Some of the more advanced VLF receivers do use frequency conversion and digital tuning as well, we are just going to. 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). The best feature of a VLF receiver is that it is already tuned for audio frequencies.
Electromagnetic relatesE-Field receivers must use a very high input impedance to detectSignals. Electrostatic pertains to theCharge voltage or current wave of the signal. What kind of recording equipment will beThere are two ways to intercept a VLF signal: electromagnetically (B-Field)Or electrostatically (E-Field). Will it be near or far from power lines?
The values of these components are most important.Here are some sample circuits to start out with. It consists of a combinationOf resistors and capacitors. Lesser degreeFiltering does improve gain somewhat, but invites out of bandSelect from short whip to long wire antennas which is a no-brainerLooking at the block diagram, the first stage starting from left goingRight is the high impedence low pass filter. More antenna area orLength means additional filtering will be needed. The front end or input ofThe receiver is the most susceptible section to unwantedSignals.Undesirable signals become more evident asThe antenna size is increased.
Silver mica capacitors would be anExcellent choice for the front end part of the circuit. TheResistors should have at least a 5% tolerance rating, the capacitorsShould have a 10% or better rating. A receiver connected to a long wire or a large verticalWill require a stout filter at the front end, whereas one with a shortVertical whip (less than 8 ft.) would work with a lesser filter. A typical long wire antenna could be up to severalHundred feet long.
Wr Natural Vlf Receiver Series With The
Digital recording equipmentIs able to handle MSK interference better than tape recordersBecause of their frequency and dynamic range which is 96 dB at20,000 Hz, a good tape recorder has 60 dB of rangeA high pass filter can be put in series with the low pass filterTo reduce AC interference as well. It also can reduce the dynamicRange of the recording equipment, especially cassette recorders. MSK transmissions can cause a reduction in sensitivityOf a VLF receiver. A more tailored filter is needed to cut this range, or atLeast reduce it greatly. MSKStations are found in the 16 through 160 kilohertz band.The front end filter reduces most of the interfering stations except stations from16 to 26 kilohertz. This section prevents interferenceAnd/or overload from MSK transmissions.
Again, you can choose betweenAn IC circuit or transistorized design. The purpose of this stage is to boost the gain lost in theThe receiver for line-level output. 10 kHz Low Pass Filter used in the X2000A INSPIRE Team 19 VLF ReceiverThe forth section is the amplifier / bufferStage. Their examplesAre designed for HF frequencies, however, the handbook provides great insight The ARRL Handbook forRadio Amateurs has a great section on filter design. Creativity can reallyBlossom in this stage, keeping in mind the values of the componentsAnd limitations of the connecting stages.
A newer, better design is on the drawing table. Usually a 3300 ohmResistor in series would follow the output of a transistorized stage going into a low or highHere is a link to the B-field receiver I built severalYears ago. Keep in mind of the impedance restrictions of theStage beforehand when designing your own filter. The LM741 can be used however, it hasA fair amount of noise, but it is very cheap.The fifth stage is basically a copy of theThird stage. A 2N2222 or 2N3904 transistor would be a good choice.Suggested IC circuits to use would be the LMC6041, LM4250, or forMinimal noise use an LMC6001.
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