[Towertalk] limitations of stacking

[Michael Keane, K1MK]

At 08:05 PM 3/25/02, Tom Rauch wrote:

>Really?

Really. Is it such a radical concept?

>Please explain how two identical-pattern antennas, each lossless
>or with the same percentage of power loss, can have different
>gains.

Because the two antennas would have to possess different radiative 
resistances. Add to that the century and a half of physics subsequent to 
James Clerk Maxwell that backs up radiative resistance as a valid concept 
in the theory of electromagnetic waves.

As an example, the classical elementary dipole (with length << wavelength) 
has a dipolar radiation pattern in the far field; that  pattern is 
independent of the dipole's length. But an elementary dipole also has a 
radiative resistance and therefore a gain that varies with its length 
squared. That's an infinite family of identical-pattern antennas possessing 
differing gains.

>Better yet, point me to a engineering reference that
>describes that effect.

If one of my students was having a lot of difficulty comprehending this 
explanation I'd send him off to review the discussions on radiative 
resistance in a basic E&M text:
Bekefi & Barrett, Electromagnetic Vibrations, Waves, and Radiation
Jackson, Classical Electromagnetism
Kong, Electromagnetic Wave Theory
Landau & Lifshitz, The Classical Theory of Fields
Staelin, Morgenthaler & Kong, Electromagnetic Waves

73,
Mike K1MK


Michael Keane, K1MK
k1mk@alum.mit.edu