ASSA Radio Astronomy Group

The ASSA Radio Astronomy Group is a new special interest group which I have become a member within the Astronomical Society of South Australia.

The aim of the group is to facilitate and encourage the sharing of skills, ideas, techniques and knowledge resources for people interested in radio astronomy.

Read more...

The Basic Radio Telescope

A basic radio telescope has the following attributes:

• A sensitive radio which can receive one of the frequency identified for radio astronomy
• The radio has "No" AGC (automatic gain control) or has the ability to turn it off.
• It uses a high gain low noise pre-amp and a directional antenna which can be pointed at the sky.
• A Data Logger is used to record total signal strength of the radio over a period of time

The signal strength of the radio is recorded over a period of time, signal levels will increase and decrease as a radio emitting object passes across the sky. Over number of days it is possible isolate radio astronomical observations from terrestrial and satellite interference.

Of cause everything is always a little more complicated. But it is possible and there are many examples of success to be found on the internet. The most famous of these is the NASA RadioJove Projects.

Radio interferometry

Radio interferometry is a powerful tool that can be used for a number of diverse applications. A radio interferometer consists of more than one antenna tuned to receive radio emissions from the desired frequency.

The antenna must be spaced more than 10 lambdas (baseline) apart East to West and following the natural rotation of the earth. The signals from the antenna is then cross-correlated in a Summing Amplifier at the input of the main radio receiver. As a radio emitting object passes above it produces a "fringe pattern" in the receivers measured signal strength, as the wave front of the radio emitting object passes in and out phase between each antenna.

From the Hans Michlmayr - Amateur Radio Astronomy Website

This fringe pattern can then in theory be analysed to produce a result ranging from an image of a distant astronomical object to the location of a nearby terrestrial or extra-terrestrial radio emitter.

The main limitation to the Amateur Radio Astronomer is not really the electronics but more the available space on your property. This is because in order to obtain a suitable fringe pattern the two antenna must be spaced at least 10 wave lengths (or 10 lambdas) apart. For example at 74Mhz VHF the distance between the to receiving antenna is more then 40 metres.

Radio Interferometry
Phase Interferometry
Interferometry

Phase-Switched Interferometer

Phase Switching is a system used to increase the discrimination and sensitivity of an interferometer; where an extra half-wave path difference is switched in, at well defined frequency, between the two interfering signal sources. In-phase signals then become out of phase and vice versa, so that the signal output becomes modulated by the switching frequency, and can be more easily filtered from the internally-generated noise. (The discrimination is highest for sources which are small compared with the interferometer-fringe spacing)

In the block diagram a phase switch is used to introduces either 180° or 360° (equivalent to 0° zero degrees) of phase shift into the right-hand transmission line, at a specific frequency easily demodulated by the receiver detector.

This switching signal generator is a square wave which changes periodically from one state to the other, many times per second, so that at one instant the first interference pattern is obtained, and at the next a second pattern. The same switching signal is applied to a phase-sensitive detector, which acts in synchronism with the phase switch and so subtracts the second pattern from the first. The resulting pattern is shown in the third part of Figure 1.; below, each maximum of the first pattern appears as a positive peak, but each maximum of the second pattern appears as a negative peak.

The instrument known as a phase-switching interferometerwas invented by Sir Martin Ryle in 1951, and is one of the major innovations in radio astronomy for which he was awarded the Nobel prize in 1974. This principle and later versions are now very widely used in radio astronomy.

You might ask, why go to so much trouble? Why not just use the simple adding interferometer and eliminate the phase switch and phase-sensitive detector? Although it is true the same information about a cosmic source is available in either type, the phase-switching instrument can be made much less sensitive to variations inherent in the receivers' electronics. Also, the phase switch causes the cosmic signal to be modulated at the switching frequency before it is amplified in the receiver. The noise generated by the receiver has the same general character as the cosmic noise, and this switch modulation acts as an identification tag helping to distinguish it from the receiver noise.

