mechanical generation of elf/vlf - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2019, 07:13:33 pm
content: https://www.militarytimes.com/news/your-military/2016/12/29/new-darpa-radio-transmitter-could-revolutionize-battlefield-communications/https://www.darpa.mil/attachments/AMEBAProposersDay_FINAL.PDFShake weight?

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2019, 08:03:33 pm
content: so I am thinking to take a spherical magnet, grind the top, insert a 1/8th inch shaft and mount it on a dremelbut how would you modulate it with a carrier given its so difficult to change the momentum? encoder on the equator, optical position detection and two electromagnets in a push-pull? so it would be a dynamic version of the mechanical pendulum control gimmicky systems that are made by mechanical engineering departments (controlled wobble injector is possible descriptor?)

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 27, 2019, 09:05:02 pm
content: Everything old is new again. In 1904, the Alexanderson Alternator was used to mechanically generate HF radio signals. It was an enormous multi-pole rotating machine. Not battlefield portable by any stretch of the imagination, but an example of what they developed before vacuum tubes made more modern oscillators practical.

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2019, 09:48:05 pm
content: Quote from: AG6QR on November 27, 2019, 09:05:02 pmhow do they modulate this ? it looks very difficult.. I would assume they would be FM only?they want this on divers as a communications system.. it needs to be really light. Marketing wank from darpa?

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 27, 2019, 10:49:26 pm
content: Quote from: coppercone2 on November 27, 2019, 09:48:05 pmI wasn't around, but the 1904 system surely used on/off keying to send Morse code. FM hadn't been invented at the time.I don't know about the modern system, but it would seem to me that, if you're generating a carrier via some form of rotating machinery, it would be far easier to modulate the amplitude than the frequency. AM could be generated by switching an amplifier, exciter, or attenuator on and off. Frequency modulation would require changing speed of a mechanism, which might present challenges due to inertia of the moving parts.

author: German_EE
author_rank: Super Contributor
timestamp: December 01, 2019, 11:03:33 am
content: Another option:http://wireless.org.uk/mechrx.htm?LMCL=dSXa6u

author: Weston
author_rank: Regular Contributor
timestamp: December 01, 2019, 11:08:18 am
content: I do not see the word "piezoelectric" used in the project proposal or article, but I think this is related to recent research on using piezoelectric resonators as VLF antennas:https://www6.slac.stanford.edu/news/2019-04-12-slac-develops-novel-compact-antenna-communicating-where-radios-fail.aspxI don't know if it is open access, but the nature article on it is here:https://www.nature.com/articles/s41467-019-09680-2It seems like a pretty cool project. Piezoelectric materials have lots of interesting uses.

author: SeanB
author_rank: Super Contributor
timestamp: December 01, 2019, 12:35:37 pm
content: Quote from: coppercone2 on November 27, 2019, 09:48:05 pmThe old method was to AM modulate the power supply, using a water cooled carbon microphone to act as a variable resistance in the power feed to the slip ring, producing a modulated AM signal. Water cooled, as the microphone could dissipate around half of the input power to produce a deeply modulated carrier.

author: Gary350z
author_rank: Regular Contributor
timestamp: December 01, 2019, 12:58:22 pm
content: Quote from: AG6QR on November 27, 2019, 09:05:02 pmI found out about the Alexanderson alternator several months ago while reading about the Haiku Stairs, also known as the Stairway to Heaven. This is a set of 3922 steps leading up the very steep slope of a mountain in Hawaii. At the top of the stairs was an Alexanderson alternator used by the U.S. Navy. There are lots of YouTube videos of people climbing the Haiku Stairs, which is illegal.Alexanderson alternator InfoThe Alexanderson alternator was a high power radio transmitter (about 200 kW), and allowed amplitude modulation radio transmission of the human voice before the invention of vacuum tubes or transistors. The alternator consists of a large steel disk spinning at 20,000 RPM. I never could find the size of the disk, but from the pictures it looks like about 3 feet in diameter, but I'm not sure. The first Alexanderson alternator was installed in 1906.Here's some info on the Alexanderson alternator:https://en.wikipedia.org/wiki/Alexanderson_alternatorHaiku Stairs InfoHaiku Stairs Wikipedia:https://en.wikipedia.org/wiki/Haiku_StairsHaiku Stairs YouTube:https://youtu.be/Lm9M0YMuqRIhttps://youtu.be/B5GTohbckEo

author: chris_leyson
author_rank: Super Contributor
timestamp: December 01, 2019, 02:48:11 pm
content: SAQ Grimeton is still operating occasionally, on off keying, OOK, at 17.2kHz and whilst probably still able to transmit 200kW they limit the ouput power to 80kW. Next broadcast is Christmas eve 24th December probably around 08.00 UTC. I think the output is modulated by controlling the generator field winding but can't find a link. The Alexander association have an interesting web site and thanks to all the volunteers they keep this amazing piece of history operating.https://alexander.n.seIt's funny, I fired up an Adafruit feather M0 with an RFM95 Lora radio for the first time yesterday and picked up four packets on 868.1MHz over four to five hours monitoring, RSSI -104dBm. Not bad given that the antenna is a sleeved dipole lying on the bench and I was only monitoring a single channel. Transmit power at the sending end only 25mW.The hardware may have changed a lot in 100 years but fundemental physics like Friis's link budget equation still remains the same, just saying.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 01, 2019, 03:07:40 pm
content: QuoteQuoteThe Swedish Alexanderson alternator was built for built for transatlantic CW Morse telegraphy AKA wireless telegrams. I can't see how a "water cooled carbon microphone" can deliver 200kW into the field windings for AM. Water cooled carbon microphone and Alexanderson alternator. Would be nice if you could post some historical references, just asking.

author: JohnG
author_rank: Frequent Contributor
timestamp: December 04, 2019, 03:45:34 am
content: Quote from: chris_leyson on December 01, 2019, 03:07:40 pmhttps://edisontechcenter.org/alexanderson.html

author: vk6zgo
author_rank: Super Contributor
timestamp: December 04, 2019, 04:48:42 am
content: Quote from: chris_leyson on December 01, 2019, 03:07:40 pmWater cooled carbon microphones were certainly used for early AM experiments, but they were not used with the field winding of an alternator.They directly varied the amplitude of the RF, & from memory, were also used with some "rotating arc" transmitters.