Using an audio amp for amateur ULF radio? - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: May 31, 2019, 12:44:51 am
content: Would it be possible to send signals using a large 100 to 250 watt mono block audio amp running to two carefully spaced ground electrodes or a reel of wire that is some ridiculously long length or a radiator wire attached to the coil? Would it matter if you used a A/B or class D amp? You could send encoded signals then use the old fashioned (by now) PC soundcard with software to speed up the recording after several minutes or hours to decode the signal. People must have tried this or is this wattage just too low? Commercial amps go up to around 1000 watts? Come to think of it they make or used to make crazy wattage class D sub amps that might work, but you would have to figure out how to make the radiating element 2 or 4 ohms. Would you ruin the output stage or does the ground have the right impedance? If you used a big coil of wire as the antenna would you put one side to ground, or would the sine wave be all wrong with one side generating a negative voltage when you would want just a range of say 0 to +100v (number is arbitrary) vs -50 to +50v? You could send morse

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2019, 04:10:33 am
content: You could do that but the output power would be limited because of the poor impedance match. A more suitable amplifier would have a higher output impedance.

author: radiolistener
author_rank: Super Contributor
timestamp: May 31, 2019, 08:30:24 am
content: For ULF band (0.3 - 3 kHz) antenna size should be 50 - 100 km length. It's almost impossible.If you use shortened antenna, it's impedance will be very high, so it needs very high voltage.Efficiency of shortened antenna will be low, so you're need to use very powerful amplifier.For tests, you can use 1 kW, but efficiency will be very low and actual radiation power will some milliwatts, all other power will be loss on heat

author: mark03
author_rank: Frequent Contributor
timestamp: May 31, 2019, 08:06:46 pm
content: To learn all about this, google "lowfer" and see the examples of home-built transmitters and antennas, then multiply the challenge by 100 (frequency ratio).Transatlantic communication was recently achieved at ~ 9 kHz; see the equipment used here:http://w4dex.com/vlf/8971Hz/Because practical antennas are so inefficient down there, the effective radiated power was 150 microwatts.

author: Beamin
author_rank: Super Contributor
timestamp: June 01, 2019, 10:08:02 am
content: Quote from: mark03 on May 31, 2019, 08:06:46 pmThat's awesome! I so miss having the space to make big antennas. This is a hobby best done with lots of land in a rural area. Where I am now is great for reception, a good distance from the city and higher then the surrounding land but no space to string wires. Does it matter if the amp is D or AB as far as trying to modulate it; does the cross over between the two push pull transistors matter in the signal?

author: Beamin
author_rank: Super Contributor
timestamp: June 01, 2019, 12:49:03 pm
content: So what did they send for a signal? They say :QuoteThey also have a GPS hooked up, so are they doing something like wspr uses? 46 microHz bandwidth?I am wondering what the tx signal looked like, how they detected it, and how weak it was, relatively speaking; it made it over to the UK.

author: radiolistener
author_rank: Super Contributor
timestamp: June 01, 2019, 08:27:03 pm
content: Quote from: Beamin on June 01, 2019, 12:49:03 pmThey transmitted just a carrier. They used extreme narrow bandwidth, just 0.000046 Hz. It allows them to improve SNR to receive very weak signal. But if you will try to send some data with such bandwidth it will take very long time, because transmission speed will be low.

author: Beamin
author_rank: Super Contributor
timestamp: June 01, 2019, 11:39:35 pm
content: Quote from: radiolistener on June 01, 2019, 08:27:03 pmSo that's why they said it was a Fournier transform that was analyzed? What was the clock / gps for?Here is the software to download if anyone is interested in: Designed for VLF radio signal processing, it also has applications for meteor forward scatter, seismographic and natural radioactivity recording, ELF and magnetometers, radio astronomy, bat detection, amateur radio, and other projects which require precision timestamps preserved through signal capture, storage, and post-processing.http://abelian.org/vlfrx-tools/It's .gz thats a tar program for linux? Says it works for pc/rpi/linux. Can you "tarball" a windows program and unzip it? My windows 10 machine doesn't know what to do with the download.

author: Yansi
author_rank: Super Contributor
timestamp: June 01, 2019, 11:47:57 pm
content: You guys should probably research a bit aboutSAQ Grimeton. That stuff is interesting and can be heard pretty well in europe, just on a length of wire to the MIC input of your laptop.

author: DaJMasta
author_rank: Super Contributor
timestamp: June 02, 2019, 12:31:09 am
content: The clock and GPS are for verification: recording the start and end of your transmissions can verify if the signal they're hearing is actually from the transmitter, since any number of noise sources could produce more noise than the received signal and just stray into the band. The GPS disciplined oscillator keeps your receiving frequency where it should be and makes sure it's in the same place as the transmitter. When your whole FFT bandwidth is 50mHz, if your oscillator is running even a tiny bit off of the transmitter's, the signal is not even going to show up in the span.

author: radiolistener
author_rank: Super Contributor
timestamp: June 02, 2019, 01:03:50 am
content: Quote from: Beamin on June 01, 2019, 11:39:35 pmWhen station bandwidth is just 0.000046 Hz, you're need really precise reference clock in order to receive it. If your clock frequency on transmitter and receiver will have just 0.001 Hz difference, you will not be able to receive it. Because such small frequency difference will leads your receiver to be tuned out of transmitter bandwidth.GPS is used to get frequency standard.

author: Beamin
author_rank: Super Contributor
timestamp: June 02, 2019, 03:08:28 pm
content: Quote from: radiolistener on June 02, 2019, 01:03:50 amExcuse my ignorance; so to compare to the SDR, this is like putting it in CW mode with a really narrow filter? But why is the fft so broad? So to address the other post , the gps is to verify precise start and stop times, you mean they are transmitting on for a certain amount of time like a second then off? Wouldn't that require a second connection or some way of the transmitter syncing up to the receiver? Unless planned in advance this would never work: when they set the transmitter up they then coordinated with people who were trying to receive it the times and frequency, but rather then saying "At 01:00 GMT I'm turning on the transmitter can you hear it?" , it's more involved then that with the receiver software being "told" to look for a signal at these exact times at this exact frequency? The more I look into this the more complicated it gets. I was hoping it was like some of the other digital modes on SW. This is a very specific and planned out experiment.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2019, 10:31:10 pm
content: The GPS timing signals are the second connection although in principle, two separate sufficiently accurate clocks could be used.The FFT is long to achieve the needed RBW (Resolution Bandwidth) to meet the noise requirements. The smaller the FFT bins are, the less noise is picked up.An alternative design uses synchronous modulation at the transmitter and synchronous demodulation at the receiver clocked by the GPS signals or two accurate clocks. This is whatlock-in amplifiersdo.