and delta loop are used as an example. You get a transmission line when folding both sides of the dipole and when stretching the delta loop.
Dipole and delta loop as a transmission line
The antenna currents can be seen as differential mode currents. The dipole behaves like a ¼ l transformer and the delta loop like a ½ l long transformer. Both give a low impedance at the feed point. So for the differential mode, the transmission line is at the end open circuited for the dipole and short circuited for the delta loop.
For the common mode current, however, both transmission lines are at the end open circuited. The common mode impedance at the feed point is then very low for the dipole, but is very high for the delta loop (transmission line length are the same for the differential and common mode).
Last update: March 2, 2007