Antennas and Receivers for Forward Scatter Set-ups

The Antenna

Generally, Yagi antennas are used for meteor observations. An example is shown below for a 70 MHz Yagi (figure 1). The dimensions depend on the resonant frequency of the aerial.

Figure 1 - the structure of a Yagi antenna for 70 MHz.

The reason why these antennas are popular is because they are easy to build and have a relatively high gain, i.e., they are sensitive in a given direction and less in others. The radiation pattern is typically like shown in figure 2.

Figure 2 - a typical horizontal radiation pattern of a horizontally polarized Yagi antenna.

The radiation pattern in the vertical direction is seriously altered by the presence of the ground. The effect of the ground depends on the height of the antenna above it. In ideal circumstances, the ground sections the main lobe of the antenna in a series of thin, pancake-like lobes. This is schematically sown in figure 3. In practice, the radiation pattern will be seriously disformed in an unpredictable manner by the ground, the surrounding relief, buildings, ...

Figure 3: the effect of a plane ground on the vertical radiation pattern of a horizontally mounted Yagi antenna.

If an antenna has a good directionality, it is important to consider the mounting of the antenna. There are two basic ways to orient the antenna. The most common is to mount it horizontally, and aim it roughly to the transmitter. The setup will then mainly observe distant meteors, and may pick up signals from very distant transmitters, up to 2000 kilometers. Another option is to mount it vertically, i.e., pointing at the zenith. Less meteors will be observed, but they will mainly be meteors in the neighborhood of your observing site. This way, you will also limit the maximal distance of the transmitters that can be received, which is useful for limiting the number of transmitters to take into account.

The calculation of the theoretical radiation pattern of the antenna above a plane and perfectly conductive ground surface is possible. It can for instance be done with the computer program YAGIMAX, which can be downloaded here.

If the receiver is not very sensitive, an external RF-pre-amplifier can be used. It is wise to combine it with a bandpass filter.

Please refer to specialized literature or a radio amateur for the building and connecting of the aerial.

The Receiver

What the receiver has to be able to do is to return the total power received at a given frequency. This output has to be rather fast, i.e., a sudden change in received power should be measurable. The exact nature of the receiver and the technical requirements highly depend on the nature of the transmitter.

Generally, existing receivers are transformed to meet the requirements. When only listening to the signal, there is no real need to make changes to the receiver. If the signal is to be registered with a pen recorder, electronic circuit or computer, the changes are strongly advised. The signal power should be extracted for the circuit, the Automatic Gain Control (AGC) and the Automatic Frequency Control (AFC) should be disabled. Note that it is not useful to register the demodulated signal (sound signal). Only the signal power is useful! For stability, it is a good idea to use a digital tuner, though it may be more difficult to adapt. An experienced radio amateur or electronician should be able to do the job. Consulting books about amateur radio astronomy can also be useful.

If no receiver can be found for the used radio frequency, pre-receiver frequency converters can be used.