Also, the phase-switching interferometer responds less to extended sources, such as the general background radiation of the Milky Way, which might otherwise obscure the fainter radiation from weaker and small-diameter sources. In summary phase switching interferometers or more current versions, the correlator interferometer, are vastly superior to a simple adding interferometer and now universally used in preference to it.

"Phase switching is a clever trick how to make a basically additive interferometer behave like a multiplicative one. A plain additive interferometer has some undesired traits, like it outputs the fringes on a big DC pedestal, and needs the channels to be reasonably amplitude balanced to give good results. The phase switching is a way to avoid these." - Marko Cebokli

Information based on articles from Sky & Telescope by G.W. Swenson & W. Swenson, Jr.

Radio Astronomy Frequencies

Natural radio emissions from space cover the total range of the electromagnetic spectrum. However, on the earth's surface the majority of this spectrum is blocked by the earth's magnetic field and atmosphere only allowing few regions to pass. In the radio spectrum the earth’s atmosphere becomes increasingly transparent above 18Mhz and then increasingly opaque at around 40Ghz.

Any frequency above 18Mhz free from terrestrial and satellite interference can be used for radio astronomy. The lower segments of the spectra are used for solar and Jupiter observations; the 73, 150 and 406 MHz segments are quite popular for pulsar, and the 1.4 Ghz band and above is used for spectral line or energy measurements.

The following frequencies below are generally accepted spectral regions for radio astronomical observations and so have just chosen list the official regions as a reference, most accessible for an amateur radio astronomer.

• 25.550 – 25.670 Mhz
• 37.5 – 38.25 Mhz
• 73 – 74.6 Mhz
• 150.05 – 153 Mhz
• 322 – 328.6 Mhz
• 406.1 – 410 Mhz
• 608 – 614 Mhz
• 1.4 – 1.427 Ghz
• 1.6106 – 1.6138 Ghz
• 1.66 – 1.67 Ghz
• 2.655 – 2.700 Ghz
• 4.8 – 5 Ghz
• 10.6 – 10.7 Ghz
• 18.28 - 18.36 GHz

Amateur Radio Astronomy Frequency Choices

Most natural cosmic sources have spectra that fall off with frequency, so even if you keep the same antenna aperture (effective area) the signals will decrease with frequency.

Consequently the lower the frequency that is still transparent to the ionosphere (e.g. above 18Mhz) the greater the energy (signal strength) that can be collected by a specific gain of antenna. Said another way the better chance you have in detecting it.

What you gain by going up in frequency is:
- a narrower antenna beam (if you keep the same antenna area),
- less man made interference,
- more transparency to the ionosphere,
- a bigger possible bandwidth (if your hardware can eat it)

The relation between gain and effective area is

G = 4 * PI * A / L2 or A = G * L2 / 4 / PI

Where G is gain (linear, not dB), A is the effective area, PI is 3.14... and L2 is wavelength squared. Units for A and L2 are not important, but both must be given in the same units. The same area means more gain at a higher frequency, and the same gain means less area at a higher frequency.

Consequently from this reasoning the best choice of frequency would then be the lowest frequency that is free of interference that can be installed on the land area available to the Amateur Radio Astronomer. Land area becomes even a greater concern with interferometry as the antenna must be space apart East to West by 15 or more wavelengths to achieve a suitable fringe pattern.

74Mhz Interferometer

I started out to build a project based along similar lines to the 21Mhz interferometer receivers used by the Fringe Dwellers..

However, I have found HF frequencies below 40Mhz are very noisy and I suspect this is the same in most suburban areas. However using a WinRadio and tuning through the bands suggested for radio astronomy observations I found that the 74Mhz band was the lowest frequency I could find that was reasonably quite.

The 74Mhz band was heavily used by commercial users in previous years, but with the advent of mobile phones and other digital technologies, I assume the band now is used to a lesser degree. This was also confirmed by investigating who uses the band by visiting the ACMA website. As a result I have found what I think is a sweet spot at 73.5Mhz for the Adelaide metropolitan area, but this will be different for other areas of Australia.

