I was wondering what it was like a hundred years ago in Amateur Radio...
So I asked Google - got a lot of guff. But I did come across some magazines from the early twentieth century concerning 'wireless'. The development of 'Radio' during the twenties was of course intent on picking up distant AM transmitters for the entertainment 'content'.
The one article that stood out was - no - is on the Minnesota University web site. This led me to read the article entitled "Is Radio at a Standstill?" Ha! We know it wasn't but it was intriguing to see what the people of the time thought about the new technology.
"that we need not look for any revolutionary improvements in radio at the present time. The chances are against any invention that will entirely upset the radio industry" - quote. We all have 20-20 hindsight.
"One of the great troubles in the United States at the present time is the heterodyning between different stations nearly on the same wave-lengths.[1] " - This quote is especially for the RAE students. Which links to the footnotes. "In Fessenden’s case, controlled heterodyning could boost a signal. But with the number of broadcast stations crowding the airwaves in the 1920s, natural heterodyning occurred when two stations operating at closely spaced frequencies produced unwanted interference for each of the signals. For more on the heterodyne principle, see The Dynamophone in this book."
This experimentation is still going on a hundred years later. Though I think no patents in the numbers like then:- "During the entire year of 1925 over nine hundred radio patents were issued by the Patent Office; and during the first six months of 1926, almost six hundred radio patents have been issued."
In my early career I was introduced at college and work to all sorts of 'new applications or components'. It was after all the 'golden years' of electronics and communications. New Integrated circuits and improved transistors were released every month. With adverts in the technical magazines of the time. Which we as young people were encouraged to read before getting down to the work of the day.
I went looking a couple of weeks ago for a 'current time' [2020s] Aircraft Transmitter specification. This would have been a simple AM Transceiver with a moderate output power [typically 10 Watts] and sensitivity. I got a surprise as the 'modulator' was described as a 'Class D' type. Providing more 'punch' in the voice modulation. Achieving 70% amplitude modulation and voice compression without distortion. [No not music quality!]
In 1926 the valves sets had an 'A' and a 'B' battery. A for the high tension voltage - 90 Volts plus maybe more. The 'B' battery was the filament supply. A few volts later 6 to 12 Volts depending on valve design. These were heavy and non-portable so the invention of a mains power supply was truly an innovation.
Shades of 'Load Shedding'
The later passage tells of the battery suppliers providing connections for the lighting circuit. Remember electric light in the home was a 'new thing' and cost more than a few oil lamps.
So I had to refresh my understanding of 'Class D'. Then it was slap the forehead time. I remember Clive Sinclair's audio amplifiers which used 'Class D' putting out a few watts. Advertised as 10 Watts per channel. Over the years this type of output stage has developed to a reasonably efficient method for power output. Be grateful it provides the 50Hz from your inverter without suffering from 'issues' as the Sinclair Amp did in the 60s.
At its basis it is a Pulse Width Modulation output stage. Providing the sine wave as the average of a switching waveform. [A QRM generator of note when unfiltered!] But it converts d.c. to a.c. reliably and efficiently.
- Raspberry Pi's core micro (BCM2835) doesn't have any audio outputs per se. Yes, it has the HDMI audio output, but the stereo 3.5mm jack outputs are fed using a pair of PWM generators rather than audio quality DACs. [Raspberry Pi - pages]
- The Sinclair X-10, introduced in 1964 by Sinclair Radionics, was the world’s first commercially available pulse-width modulation (PWM) / Class-D audio amplifier sold to the general public.
Designed by Clive Sinclair and Gordon Edge, it used high-speed switching instead of linear operation to achieve high efficiency. [Wikipedia]
The Sinclair amplifier used a set of low frequency transistors to switch the output supply voltage at a super-sonic frequency. Theoretically providing the real power output. Others have tried and failed to provide an efficient audio power amplifier over the years since then. Recently radio amateur designs have with high speed clocked processors managed to achieve AM or SSB modulation of an r.f. signal at high powers. Most new 'linears' use PWM to achieve 1 to 2 kilo Watts output.
Check the RSGB's recent conference videos on YouTube for more information.
[a few links:-
https://youtu.be/gXx5sa339i8?si=GyU1xxTh2KEqTPjm
https://www.youtube.com/live/6FheeAv3src?si=VU8WNCsAKpXkpRMU
https://youtu.be/21uE341GtGU?si=ccCg6WwPTbVYZrKR
]
From the 'inspiration' :-
The Perversity of Things: Hugo Gernsback on Media, Tinkering, and Scientifiction
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