Overview

The very low frequencies (VLF) are located between a few kilohertz and around 300 kHz, depending on whose definition is used. For purposes of this chapter VLF represents the 5- to 100-kHz region. The reason for this seemingly arbitrary designation is that many ham-band and SWL communications receivers operate down to 100 kHz and only a few operate below that limit.

A lot of radio stations are active in the region below 100 kHz. Perhaps the best-known station is WWVB on 60 kHz. This station is operated from Colorado by the National Institutes for Standards and Technology (NIST). WWVB is a very accurate time and frequency station and for many purposes is preferred over the high-frequency WWV and WWVH transmissions. The U.S. Navy operates submarine communications stations in the VLF region, NSS on 21.4 kHz from Annapolis, Maryland (400 kW) and NAA on 24 kHz from Cutler, Maine (1000 kW) being two most commonly heard. Other stations, in both the United States and abroad, are found throughout the VLF region.

But DX-ing in the VLF band is not all that easy. Besides the fact that propagation doesn t support skip the way the 20-m ham band does, huge noise signals are in the VLF region. Two sources seem to predominate. First, the 60-Hz power lines are terrible offenders. Although it might seem counterintuitive that a 60-Hz signal could be of much concern at, for example, 30 kHz, it nonetheless is. The high harmonics are caused by harmonic-laden alternating current from...