One may wonder whether the reasonably widespread adoption of 1.4 MHz as a final IF for professional upconversion HF receivers, apparently from 1969, was in any way influenced by the fact that Rees Mace had used this number as a 1st IF for its CAT receiver, designed to an admiralty specification. By the way, that receiver was described in Wireless World (WW) 1954 July, p.333ff.
As previously mentioned, Redifon, with its R550, announced in WW 1969 March, p.136, looks to have been the first upconversion model to use the 1.4 kHz final IF, and soon thereafter this number was supported by the filter makers. Redifon was a major supplier of marine HF receivers, and perhaps unusually during the valve era, used common designs for marine and general-purpose models, whereas for example IMR and Marconi designed dedicated marine HF receivers. With Redifon, the commonality goes back at least to the R50, which was advertised at least as early as 1947 (WW 1947 November p.41), and which was described in WW 1949 July p.251ff. The marine version of the R50 was the R50M; the marine version of the R550 was the R551, announced in WW 1970 January p.41. So Redifon would have had marine applications in mind when it was designing the R550/R551, and like Rees Mace, would have wanted an IF that was outside of (below) the upper MF marine band in which vessels might transmit.
I don’t think that Racal was much concerned about the marine market – although it certainly had sold its RA17 receiver into marine applications, albeit for ancillary purposes. Rather it said that the choice of 1.4 MHz as final IF for its RA1772 was due to the ready availability of suitable filters; an example of using an established and generally suitable number rather than starting from scratch.
More-or-less a contemporary of the Redifon R550 was the Eddystone EC958, announced in WW 1969 July p.322 (and I understand earlier in other trade journals). Whereas previously Eddystone had stayed away from the marine HF receiver market (except for designing and initially building the IMR54), the EC958 was designed with marine applications in mind. The marine version was the EC958/5, also rebranded as the Marconi Nebula, which thus competed with Marconi’s own Apollo design. Eddystone had opted for a more traditional design rather than upconversion, albeit one that incorporated what was more-or-less a narrow-band Wadley loop. It was triple conversion with IFs of 1235 to 1335 kHz (tuneable), 250 kHz and 100 kHz. The tuneable 1st IF was well established in Eddystone practice, having been used in the valved 830 series, where the range was 1250 to 1450 kHz, 1350 kHz nominal, evidently chosen to allow its use on incoming frequencies down to 1.5 MHz.
In the case of the EC958, the tuneable IF was used on incoming frequencies down to 1.6 MHz. It was 1335 kHz nominal, tuneable in the downwards direction only. It looks as if it may have been a carryover from the 830; perhaps the 5 kHz offset was something to do with the Wadley Loop implementation. Anyway, it was in the right place, that is below 1.6 MHz, for the marine application. Also probably connected with the Wadley Loop was the need for an intermediate IF of 250 kHz, whereas the 830 had converted directly to the 100 kHz final IF from the 1st 1350 kHz (nominal) IF. 250 kHz looks to have been an ad hoc number, perhaps chosen on the one hand to keep the 2nd mixer local oscillator injection frequency – which was on the high side - below 1.6 kHz, and on the other to keep any images from the tuneable 1st IF far enough away, something that may have been more difficult with conversion from 1335 kHz (nominal) direct to 100 kHz.
But even 250 kHz had a precedent. Recently I read an article about the use of SSB in aircraft HF communications, in WW 1958 October, p.460ff. The transmitter and receiver block schematics shown had IFs of 250 kHz. I suspect that these block schematics were representative of Marconi equipment, given that the article was written by a Marconi staffer. Whether 250 kHz was a norm in aircraft HF practice I don’t know. The only other reference point I have is that of the earlier Marconi AD108 aircraft HF receiver, which was single-conversion with a 600 kHz IF.
All of the above-mentioned WW items and articles are available at: http://www.americanradiohistory.com/Wir ... gazine.htm