Exceptional tuning ranges in vintage broadcast receivers

[Julesomega]

Without up-conversion, tuning ranges are limited by the ratio of maximum tuning gang capacitance to stray capacitance, usually to not much more than 10:1 giving a maximum tuning range around 3.3:1. This suited the original "Broadcast" band, or more likely dictated it, of about 520 - 1620 kHz. The IF is added to the LO tuning range so that >3.3:1 is easily attained but at the expense of compromised tracking of the signal frequency circuits.

I was thinking about this while watching the latest of Chris Fletcher's AllTheGear videos, where he has a penchant for buying immaculately preserved vintage radios and gently tickling them back to peak performance and cosmetic presentation

In the video he has been so lucky as to buy an HMV 1121 (IF = 465kHz) which has two SW bands including one covering the 'Trawler Band' with a range of 50 - 187 metres. This equates to a tuning ratio of 3.75:1 - is that a record? AllTheGear YouTube

[turretslug]

Quite a few "export" radios used decadic capacitance swing to give SW coverage of 2.3 to 7 and 7 to 23MHz, having a pleasing reciprocity to it and covering the principal broadcasting bands (not sure when 11m came into use but it was never very busy and something of a hostage to the sunspot cycle). I guess that 10:1 capacitance ratio represented about the limit of what was easily and repeatedly possible without getting finnicky about component and wiring layout and consistency of active device behaviour.

3.75:1 tuning ratio might be pretty much the record- apparently there was a version of the PCR that stretched from c.6 to 22MHz, most (ISTR) giving the "standard" 5.8 to 18ish MHz found in a great many domestic sets. Bush's EU4 boasted 540-1875kHz MW coverage-a lower ratio, but not bad considering the inevitable extra self-capacitance of lower-frequency coils.

Marconi's CR300 and Atalanta receivers offered LF ranges of 15-85kHz and 25-100kHz respectively but with RC rather than LC filtering, so sort of cheating in the context!

Maybe Synchrodyne can shed some light on this question.

[Skywave]

During the early-1960s *, I had a Mc. Michael 'woodie' that apart from the traditional LW and MW bands coverage, had two SW bands, (coverage from approx. memory), one tuning from 3.0 MHz to 10 MHz, the other 10 MHz to 31 MHz. (That last figure is for certain ) I can't recall the model number, but it had a tunable RF amp.; a 4-gang tuning condenser; 465 kHz I.F. and, I think, two I.F. amplifier stages. I distinctly recall listening to American police radios on some occasions. (Presumably when propagation was favourable). It must have been an expensive radio in its day.

*During that period, I used to collect old radios. However, during the early 1970s, I discovered communications-type radios, so my interest in 'ordinary' domestic radios faded away.

Al. / Apr. 27th.

[Synchrodyne]

Originally Posted by turretslug

Maybe Synchrodyne can shed some light on this question.

Regrettably, I can’t add anything material. I have long been under the impression that the maximum MF and HF tuning range that in practice could be covered with a regular variable capacity was nominally √10:1, say 3:1 in round terms. The widest I have actually encountered “in the flesh” were 524 – 1750 kHz MW on the Bush EU24 (3.34:1) and 510 – 1620 kHz MW on the Quad AMII (3.18:1), also 5.8 - 18.5 MHz SW2 on the same unit (3.19:1). And I don’t think that I have seen any range materially greater on specifications and data sheets that I have seen.

On that basis, I’d say that the HMV 1121, with a 3.75:1 range, is probably a leading contender for the greatest range with tracked RF and oscillator tuning.

Re the Atalanta (MIMCO 2207C, MWT NS702), in Marconi’s own words:

>>>>>>>>>>>>

The single frequency changer technique used on ranges 1, 3, 4 and 5, with an I.F. of 85 kc/s, cannot be employed on range 2 since the frequency of 85 kc/s is included in this band. Accordingly a double frequency change, as used on ranges 6, 7, 8, 9 and 10, is also provided for range 2.

The fact that first I.F. is thus fixed at 700 kc/s would give rise to tracking difficulties if an attempt were made to tune both the signal frequency and oscillator circuits by means of ganged capacitors. This can be seen from the following table which shows that the required signal frequency change is more than three times the corresponding oscillator frequency change.

I.F. = 700 kc/s
Signal frequency 25 – 100 kc/s fmax/fmin = 4
Osc. frequency 725 – 800 kc/s fmax/fmin = 1.1

For this wave-range, therefore, the signal frequency circuits are untuned and consist of an untuned transformer input with high and low pass filters resulting in a broad band R.F. circuit covering the whole range 25 kc/s to 100 kc/s.

Frequency selection in this range is provided solely by the manual tuning of the oscillator.

>>>>>>>>>>

I think that Marconi could “get away” with that arrangement as to the best of my understanding, the 25 to 100 kHz band was not part pf the GPO specification, whereas 15 to 25 kHz was. The GPO appeared to have quite tight RF selectivity requirements for marine main receivers, whether directly or indirectly specified.


Cheers,