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.
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

Wednesday, March 09, 2005

Pentax Takumar lens

I bought a 55mm f/1.8 Pentax screw mount lens off of Ebay last week to use as a wide-field imager in front of the Starlight Express SXV-H9 CCD camera. The total cost was $11. What I didn't expect was that the rear element of this lens was made with a thorium-rich glass. The radiation might preclude use of this lens. I get counts of 6000 microrem/hr with the counter at the surface of the element. I did some counting last night:

With the detector 2 cm away and 10 samples of 10 seconds each:
Bare: 2743
Single sheet of paper: 2466
Aluminum (1/16 inch thick): 577
Aluminum and paper: 563
With geiger counter reversed: 80
With geiger counter reversed and Al: 60
From side of geiger counter: 96
From side with Al: 87
Background count was 12.

Then, I did some distance counting.
Distance Count rate
2cm 2721
4cm 1145
6cm 647
8cm 391
10cm 282
12cm 203
14cm 165
16cm 130
18cm 98
20cm 87
22cm 80
24cm 57
26cm 59
28cm 47
30cm 38
32cm 42
40cm 26

I also measured the rates with the 1/16 inch thick Aluminum sheet in the way--this way, I was hoping for only the photonic component of the radiation, so no air absorption.
6cm 156
8cm 88
10cm 64
12cm 48
16cm 30
20cm 18

Graphs are needed, I know. I just wanted to post something. Everyone else is busy with end of quarter work.