Build Your own VLF ReceiverThe 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
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 VLF Receiver 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 amplifier section: The third 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 pass filter. Schematic samples will be added soon...
B-Field Receiver 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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