The only down side in using this frequency is that it is not suitable for Jupiter observations as the natural signal emitted signals peak at around 8 to 11Mhz and reduce at higher frequencies rarely emitting above 30Mhz.

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Amateur Radio Astronomy Websites

• ASSA Radio Astronomy Group
• Amateur Radio Astronomy - Fringe Dwellers.
• Amateur Radio Astronomy - Hans Michlmayr
• Amateur Radio Astronomy - Marko Cebokli - S57UUU
• Slovak Amateur Radio Astronomy Group
• www.CosmicRays.org

Antenna Aperture

A receiver antenna aperture or effective area is measured as the area of a circle to incoming signal as the power density (watts per square metre) x aperture (square metres) = available power from antenna (watts).

Antenna gain is directly proportional to aperture and generally antenna gain is increased by focusing radiation in a single direction, while reducing all other directions. Since power cannot be created by the antenna the larger the aperture, the higher gain and narrower the beam-width.

The relation between gain and effective area is

G = 4 * PI * A / L2 or A = G * L2 / 4 / PI

Where G is gain (linear, not dB), A is the effective area, PI is 3.14... and L2 is wavelength squared. Units for A and L2 are not important, but both must be given in the same units. The same area means more gain at a higher frequency, and the same gain means less area at a higher frequency.

Simply increasing the size of antenna does not guarantee an increase in effective area; however, other factors being equal, antennas with higher maximum effective area are generally larger.

It seems obvious to optical astronomers that a parabolic dish antenna that is many wavelengths across, will have an aperture nearly equal to their physical area. However other antenna such as a Yagi and Collinear arrays my not look to be the same at first glance but they do achieve the same result using other means at radio frequencies.

Basic VHF Radio Telescope

A few years back I bought some large VHF Broadside Antennas at a stock-take sale. So during one weekend, I thought I'd just weld up a stand and see if I could use them as a simple radio telescope.

Using a WinRadio WR-3700e Wide-band receiver and small preamp, I scanned the VHF spectrum within the antenna's designed frequency range for a radio quiet area where I could conduct a meridian drift scan of the sky.

After a week of scanning and logging, the quietest frequency identified was 231.58Mhz which was reasonably clear either side by about 1Mhz. The scans where conducted at a receive sensitivity less than 1uV, the bandwidth was 17Khz and was logged at 1khz steps, AGC (Automatic Gain Control) was disabled.

The purple line indicates the maximum signal strength detected over the 24 hours and similar the green the average and orange minimum. Note that over the 24 hour period, some RFI interference occurred at different times of the day. Nevertheless this could be reduced using a band-pass filter at a latter stage.

The audio of the receiver was connected to a computer using the sound card and was logged over 24 hours from 7pm to 7pm using the the famous amateur radio astronomer software Radio-SkyPipe and the results so far are not to bad for a first attempt.

The antenna was aligned approximately by eye to line up with the sun as it passed over, if there are any extraterrestrial objects it is difficult to verify, as no planning at all was attempted.

The next trace was recorded from 9pm to 9pm where the antenna was tilted directly vertical to zenith.

The last is an example of the noise generated by heat in the equipment without an antenna over a similar 24 hour period. Note that the noise floor has been amplified significantly.

The two antenna where stacked to offer a 32 element collinear phased array and should in theory offers a gain of around 20dbi and a beam width of about 18 degrees. Although the gain may seem low compared with microwave radio telescopes the all important aperture size is over 4 metres square. Also note that galactic noise is very bright compared to microwave in the VHF spectrum.

Direct Conversion Software Defined Receiver

I recently purchased and built a low cost Soft66Lite kit which is a Direct Conversion (DC) Software Defined Receiver (SDR) the simple cousin of the well known Software Defined Receivers and unlike the early Direct Conversion Receivers of the past, as the mixer stages are based on a Quadrature Sampling Detector (QSD).

