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Meteor RF Scattering Receiving System

Abstract : in this post we want to describe a “radar-astronomy” project. By exploiting the reflection capacity of radio pulses by meteor trails, it is possible to study their characteristics and number with radar techniques. At an amateur level it is practically impossible, even for legal reasons, to have adequate radio powers in transmission, but it is possible to exploit existing radio stations (commercial or aeronautical broadcasters) with the bistatic radar technique where transmitter and receiver are separated by distances of hundreds of kilometers . In this way it is relatively easy to set up an apparatus capable of receiving and recording the radio echoes produced by the meteors.

Introduction

Most of the meteoric particles are really tiny in size: they are comparable to a grain of sand. In some cases they are larger, like a pebble or even larger, but this rarely happens. In spite of their size and thanks above all to their high speed, the impact with the atmosphere generates, by friction, very high temperatures capable of rapidly vaporising the meteoroid and generating a trail of ionized gas (plasma) hundreds or thousands of meters long. The luminous trail we see from the ground is precisely the trace of this ionized gas.
As we know, ionized gas is an excellent electrical conductor and is able to mirror incident radio waves. We can therefore exploit this characteristic of meteor trails in order to detect them with the bistatic radar technique.
The principle of the bistatic radar is simple: an active transmitter element sends radio pulses that are reflected by an object in the sky at an altitude of about 100 km and are received by a passive receiver element. The figure below illustrates how the process takes place.

The active element is usually a powerful radio transmitter located at a distance of 400 – 1000 km from the receiver. Direct wave receiving is not possible at this distance; receiving can only occur through an intermediate reflection such as that caused by a meteor trail. The radio-echo manifests itself as a sudden increase in the received signal that lasts from a few fractions of a second up to a few seconds in the cases of the most intense meteors. The study of the signal, in intensity, in time and in frequency (doppler-shift) provides information on the source (meteor trail) that caused the scattering.

A radio station suitable for use as an active transmitting element is the GRAVES radar station. The GRAVES radar is a space surveillance system based on a French radar, similar to the US Air Force space surveillance system. Using radar measurements, the French Air Force is able to locate satellites orbiting the Earth and determine their orbit. The radar transmits at the frequency of 143.050 MHz with a power of the order of tens of KW (below there is image of one of the transmitting antennas).
The images below show the geographical location of the radar and the area that is “scanned” by the beam emitted by the antennas. Our receiver is located in the Trentino region (Italy) and therefore we are located in the SOUTH-EAST direction, at a distance of about 450 km from the transmitting antennas. The useful area for receiving meteoric echoes is the one that is placed around the line (indicated on the map) that connects the transmitting antenna with the receiving one.

Antenna

As with any well-built radio system, also for the reception of meteoric radio-scattering, the main element is the antenna. However, we are in a rather fortunate situation: the frequency of the Graves radar is 143.050 MHz, very close to the frequency for radio amateurs of 144 MHz, so it is quite easy to find antennas, new or even used, suitable for reception on this band. If instead we want to follow the path of self-construction, there are numerous examples and projects of Yagi-Uda antennas that are easy to build and perfectly suited to our purpose. We too have opted for self-construction by choosing to build a three-element Yagi-Uda antenna.

The design of a Yagi-Uda antenna is characterized by three components: the actual active part (the dipole) and two parasitic elements; the reflector on the back and the director on the front. The lengths of the elements and their mutual distances must be chosen carefully because the two parasitic elements must “resonate” in phase with the dipole. The mutual distances determine the travel time of the wave and the lengths of the elements determine the resonant frequency. Signals from other directions will be more or less out of phase and are therefore less amplified or even attenuated.
In this way the received signal is strengthened along a certain direction and, in the case of a 3-element antenna, amplified by 5 dB (3x). This principle was discovered in 1926 by the Japanese Uda and Yagi, the latter of whom published an article in English on the antenna. Directional antennas that work according to this principle are therefore usually called Yagi antennas.

