UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

turretslug · 18th Nov 2017, 9:53 pm

In the UK at around that time, RSA on 25.79 MHz, RFI on 25.82 MHz (the French broadcaster, not the interference!) and VOA on 26.04 MHz were all strong, clear and dependable presences during the day, with a few others appearing more rarely and faintly. Further into the formal VHF definition between 30 and 32 MHz, harmonics of 19m stations could be found, usually faint but often clear. The strength and readability of the fundamental having little relationship to that of the harmonic, presumably propagation from site and the harmonic suppression (or otherwise) of sender and aerial systems having greater bearing.

Re. circuit techniques, I can't help thinking that VHF considerations could be said to come into play by 20 MHz or so- many HF receivers that followed traditional LF/MF/HF architecture of multiple band-switching around a single set of active devices looked to be pushing things by this point with stray capacity, stray inductance and poor L/C ratio of a system centred around a large-value, wide-ratio gang capacitor all presenting design headaches. The very few turns of the highest frequency band inductors set against a mass of band-change wiring links shows that this format was on its edge. After this point, the idea of dedicated front-end modules for a particular frequency span with hard-wired tuning elements and active devices looks wise- the wartime GEE receivers with a choice of 4 plug-in modules spanning around 20-85 MHz being a classic example. Having said that, turret formats could allow a single set of active devices to give at least adequate performance with choice of several sets of tuned circuits to 200MHz+ but frequently with bespoke expense and bulk, the (I think) US-originated Band I/III TV tuners being a mass-produced exception. Perhaps, as you say, circuit technique boundaries could be said to be more like 20-200MHz for VHF with UHF above this point- thus we have the 225-400 MHz "UHF" airband. A further delineation might be where stripline techniques give way to cavity techniques.

18th Nov 2017, 9:53 pm	#25
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,395	Re: VHF or UHF? In the UK at around that time, RSA on 25.79 MHz, RFI on 25.82 MHz (the French broadcaster, not the interference!) and VOA on 26.04 MHz were all strong, clear and dependable presences during the day, with a few others appearing more rarely and faintly. Further into the formal VHF definition between 30 and 32 MHz, harmonics of 19m stations could be found, usually faint but often clear. The strength and readability of the fundamental having little relationship to that of the harmonic, presumably propagation from site and the harmonic suppression (or otherwise) of sender and aerial systems having greater bearing. Re. circuit techniques, I can't help thinking that VHF considerations could be said to come into play by 20 MHz or so- many HF receivers that followed traditional LF/MF/HF architecture of multiple band-switching around a single set of active devices looked to be pushing things by this point with stray capacity, stray inductance and poor L/C ratio of a system centred around a large-value, wide-ratio gang capacitor all presenting design headaches. The very few turns of the highest frequency band inductors set against a mass of band-change wiring links shows that this format was on its edge. After this point, the idea of dedicated front-end modules for a particular frequency span with hard-wired tuning elements and active devices looks wise- the wartime GEE receivers with a choice of 4 plug-in modules spanning around 20-85 MHz being a classic example. Having said that, turret formats could allow a single set of active devices to give at least adequate performance with choice of several sets of tuned circuits to 200MHz+ but frequently with bespoke expense and bulk, the (I think) US-originated Band I/III TV tuners being a mass-produced exception. Perhaps, as you say, circuit technique boundaries could be said to be more like 20-200MHz for VHF with UHF above this point- thus we have the 225-400 MHz "UHF" airband. A further delineation might be where stripline techniques give way to cavity techniques.