In the Direct Conversion SDR version, the radio frequency (RF) signal is first down converted to an audio frequency (AF) where it is then sampled by a high performance stereo audio card or Analogue-to-digital converter (ADC). Then through the use of digital signal processing (DSP) it can be filtered and enhance to demodulate many modulation systems including AM, CW, SSB, FM and a variety of digital modes.

Quadrature sampling detector (QSD)

A QSD is a system that switches the incoming RF signals into in-phase signals (I) and quadrature signals (Q) by the frequency of the local oscillator. The in-phase signal is the first 90º of the RF signals waveform (I) and the quadrature signal is the second 90º segment of the RF signals waveform (Q).

Mathematical functions can be used by the software to calculate the phase and amplitude of the original signal by measuring the values of I and Q simultaneously which has all the information contained about the original RF signal in it.

SDRs have become increasingly popular in recent years due to their relative low cost; the ubiquitous availably of high speed computers and they are significant flexible in terms of bandwidth and demodulation in comparison with traditional superheterodyne receivers. So I thought I might be good to build a few different units to gain an understanding of these receivers and see if these low cost radios can be employed in a Radio Telescope project at least in the latter IF and Detector stages.

Examples of software used include:

• Rocky
• Winrad
• SDRadio

Interferometer Baseline

Most interferometers for radio astronomy are built on an east-west baseline and although this is not strictly necessary the operation of an east-west interferometer is the easiest to visualise. Also it is the easiest to interpret as the fringes recur more rapidly than those of any other baseline.

So how long should the baseline be?
In the vicinity of the meridian a baseline n wavelengths long would produce n/4 fringes per hour when a source on the celestial equator is observed. For example if the wavelength was 4 Meters (e.g. 74Mhz) then a baseline of 80 metres or 20 wavelengths would have a fringe rate approximately 5 fringes per hour or a fringe every 12 minutes.

Mapping the Radio Sky

Using a method called “Meridian Drift Scan Observation” it is possible to build up an image of the sky at radio frequency not visible using optical telescopes. As seen below.

The Radio Sky: Tuned to 408MHz Credit: C. Haslam et al., MPIfR, SkyView

Drift scans plot the sky line by line using the earths rotation from East to West then adjusting the antenna every 24 hours over a series of elevations separated by somewhat less than the angular beamwidth of your antenna. If you had a beamwidth of say 10 degrees, you would then lower the elevation by about five to seven degrees and making a strip chart for that elevation. You would continue the process until the beam was point a bit above your horizon and then combine the data to make a 2 dimensional map of the sky. In reality, there is quite a bit more to do this, but this is the basic Idea.

Here are some more examples:

The Infrared Sky (and more)

Polarisation of Radio Astronomy Antennas

Most natural signals (i.e cosmic sources) are almost always non-polarised (which is the same as "random polarised"), so the use of any single polarisation method either linear or circular will achieve the same result. The slight polarisation present in such signals do not bring any significant "power advantage" so in practice linear polarised antennas are preferred more in Radio Astronomy as they are more practical to construct for a specific gain over a circular polarised antenna.

Polarisation can however carry interesting information about the source, so radio astronomers sometimes want to measure this. However it is quite difficult to do, because the signal characteristics are so weak, and below a few 100 MHz, the polarisation information is usually too mixed up by the ionosphere to be of any practical use.

Information about Antenna Polarisation.

What are Cosmic Rays?

Cosmic rays are energetic particles originating from deep space that hit our atmosphere at high speed.

There are a variety of sources including our own Sun but most come from interstellar events like Supernova, Black Holes and yet unknown happenings in the outer most reaches of our universe.

Although commonly called cosmic rays the term "ray" is a misnomer, as cosmic particles arrive individually as a primary particle, not as a ray or beams of particles. 90% are Protons, 9% helium and iron nuclei and remainder electrons or other sub-atomic particle remnants.

