Topband: beverages: Diversity / Phasing

[Michael Tope]

> Hi Tom,
>
> It seems like people are using the term "diversity" incorrectly. I don't
> have an authoritative definition. It would be interesting to know what
> people think diversity is. I can think of a few things which are NOT
> diversity. Adding the RF from two or more antennas (perhaps using
> combiners, phasing lines, variable attenuators, or phase shifting
> networks) is a "phased array" and is not diversity. It is my
> understanding that "diversity" choses the signal from the antenna which
> has the best S/N ratio or bit error rate. This is often done using
> separate receivers AFTER the signals have been demodulated and are
> audio, or a data stream. If some kind of control is selecting the best
> antenna going to a single receiver, it might also be called diversity.
> In microwave communications the terms "space diversity" and "frequency
> diversity" are used. Space diversity uses two antennas, two receivers
> and some sort of "voting" system to select the better signal. Frequency
> diversity requires all of that and two transmitters at the other end on
> two frequencies. If the "selection" of best signal is done in the
> operators head, using separate signals in each ear, maybe that is also a
> form of diversity. Perhaps a phased array with some kind of control loop
> dynamically adjusting the relative phase and amplitude from the two or
> more antennas would also be a correct application of the word "diversity"?
>
> I would be interested to know what is the real defintion of the word
> diversity in the context of RF communications?
>
> Ken N6KB


Just to add to what Ken has said, another key distinction
between phased arrays and diversity systems is correlation.
Phased arrays rely on combining correlated signal to increase
the overall "clear sky" SNR. Diversity systems rely on creating
redundancy by capturing signals which are "uncorrelated" with
respect to whatever system degradation the designer is trying to
overcome. For satellite systems at microwave frequencies, this
degradation is typically atmospheric effects like rain induced
fades. At HF frequencies, the degradation that is of greatest
concern is typically ionospheric induced fading.  Thus to achieve
diversity at HF, one needs antennas that are far enough apart
(in terms of distance or polarization) such that the fading effects
in the two respective transmission paths aren't strongly correlated.
Otherwise, both links will fade simultaneously. If the two antennas
get too close together (in either distance or polarization), then
"diversity system" simply becomes a phased array that uses
baseband combining.

In fact, just having two different receive antennas and an A/B
switch is a very simple form of diversity reception. Sometimes
my 80 meter dipole hears better than my trapped vertical, and
at other times, the opposite is true. My brain in conjunction
with the antenna switch is the voting system. This crude
system increases the probability of link availability in the face
of degradations which don't always correlate between the
two receive antennas (local noise sources, polarization match,
angle of arrival match).

73 de Mike, W4EF...........