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 (Projects->VLF Reception above), and I simply reference those designs here.
Last Update: 2019-02-23
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.
Current E-Field System. Click for larger image.
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.
The content presented here is the original work of Mike Smith unless
otherwise shown. Please contact me
for comments or errors.
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