When these primary particles hit our atmosphere 30km above the Earth’s surface they hit with such tremendous energy they cause a nuclear reaction producing a shower of subatomic particles called pions. The charged pions decay into muons and muon neutrinos whereas the uncharged pions decay into pairs of high energy photons which become the starting points of large cascades of electrons, positrons and gamma rays. The resulting flux of particles at ground level consists mainly of muons and electrons/positrons in the ratio of roughly 75 to 25 percent and hit the Earths surface at near the speed of light ~0.998c...

Another interesting phenom of cosmic rays is that they are an everyday demonstration of Einstein's theory of relativity. This is because a muon at rest will disintegrates in about 2 x 10-6 seconds and so should not have the time to reach the Earth's surface given their travel distance. However as they move at close to the speed of light, time dilation extends their life span as seen from Earth and so can be observed at the surface before they disintegrate.

This is apparently happening 200 times every second, on every square metre, across the entire surface of the earth. With so much energy behind them they pass through just about everything, penetrating deep into the Earth’s surface, without anyone really being aware of their existence.

Below is a segment from an episode the TV program called "Cosmos" with Carl Sagan a legend in my youth, sadly now no longer with us. Although a little out of date, I could never have put the mystery of cosmic rays better.

Theory of Detecting a Muon

Unfortunately a muon is not easily seen, but their after-effects when passing through is a little more easier, typically most forms of radiation detectors will do the job. The oldest and most famous example of this is the Cloud chamber. There is an operational cloud chamber installed and running at the South Australian Museum and is well worth a look (I think its fascinating).

Other radiation detectors can be used like Geiger Counters, Spark Chambers, Resistive Plate Chambers and materials called Scintillators which give off light when an ionizing particle passes through them.

The problem using a radiation detector for a cosmic ray observation is that there will be equal amounts of terrestrial radiation known as background radiation due to the natural decay of matter. Although in small quantities it is sufficient to make it difficult to discriminate between a terrestrial or cosmic source.

Consequently two detectors are needed placed one above the other, feed into electronics that can monitor coincidence quickly thus effectively filtering out terrestrial noise.

Cosmic particles travel at nearly the speed of light and so do not ionise very efficiently and hence can travel through matter very easily passing through both detectors without effort, whereas the terrestrial radiation may not. Consequently anything detected in both detectors simultaneously is more likely to be a cosmic event than terrestrial.

Well almost simultaneously, if a muon is travelling at 0.998c and the detectors where spaced 5cm apart the actual flight time of a muon would be just 0.16ns. However as the detector and electronics response and delay times would be much slower than this, we can say in "real-life" terms it is simultaneous.

The main idea of 'coincidence detection' in signal processing is that if a detector detects a signal pulse in the midst of random noise pulses inherent in the detector, there is a certain probability, p, that the detected pulse is actually a noise pulse. But if two detectors detect the signal pulse simultaneously, the probability that it is a noise pulse in the detectors is p². Suppose p = 0.1. Then p² = 0.01. Thus the chance of a false detection is reduced by the use of coincidence detection.

Issues to consider in the design of DIY Muon Detectors

Muon Energy
Muons created by the interaction of cosmic rays and our atmosphere lose their energy gradually by ionisation of the material through which they pass. As they start with high energies they have the capacity to ionise many atoms before their energy is exhausted. Also, as they travel at nearly the speed of light, they tend not to ionise very efficiently and hence can travel through substantial lengths of matter, some metres of lead, before being stopped. Consequently, coincidence detection methods are the only real reliable way to discriminate between terrestrial radiation and cosmic sources.

Penetrative Terrestrial Radiation
I've been very surprised how penetrative local terrestrial radioactive sources can be. For example Cobalt-60 gammas which can be found just about everywhere have energies up to 1.3 MeV and so can penetrate 10mm of lead. In all detector arrays designs either Geiger–Müller or Scintillator-Photomultiplier configurations, as this can cause a substantial number of false detections. This particularly becomes a problem of detectors with small surface areas (aperture). Consequently, it is recommended that radiation shielding be included in your design to reduce the problem and increase reliability.

