[Elecraft] Diversity modes

[Fred Jensen]

On 8/8/2015 6:23 AM, Bill Frantz wrote:

> The other interesting diversity mode I know of is the way the costal
> commercial communication radio stations worked in the 1930's and later.
> They were using CW to send messages across the oceans. Each station
> transmitted on 3 separate bands. The receivers listened to all 3 bands
> and routed the "best" signal to the operator. (I don't know how they
> determined the best signal using 1930s technology.) Obviously a two
> channel version could be assembled using a K3 with a sub-receiver.

Well, at least in 56-57, it wasn't exactly diversity [I worked at a 
coastal marine station for 11 months during that period when I was a HS 
senior].  Coast stations [three letter calls] were assigned specific 
ITU-defined channels [freqs] in each of the maritime bands, both MF and 
HF.  Everyone could use 500 Kcs.

A coast station would transmit its "wheel" on all of its assigned HF 
frequencies, except those for which there was no propagation, such as 22 
Mcs at night.  The wheel might have comprised:

VVV VVV VVV DE KOK KOK KOK QTC <list of ships for which KOK was holding 
traffic> QSX <list of MF/HF maritime bands on which KOK was standing 
watch for calls> AR.

The QSX was followed by the band designators only, such as "4 6 8 12", 
the ship operator had a printed directory of the assigned frequencies in 
each band.  HF channels were full-duplex, coast station on its assigned 
frequency, ship on the second frequency in that band paired with the 
coast station.

When the operator afloat opened his watch, he would start the log and 
listen for his company's station wheel.  If his call was in the list, he 
called the station, the traffic was passed, his call was removed from 
the wheel [usually paper tape] and the wheel continued.  In most [but 
not all] cases, the operators on the ships worked for the same companies 
that owned the various coast stations.

So, while multiple frequencies were involved in this process, the ship 
was almost always on only one, so it wasn't really diversity reception 
in the technical sense.  500 Kc [600 meters, the "Holy Frequency"] was a 
huge, simplex, world-wide party-line.
>
> A system where you can automatically determine which receiver has the
> better signal, could be useful for RTTY. Measuring the difference in
> signal strength of the mark and space signals compared with the signal
> strength half way between them might be a useful technique.

In the mid-60's [while in the US military in SE Asia], we used real 
diversity on multi-channel troposcatter systems.  The AN/MRC-98 was an 
example.  In the 450-550 MHz range, 2-10 KW transmitters to two 9.1 
meter dishes, each with two cross-polarized feed horns.  4 receivers, 2 
each on each of the two frequencies with phase locked local oscillators 
for pair.  60 good quality telephone channels over 400-500 km paths. 
Modulation was FM, the combiner watched the SNR from each RX [which 
varied all over the place], and added the basebands together using all 
analog circuitry for best output SNR.  The two PA's were Eimac 
klystrons, about 1.6 meters long.  Space, polarization, and frequency 
diversity.

It was somewhat mesmerizing to sit in in front of the receiver bank and 
watch the SNR on the single receivers fluctuating wildly while the SNR 
on the baseband would drift very slowly up and down in very small 
increments.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015
- www.cqp.org