On Sun, 2 May 2010 21:44:30 -0500, (Richard
Harrison) wrote:

Jeff Liebermann wrote:
"In my never humble opinion, there`s no way to provide any form of
diversity reception improvement with a single antenna, unless one also
uses two feeds, going to different receivers, and ending in either a
decision awitch or an intelligent combiner."

That is my experience too. Space diversity requires 2 or more antennas
and receivers. One antenna can serve separate receivers which are
connected to cross-polarized feeds using a single reflector for
polarization diversity.
Or, multiple receivers can be used on a single receiving antenna, but
transmission of more than one copy of the desired signal is required,
This is how frequency diversity is usually achieved. Two copies of the
same program may be modulated on the same carrier if it is shown that
the medium treats the sidebands differently so that when one is treated
badly the other may be solid. I`ve seen this done with selection of
upper or lower sideband from a double sideband transmission.

Best regards, Richard harrison, KB5WZI

Thanks. It's an unusual experience when someone actually agrees with
me.

Part of the problem is that HF and microwave diversity have different
purposes and therefore different methods. I'll try to describe some
of these (until the epoxy dries and is safe to handle).

For example, the common Wi-Fi 2.4Ghz access point, uses diversity to
mitigate the effects of frequency selective fading. With two
antennas, one receiver, and a PIN diode switch, the access point
normally has one MAIN antenna selected. However, when the error rate
climbs to the point where the MAIN antenna is hearing garbage, the PIN
switch selects the AUX antenna in the hope of an improvement. With
frequency selective fading, the MAIN antenna could easily be sitting
at a location, where the direct and incident paths from the client
radio are 180 degrees otto phase, and therefore would cancel. By
switching to the AUX antenna, the assumption is that it is not located
in a place where the signals cancel.
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml
http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16501888
http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16500279

For VHF/UHF, a form of diversity that is very commonly used is a
receiver voting system. These are heavily used by municipal services
to cover wide areas with HT's. The HT can easily hear a single
central dispatch transmitter, but the return TX power is limited,
requiring multiple receivers at difference geographic locations to
cover a city or county. For such systems, remote receivers are
located at various locations. The backhaul returns the audio and data
to a central location, where a voting system equalizes the backhaul
delays, determines the best SNR, and provides the dispatcher with the
best possible receiver audio or data. There are various patented
schemes to make the SNR selection. While not normally considered a
diversity reception system, I consider it to be a form of diversity.
http://www.repeater-builder.com/tech-info/votingcomparators.html

For HF, the problem is fading caused by atmospheric and ionospheric
phenomenon. There are several types of fades (flat, frequency
selective, multipath, Faraday rotation polarization change,
absorptive, fast, slow, etc). Some of these are detailed in:
http://en.wikipedia.org/wiki/Fading
The assumption is that for most (not all) types of HF fading,
geographic separation of the antennas will result in one of the two
antennas being in a location where the fade is minimal. In order to
utilize this advantage, some manner of voting system needs to be
implemented to decide which antenna is best. This is usually done
with two receivers, but can be done with a single receiver and an
antenna switch, if one is willing to tolerate some data loss when the
receiver is switched to the wrong antenna.
http://www.navy-radio.com/rcvr-div.htm
(Also search for "dual diversity HF reception")

Locating the two HF antennas at a single location has some benefits
when dealing with polarization diversity, but is generally a loser
when dealing with most of the others, where both antennas (and both
polarizations) are likely to simultaneously experience the same fade
mechanism. For such contrivances, I suspect it might be equally
effective to setup the HF antenna for circular polarization.

Ok... the epoxy is sorta dry.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558