[TowerTalk] Directivity vs gain vs S/N

[Tom Rauch]

> What's an FOM?

Figure of merit.

> And, for us non-engineers, bottom line, can we still
safely say more
> efficient antennas with more gain are going to tend to be
better, in
> general, than less efficient antennas with less gain?

For transmitting. Not for receiving.

> Realizing that, though it seems intuitive that more
efficient is ALWAYS
> better than less efficient, that there are exceptions,
such as low noise
> receiving antennas that may be less efficient.

That's right. When receiving, antenna system gain doesn't
matter until the system's internal noise starts to affect
S/N ratio. Normally external noise limits S/N. It's all a
response issue caused by pattern (which includes
polarization effects) and directions.

> Resulting in the conclusion we often hear, "You can't have
too many
> antennas," meaning it's nice to be able to switch to
different antennas to
> find the best one in the circumstance.

That's right too, and the best S/N may not be on the one
that has the best signal level.

That's why we can't assign a FOM to receiving based solely
on antenna gain.

73 Tom