Hi David:
Thanks for that.
The BBC situation was described in BBC Engineering #84, 1970 October (
http://downloads.bbc.co.uk/rd/pubs/a...neering_84.pdf) which noted that the BBC was expanding the use of HF SSB program links, having started in 1964, with use of ISB receivers for HF AM reception predating that. But the article deals mainly with the SSB transmission, and does not mention the receivers that were in use.
I imagine that the ISB receivers were used both to provide sideband diversity when receiving AM broadcasts, and perhaps more so to mitigate selective fading distortion. As far as I know the BBC did not use ISB program links, although the VOA did (and perhaps still does). Way back then sometimes I used to listen to them using the Liniplex F2.
The Quartz Hill operation is described here:
http://www.zl6qh.com/000468.html. From the 1960s, the primary receivers for the BBC World Service were of the Marconi HR21 ISB type. To quote from that website:
“Two Marconi HR21 independent-sideband HF receivers were the mainstay of the station’s receiving equipment. They were used primarily for maintaining a round-the-clock feed of the BBC World Service to Broadcasting House. Designed for use on international radio-telephone circuits, they were purchased in the early sixties when the advantages of using the sideband reception technique for short-wave broadcasts were finally recognised. These were the ability to select the sideband with the least interference (especially useful in eliminating heterodynes or whistles), and a substantial reduction in the audio distortion caused by selective fading. Each HR21 had separate 6kHz bandpass filters and amplifiers for the upper and lower sideband, and its two audio output channels were fed to faders on the control panel, thus enabling a smooth changeover to the better channel. Each receiver employed about 65 valves (vacuum tubes), and comprised several separate units occupying a complete rack about two metres high. They were relatively difficult to tune and to service, but their shortwave reception quality was superior to that of the nine GEC BRT-402 general-coverage receivers that continued to be used for medium-frequency and less important short-wave reception.”
I drove past the Quartz Hill site a few times back in the old days when it was fully operational, and once I think up the hill to the gate, but never saw inside it. It certainly had a bleak, remote feel about it, even though it was maybe only 40 minutes from the Wellington suburbs.
Cheers,