The design goals for this observatory include the ability to receive radio signals in the frequency range 10-100kHz from terrestrial transmitters located in many different compass directions. Initially observations will be made using a single frequency and therefore transmitter but in time it is planned to observe several transmitters simultaneously. The antenna system must provide for both the initial and later observations.
The antenna used to accomplish these goals is an untuned, electrically-short, vertical monopole. It is made from a commercially available CB whip antenna (Radio Shack Part# 21-903) and is 2.6m (102") in length. An antenna of this type possesses an omnidirectional azimuth sensitivity pattern in the horizontal plane which conforms to the design goal. In contrast, the altitude dependence is decidedly directional favoring low angles.
To call the antenna "electrically-short" is an understatement: the desired radio signals with frequencies of 10-100kHz possess wavelengths in the range of 30-3km. At most this antenna is 0.001 wavelength. As a consequence, the behaves like a voltage source with fairly high source impedance. This means that the antenna cannot drive a low-impedance transmission line or low-impedance receiver directly as would be the case for a tuned and matched antenna.
To couple the antenna to the receiver system a mast-mounted preamplifier has been designed and built. The preamplifier serves a number of purposes:
- Provides very high-impedance load for the antenna (0.5MOhm)
- Provides a low-impedance, balanced output to drive the long wires into the workshop/laboratory (8Ohm)
- Provides voltage gain (approximately 450)
- Provides frequency band pass filtering (-3dB at roughly 1 and 100kHz)
- Provides protection for the workshop/laboratory equipment using several different techniques:
- spark-gap
- shunting-diodes
- sacrificial capacitor
The image to the right shows the combined antenna/preamplifier mounted on a wooden fence. The antenna is quite narrow and can be seen as the bright, white roughly vertical line in this image. The preamplifier box is in the shadow of the fence.
The physical configuration and assembly of the antenna mount and preamplifier can be seen in the next several images.
The antenna is held to the side of the preamplifier box using a a clamp made of two three-inch-long pieces of one-inch-square acrylic plastic stock and a half-dozen brass machine screws. A groove cut into each piece of plastic holds the antenna securely. A generous amount of silicone RTV filler is applied to the joints between the plastic and the metal. The antenna terminates with a threaded stud and a correspondingly threaded nut is used to hold the electrical connection.
The bottom of the box has three distinct items located on it:
- A feed-through capacitor for antenna connection
- A brass stud for earth ground connection
- A Mini-DIN connector for DC power input and VLF signal output
The antenna has a spade lug attached to it with the nut and a fine, flexible, stranded, insulated wire is soldered between it and the feed-through capacitor. The wire provides a solid electrical connection with some mechanical flexibility to ensure reliability. It is intentionally made relatively fine to serve as a first-line sacrificial fuse in case of significant electrical assult.
The middle connection is a heavy brass stud, firmly attached to the metal box. It serves as an electrical and mechanical connection to the heavy earth ground wire. In practice, a length of the ground wire extends past the stud and its sharpened end is drawn close to the base of the antenna to form a crude spark gap. This will help limit accumulated voltage on the antenna.
The right-most connection is an 8-pin Mini-DIN female connector. Four pins are used: DC power input, DC power ground return, and a pair for the balanced VLF output. This connector is not a fully-qualified, all-weather item but will serve adequately for the limited lifetime expected of this prototype. Every opportunity to provide water shedding capability has been provided but in this climate there simply isn't much moisture so this does not represent a high-risk issue.
The figure at the left shows a visually-enhanced view of the ground wire which runs directly beneath the metal box and terminates in a long metal grounding rod. Please keep in mind that none of these precautions are considered to be intended to protect the equipment from a direct lightning strike - that is a very difficult goal to accomplish. Rather, these are precautionary measures designed to increase the survivability of the equipment under less adverse circumstances.
The figure at the right shows the interior of the metal box. A general-purpose, perforated circuit board is mounted using metal standoffs. The electrical signal from the feed-through is conveyed to the preamplifier input with a free-standing capacitor designed to serve as a second-line sacrifical fuse in case of significant electrical assult. The DC power is conveyed from the connector to the circuit board via a pair of fine wires taken from a larger ribbon cable and are carefully routed away from the preamplifier input. A second pair of similar wires which carry the VLF output are similarily routed. The gain of this preamplifier is quite high and its behavior is unstable and prone to oscillation when the metal box's cover is not present. Layout is very important in high-gain, wide-bandwidth devices such as this.
A schematic for this antenna/preamplifier system is shown in the figure to the bottom right. The circuit consists, from input to output, of protective diodes, biasing resistors, two independent AC amplifier stages, and a transformer output. The single voltage rail is protected using a zener diode and is decoupled using two significantly different-sized capacitors.
The biasing resistors at the input are equal-valued and hold their junction at one-half the supply voltage. The protective diodes at the input are then normally reverse biased. In this state they each appear as a very small capacitor with a value of around 4pF so they are effectively out of the circuit. If a large voltage transient were to appear at the input then one or the other would become forward biased, i.e., heavily conducting, and would conduct the transient safely into the power supply decoupling capacitors and/or the zener diode.
Each op-amp stage is configured as a single-supply, non-inverting amplifier with a high frequency gain of around 21, a DC gain of 1, and -3dB cutoff frequencies of 7kHz on the low side, and roughly 190kHz on the high side. The LF356 op-amps are especially well-suited for this application. They offer low noise performance with plenty of high frequency response. Their input impedance is governed by their JFET input circuitry and is so high that the input impedance presented to the antenna is dominated by the biasing resistors. They are also readily available at low cost.
The zener diode is intended to provide over-voltage protection rather than voltage regulation. Not only will the diode limit the maximum positive voltage to 15V but if it becomes reverse biased acts like a forward biased silicon diode and will limit the maximum negative voltage to only 0.6V which is below that which would cause damage to the op-amps. The combination of a 300 uF electrolytic and 0.15uF dipped capacitors serves to decouple the DC power rail over a wide frequency range which could not be accomplished by either one alone. The op-amp output is DC decoupled by a series capacitor before being delivered to the output transformer. The op-amps operate with a quiscent voltage equal to one-half the power supply voltage and if the transformer were to be directly connected to the op-amp there would be unnecessary additional DC current draw.
The use of a transformer in the output circuit deserves special explaination and serves two purposes. First, there is no DC path in the signal path so the opportunity for the formation of ground loop is eliminated. Second, the pre-amplifier is located at a significant distance from the remainder of the receiver and the VLF signal must be conveyed with low loss, minimal signal degradation and interference. A balanced, twisted pair is used to convey the signals to the rest of the receiver. The details of this segment of the system is described on another page.
