VLF E-Field ReceiverContents
The Reception Environment
Receiver Design And Operation
Processing The Signal
A note before you begin: This page is a summary of my current E-field receiving system, and is constantly being updated as I try different experiments to improve the system. Much of my current system is a combination of previous attempts (see menu to the right), and I simply reference those designs here.
Last Update: 2014-04-02
You can listen to my current receiver while you read.
The Reception Environment. The part of central Virginia in which I live is located in the foothills between Lynchburg and the Blue Ridge Mountain Range. The area is mostly forest and farmland, woven with many streams, rivers and several large lakes. It is dotted with low density housing and sparsely placed small cities. It has the occasional high voltage power line, but most of the power lines follow streets and highways. The soil is predominately a red clay, with granite, limestone and volcanic formations underneath.
My current home is in a development with underground power lines, with above ground transformers set on pads at ground level. The nearest above ground power lines are about 1000ft to my north, behind a thick patch of forest on the north part of our property.
Receiver Location. The AC mains hum, it's harmonics, and interference from other local sources can set up standing patterns with 'local' maxima and minima. Through careful study and observation, you can select a location that minimizes (relatively speaking) that interference. Following the technique outlined here, I found a location to the north east of the house, which places the house between the receiver and the nearest above ground transformer. This particular receiver site is good enough, in fact, to be able to listen to the VLF band using a portable receiver without a filter! I was quite amazed, given my prior experiences with VLF reception at home, to be able to stand there and listen to tweaks and sferics without fatigue.
Receiver Design and Operation. In order to optimize my reception as much as possible, I installed my optical transmission system (local copy) to prevent domestic interference from entering the receiver via the transmission line and spoiling my low hum levels. Given so little AC mains interference, I used only the receiver module detailed here, which directly drives the optical transmitter. No additional filtering was needed, and unlike previous installations, I am not using the filter module (shown on the bottom half of the receiver page).
System Description. Currently, I have the receiver and optical transmitter placed in a waterproof tub (which is quite handy for easy experimentation). A 50 meter run of optical cable (placed in 3/4 conduit for protection) is used to get the signal indoors. The optical receiver directly drives the input of the sound card.
The whip antenna is approximately 10 feet tall, and made from 3/4" PVC pipe. Internally, it is 18ga hookup wire glued to the top cap, run through the length, and brought out through a "T" fitting and SO-239 connector. The body is also filled with dry sand. It is mounted to a piece of pressure treated lumber, which is in turn clamped to two ground rods.
Here is a small gallery showing the external parts of the system (click for larger images):
System Performance. The noise performance of the receiver is very, very good, and well below the desired VLF signal. You can see more details about it's noise performance here. Coupled with the optical link, the entire system is extremely quiet, with excellent dynamic range. I am using the ASUS Xonar D1 to digitize the VLF signal. The D1 It it widely available, and can be picked up on eBay quite inexpensively. There is full ALSA support, and the sample rate for all of the D1s I've purchased so far is quite stable.
Processing The Signal. Once you get the signal indoors, there are quite a number things you can to do with it. Right now, I'm listening for whistlers, watching the VLF band, looking for SIDs with my SID Monitoring system, and enjoying myself running various experiments.
For processing the VLF signals, I continue to use Paul Nicholson's vlfrx-tools. In my opinion, you just can't beat the reliability and configurability of using a Linux-based system. For information on my vlfrx-tools setup, please see this page.
Handy VLF Links