Build Your own VLF Receiver
The information here is most helpful to a person with a
knowledge of electronic circuitry and fundamentals.
Most radio receivers require complex
frequency conversion to change and then demodulate the radio signal into an audio signal which
is passed on to the amplifier stage to drive the speaker or headphones. The best feature of a VLF
receiver is that it is already tuned for audio frequencies. Most Natural Radio or VLF Receiver circuits are tuned
for ULF, ELF and VLF bands or 100 hertz to 15 Kilohertz (see band allocation chart)
. Some of the
more advanced VLF receivers do use frequency conversion and digital
tuning as well, we are just going to keep it simple here for now. These circuit
examples are optimized to design a receiver specifically for Natural Radio recording and listening.
The first step in building a receiver is to consider these questions
- What kind of antenna will be used?
- Will it be near or far from power lines?
- Is power consumption a major factor?
- What kind of recording equipment will be
used? Analog or digital?
There are two ways to intercept a VLF signal: electromagnetically (B-Field)
or electrostatically (E-Field). Electrostatic pertains to the
charge voltage or current wave of the signal. Electromagnetic relates
to the magnetic influence of a signal.
E-Field receivers must use a very high input impedance to detect
signals. The reason for this comes from the ratio of actual antenna
length to the resonant frequency wavelength. A typical vertical
antenna for VLF reception is usually less then 60 feet long; some
are even less then 10 feet. A resonant, full wavelength antenna is
much longer than 60 feet. In fact, a full-length VLF antenna would
be about 60 MILES long! A small antenna produces very little signal
current for reception, however, a high gain, high impedance
amplifier can make up for the difference. To construe how to go
about building a receiver we must first get the basics down.
The picture here is a block diagram,
which is very useful in
looking at the overall structure of a good receiver design. Each section
of the blocks are explained in detail. The circuit samples below are designed
to work together; one block section will connect directly with another
section, providing it is put in it's proper place.
The most important factor in VLF radio design is the use of low
pass filtering throughout the circuitry. The front end or input of
the receiver is the most susceptible section to unwanted
signals.Undesirable signals become more evident as
the antenna size is increased. More antenna area or
length means additional filtering will be needed. Lesser degree
filtering does improve gain somewhat, but invites out of band
signals. The INSPIRE RS-4 (see
picture) VLF receiver has a switch to
select from short whip to long wire antennas which is a no-brainer
for versatile receiver design.
Looking at the block diagram, the first stage starting from left going
right is the high impedence low pass filter. It consists of a combination
of resistors and capacitors. The values of these components are most important.
Here are some sample circuits to start out with. In designing your own,
keep in mind that more complex filters are needed for receivers with long
or large area antennas. A typical long wire antenna could be up to several
hundred feet long. A receiver connected to a long wire or a large vertical
will require a stout filter at the front end, whereas one with a short
vertical whip (less than 8 ft.) would work with a lesser filter. The
resistors should have at least a 5% tolerance rating, the capacitors
should have a 10% or better rating. Silver mica capacitors would be an
excellent choice for the front end part of the circuit.
The second stage of the block diagram is the
high-gain amplifier. This section is very important. A low noise design is essential for optimum sensitivity. Operational amplifier
integrated circuits, or Op-amp ICs have been improved much over the years. Suggested
IC circuits to use would be the LMC6001CIN, LMC6041, LM4250. For minimal noise
and best performance use the LMC6001AIN. The FET transistor circuit uses a 2N3819,
which Radio Shack no longer carries, is still available from internet sites.
Here are links to a detailed look on design of the high-gain
stage is the low pass filter section. This section prevents interference
and/or overload from MSK transmissions. MSK
stations are found in the 16 through 160 kilohertz band.
The front end filter reduces most of the interfering stations except stations from
16 to 26 kilohertz. A more tailored filter is needed to cut this range, or at
least reduce it greatly. MSK transmissions can cause a reduction in sensitivity
of a VLF receiver. It also can reduce the dynamic
range of the recording equipment, especially cassette recorders. Digital recording equipment
is able to handle MSK interference better than tape recorders
because of their frequency and dynamic range which is 96 dB at
20,000 Hz, a good tape recorder has 60 dB of range
and a frequency limit of 14,000 Hz.
A high pass filter can be put in series with the low pass filter
to reduce AC interference as well. Both types of filters are made
from inductors, capacitors and some resistors. Creativity can really
blossom in this stage, keeping in mind the values of the components
and limitations of the connecting stages. The ARRL Handbook for
Radio Amateurs has a great section on filter design. Their examples
are designed for HF frequencies, however, the handbook provides great insight
on filter design.
Here are some filter schematic samples:
The forth section is the amplifier / buffer
stage. The purpose of this stage is to boost the gain lost in the
filter section and to ready
the receiver for line-level output. Again, you can choose between
an IC circuit or transistorized design. The transistor design would be the low-cost
solution. A 2N2222 or 2N3904 transistor would be a good choice.
Suggested IC circuits to use would be the LMC6041, LM4250, or for
minimal noise use an LMC6001. The LM741 can be used however, it has
a fair amount of noise, but it is very cheap.
Schematic samples will be added soon...
The fifth stage is basically a copy of the
third stage. Keep in mind of the impedance restrictions of the
stage beforehand when designing your own filter. Usually a 3300 ohm
resistor in series would follow the output of a transistorized stage going into a low or high
Schematic samples will be added soon...
Here is a link to the B-field receiver I built several
years ago. A newer, better design is on the drawing table.
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