Dual-diversity reception is a configuration which allows the operator to use two receivers, connected to separate antennas but tuned to precisely the same frequency, to select the signal with the best S/N for uninterrupted reception in the presence of selective fading etc. Its main use is for the reception of RTTY or other data modes, but it also works with SSB signals.
The two antennas are physically separated and/or pointed in different directions (space diversity), or vertically and horizontally polarised respectively (polarisation diversity). This antenna arrangement increases the statistical probability that a selective fade will only affect one, but not both receiver systems. The receiver audio outputs are fed to a circuit known as a baseband diversity combiner, which automatically switches the signal with the best S/N ratio to the line output.
Usually, the mixers of both receivers are fed from a common local oscillator (LO) to ensure phase coherency between the two received signals. From a first look at the IC-7800 user manual, it is apparent that the IC-7800 does not offer this LO routing feature - at least not for the 1st LO. The 2nd LO sources feeding the A and B second mixers (I/Q down-converter) are derived from a 40 MHz source which is common to both receivers. (Refer to simplified block diagram of IC-7800 front end.)
All the DDS frequency sources in the IC-7800 are clocked from the precision (5 x 10-8) internal reference oscillator or an external reference. I believe that the DDS LO system in the IC-7800 is so stable that equalising main and sub frequencies by pressing the [M=S] key should be sufficient to allow good dual-diversity operation. The main and sub (A and B) receiver audio outputs are fed to separate audio channels. Thus, the operator can listen to the main and sub receivers on stereo headphones. An external diversity combiner can also be connected.
In a typical fixed-station operation, the two antennas used can be two horizontal dipoles 90 degrees apart, or a horizontal dipole and a vertical ground-plane. Diversity reception is probably of greatest value on 7 MHz and below.
Q: Do I understand that you would not be able to tune the band with diversity active, but would need to press the [M=S] button to put both receivers on the same frequency?
A: Yes, this would be a limitation - but not a serious one. The main purpose of dual-diversity reception is to be able to select the receive channel with the best S/N, as a means of offsetting signal-quality degradation due to selective fading. Thus, you would only need the diversity function once you had tuned in the desired station. In practice, you would tune the main receiver to the desired frequency, then press [M=S]. In the commercial HF installations with which I was involved in the past, the diversity combiner was enabled only once the receivers had been correctly tuned to the frequency of the distant station.
Google search on "dual-diversity HF reception"
The Racal MA168B IF Diversity Switch (on Keith's Vintage Racal Site)
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Last updated: 06/15/2018