Compton Scattering
Compton Scattering is an effect where an interaction between charged electrons within the detector and high energy photons result in the electron being given part of the energy, causing a recoil effect of another high energy photon, which may enter into the adjacent detector causing a false coincidence detection.

In other words placing detectors too close to each other may cause cross-talk interference in coincidence mode, and so radiation shielding should be added or the detectors spaced further apart, but increased spacing also has the negative effect of decreasing the aperture of the detector.

Geiger–Müller Tube Detector Pulse Width
The Geiger–Müller tube is a very good detector of Muons however it would seem that filtering out background radiation using a simple coincidence detector systems alone is problematic due to the Geiger–Müller tube response and decay time (Pulse Width) when a muon has passed through and is detected.

Consequently, the wider the Pulse Width the greater the number of false positives. The means a pulse shorting or quenching circuit is also needed to shorten the Pulse Width to a period closer to the expected flight time of the Muon between tubes, but not too narrow that the electronics cannot measure relative coincidence. Some improvement might also be achieved by spacing the tubes further apart, but this also has the negative effect of decreasing the aperture of the detector.

Detector using Scintillators
As muons travel at nearly the speed of light, they tend not to ionise very efficiently and hence can travel through substantial lengths of matter, some metres of lead, before being stopped. This means that although a Scintillator-Photomultiplier detector has the potential to measure the energy of an ionising particle they can not discern between a muon and any other radiation caused by terrestrial sources and so must be used in a coincidence detection mode.

The major advantage of Scintillator-Photomultiplier detectors over a Geiger–Müller Detector is that a photomultiplier has a very fast response time and so more accurate than Geiger–Müller Detector in coincidence mode. Also as Scintillator panels can be made to have a much larger surface areas means a greater number of muons can be detected compared to other radiation caused by terrestrial sources, further increasing accuracy.

The major disadvantage of Scintillator-Photomultiplier detectors is cost and complexity.

Lead Shielding
Lead plays an important role as a material to shield against environmental radioactivity due to its high density and atomic number together with reasonable mechanical properties and acceptable cost. This role is however hindered by the unavoidable natural presence of Pb-210, which undergoes beta decay, with consequent emission of gamma and Bremsstrahlung radiation.

Again why coincidence detection methods are the only real reliable way to discriminate between terrestrial radiation and cosmic sources.

Muon Detector using Geiger–Müller Tubes

This detector is to be used as test unit, to measure the performance of my other project using Fluorescent Tubes against in order to clarify and identify any issues, also to better understand and to also demonstrate the principles of a cosmic ray telescope.

Above is one of the first visual test of the circuit, which has been updated a number of times since this video. Although the new circuit produces less false positives and hence less flashes, the video here offers a good demonstration of what the output data looks like. The LED flash times are slowed by a one-shot timer, as the pulses are so short they would not be visible if the LEDs where driven directly.

After a number of different configurations and tests, I have distilled the design down to a simple circuit seen here. The outputs 4,5,6 give a positive 5V logic pulse when a coincidence is detected in two or more of the tubes.

Although Geiger–Müller tubes are sensitive to Muons, the response time to decay (Pulse Width) when a muon is detected is relatively long for measuring the probability of coincidence in two or more the tubes. This means the wider the Pulse Width the greater the number of false positives. Consequently some means of pulse shortening is required to shorten the Pulse Width to a shorter period to decrease the probability that it is terrestrial radiation hitting the tubes in close succession.

The CRO trace above demonstrates what happens in this circuit when a wider pulse in feed in. The bottom trace is the output and only responds to a negative travelling pulse regardless of pulse shape. This is also important as the Geiger–Müller tube is positively biased and when a particle is detected the output swings negative, so the circuit ensures that only the first micro second of the detector pulse is processed in the coincidence circuit.

