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Analog Angle

Antenna Diversity: Blessing, Curse, or Both?

Bill Schweber
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4/28/2016 5:20:06 AM
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One of the most interesting of components
I'm one of those dinosaurs who likes using (or thinking about using) antennas for ancient applications, from MF frequencies on up, often with significant compromises involving size and location.  One of my current projects is to think about an antenna that is efficient over a very wide range of frequencies on HF, in limited space, close to the earth, might be stealthy, and I get to choose whether most of the energy goes up or outwards to the horizon.  Hopeless, I know.  EM simulation is quite illuminating, even if it is not altogether accurate.  Many attempts with the field-solver have resulted in impressive failure, even when attempting something known to "work."  This implies that either the modeling is wrong, or that radio transmission over variable distances in an informal setting (e.g., without a loss budget) may work even when the efficiency is poor, and you don't know (and maybe don't care) how bad it is because it works.

Antennas are not nearly as simple as "just a piece of wire" would lead you to believe.  They have L and C -- to themselves, to the earth and everything else nearby, and to "infinity".  All of it dependent on position on the wire.  They are the epitome of the non-lumped circuit, where even the approximations fail to show how they work.  And there are the E-M fields themselves, which behave so differently in near- and far-field regimes.  Some antennas seem like a coil of wire, coupling just to the magnetic field (or so you might think), but that simplicity fails to show how they really work as E-M transducers.

Can we wrap our heads around Maxwell's equations and truly visualize what they say about what an antenna is doing?

I am fascinated by the variety of antennas.


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