Friday, March 11, 2005

VLF radio part 1

A long time ago I dabbled in Very Low Frequency radio. These are radio waves with frequencies under 20kHz. They have obscenely huge wavelengths and as a result can penetrate into "conductors" a significant distance before being absorbed/reflected. The Navy used to use VLF for communicating with submerged submarines at 76Hz and 58Hz. There is a navigational system located down near 100kHz called LORAN-C, and the WWVB signal at 60kHz. Lightning is the dominant natural signal in this wavelength band. But, nearly everything creates electrical noise down there. For instance, let's talk about switching power supplies.

Nowadays nearly everything is powered via a switching power supply, as opposed to a linear supply. What's the difference? A linear supply uses transformers to convert the AC running at 120 volts and 60Hz into a lower, often 13.8 Volts 60Hz AC. This is then rectified into pulsing DC via a set of diodes and smoothed out with capacitors. It's wasteful--a percentage of the power is lost via heating in the transformers. They have to be very large to keep the 60Hz oscillating magnetic field inside the transformer.

Switching power supplies first convert the AC into a high voltage DC (around 200-300V). Then, the DC is switched quickly on and off at a high frequency, often 50kHz or higher. This is then put into a much much smaller transformer since high frequency transformers can be much much smaller and still be efficient as a big transformer running at a lower frequency. On the other side, power requirements are measured and there is feedback to provide a pulse-width-modulated approach to supply the power needed.
http://mrtmag.com/mag/radio_switching_vs_linear/
gives a good overview.

Anyways, they are noisy. The world is a very noisy place in the VLF band.

Long ago, my friend John Crocker ran a 1000ft wire down the length of the Midway, stuck a ground rod into the earth, and placed a pair of high-impedance headphones in between. The VLF radio waves are directly converted into audio frequencies.

Read VLF Radio Part 2: Sferics

3 comments:

Steven said...

What did he hear?

Dean W. Armstrong said...

For that, dear reader, you must wait for part 2.

Anonymous said...

It's April when is part 2! :)