Our antenna is based on the data in the following table. The directivity of an antenna with only three elements is limited, this means that the receiving lobe of the antenna is quite large but this is suitable for our purpose as we want to cover a large area to receive the greatest number of meteoric events. To increase the directivity, and therefore the scale, it is possible to opt for antennas with a greater number of director elements.


The elements of the antenna consist of an 8 mm diameter aluminum tube, cut to size and fixed on a wooden “arm” of suitable length. The dipole element obviously consists of two halves of equal length. the following drawing (taken from an internet website) shows the arrangement of the elements on the wooden structure.

Various methods can be used to attach the elements to the wooden arm. We drilled grooves at the correct positions, positioned the elements, and fixed wooden blocks over the elements to keep them fixed in place. The wooden arm must be drilled in correspondence with the dipole in order to pass the two connecting cables. On the opposite side, a sealed electrical box will be fixed, with inside the balun and the BNC connector for connecting the cable. The antenna was then fixed with a clamp to an iron pole, as seen in the image below.

The image below shows the inside of the electrical box. The two cables, black and red, are the connecting cables with the two elements of the dipole. The balun, inserted inside the box, has the function of adapting the dipole antenna to the connection cable. The antenna is a symmetrical and balanced element, while the coaxial cable is not. This circuit element serves precisely to eliminate the “unwanted” effects that may occur due to this imbalance. In reality, the greatest problems arise when the antenna is used as a transmitting element, when the antenna is only a receiving element, the problems are gain reduction, change in directivity and interference reception.

The balun is built by drilling two 6 mm holes on both ends of a 50 mm long PVC pipe. Pass a length of RG-58 coaxial cable through the first opening. Wrap a six-coil coil around the pipe and insert the last piece of cable through the second 6mm hole. The coil of the RG-58 coaxial cable around a PVC pipe serves to suppress so-called common mode currents. An alternative is a ferrite ring or clamp suitable for VHF. Without these filters, the antenna still works, but there is a possibility that environmental interference will reach the antenna through the outer casing of the coaxial cable.

Receiver

Our receiver is based on an SDR device. We have adopted the Nooelec SDR receiver, Smart model, based on the RTL2832U & R820T2 chips, characterized by good performance and low price. It has good frequency stability (0.5ppm tcxo) and is enclosed in an aluminum case to minimize external interference. Between the receiver and the antenna we have inserted a low noise amplifier with a band pass filter centered on 145 MHz. In this way we want to attenuate all out-of-band signals (especially the FM band) and only amplify our Graves radar signal at 143 MHz.
In the image below we show the SRD receiver connected to the PC and the filtered amplifier.


The pre-filtered amplifier is from Uputronics and provides a gain of 20 dB with a noise figure of 0.75 dB. The SAW band pass filter is centered at 145 MHz and has a band of about 10 MHz, the 143MHz signal is therefore within the passband of the filter. The images below show the amplifier and a diagram showing the frequency response of the device.

Setup

Our system for receiving radio meteoric echoes also includes software for data acquisition. A variety of software can be used for the SDR receiver: one of the most popular programs is SDR#, it is a very complete program that, however, makes it difficult to automatically record events of interest. One possibility is to redirect the audio output to a program that makes it possible to configure a threshold and record events. An alternative is the development of software on the GNURadio platform. Since it seems useless to “reinvent the wheel” we have adopted a ready-made open software: it is the multiplatform program echoes. It is a very complete program thanks to which it is possible to automatically record radio meteoric events that exceed a configurable power threshold. The program has many configuration parameters so we recommend that you read the documentation thoroughly (really complete documentation).
The image below shows the PC connected to the receiver with the echoes program running.

Conclusions

The system we have described is really “low-cost”: the antenna can be easily self-built, the receiver is a very low cost SDR device. Even the amplifier has a low cost and could, in an initial test phase, not even be used. Finally, the software part is open. With a very low financial commitment and with a bit of patience and will, it is therefore possible to prepare an excellent system for receiving radio meteoric echoes.
In the next post: Radio Observations of Meteors, we will describe some observations made with our system also on the occasion of the passage of meteor showers.

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