Why 3 tubes? I hear you say, well it is all about increasing the detectors aperture size to muons as most Geiger–Müller tubes available are longer than they are wide. So as an attempt to achieve a larger aperture as well as coincidence detection without using rows of tubes in each layer I am combining the coincidence outputs from top-middle and middle-bottom detectors.

The prototype provides six positive 5V logic outputs to a din socket.

1) Top tube - all detections
2) Middle tube - all detections
3) Bottom tube - all detections

(all detections) meaning both cosmic and background radiation

4) Top and Middle - coincidence detection
5) Bottom and Middle - coincidence detection

6) "Top & Middle" or "Middle & Bottom" - coincidence detection

(coincidence detection) meaning a stronger likelihood of a cosmic source than terrestrial source being detected.

Outputs 4 and 5 are the main outputs, but to give some visual information that the detector is working I've added some LEDs driven by one-shot timers to give a 1/8 second flash, as the output pulses are only a micro second and wouldn't be seen if used to drive the LEDs directly. Output 6 drives the blue LED indicating a confirmed coincidence in two or more tubes and where outputs 1,2 & 3 drive the red LEDs indicating any detection in the adjacent tube.

Coincidence Detection
The main idea of 'coincidence detection' in signal processing is that if a detector detects a signal pulse in the midst of random noise pulses inherent in the detector, there is a certain probability, p, that the detected pulse is actually a noise pulse. But if two detectors detect the signal pulse simultaneously, the probability that it is a noise pulse in the detectors is p². Suppose p = 0.1. Then p² = 0.01. Thus the chance of a false detection is reduced by the use of coincidence detection.

Muon Detector using a Geiger–Müller Array within Lead Shielding

I am currently building a Geiger–Müller Array with the aim of exploiting an effect called Electromagnetic Cascade or Particle shower as a means of significantly increasing the effective aperture and reducing other issues identified in my DIY experiments.

The circuit for this design is not that dissimilar from my three tube Geiger–Müller array however this design has nine coincidence outputs which will be feed into a Arduino embedded CPU for further processing.

It should be mentioned at this stage that this idea is very much an experiment and I really have no idea if it will really work. However if muons do induce an Electromagnetic Cascade in Lead as demonstrated in other experiments there should be an improvement compared to using Geiger–Müller tubes only on their own.

The other expected benefit using this method is that coincidence detection is also moved into the vertical where in a conventional detectors coincidence detection requires layers of detectors at the top and bottom.

Muon Detector using Scintillators and Photomultipliers

Recently I obtained some blocks of Plastic Scintillator BC412 which measure 89mm x 89mm x 38mm and is ideally suited for detecting muons.

Scintillation occurs in the BC412 when exposed to ionising radiation with an energy between 100 KEV to 5 MEV and emits light between 420nM and 450nM (i.e. blue light)

The scintillator block will be coupled using Dowel Corning DC4 to a 10 stage photomultiplier BURLE S83020F which is very sensitive to light between 350nM to 500nM.

Dowel Corning DC4 is mainly used in Aviation as an electrical insulator but because it is clear with a refractive index range of 1.4 to 1.5 it is similar to glass making it an ideal low cost choice Optical Coupling Grease between the scintillator plastic and photomultiplier glass envelope.

More information soon...

Muon Detector using Fluorescent Tubes

This project is deliberately aimed at developing a very low cost cosmic ray detector using common Fluorescent Tubes. It is based on variation of an experiment performed in 2000 by the CERN (European Organization for Nuclear Research) laboratories by Dr. Schmeling which found a simple method for detecting and visualizing cosmic rays using everyday fluorescent tubes inside a wire mesh of feed with a high voltage. I found information about this on the CosmicRays.org website called a Spark Chamber.

There is another link here at the Teachers CERN Website at the bottom of their page, unfortunately there is little/no information about how this actually works, but it should be fun finding out.

(C) www.cosmicrays.org

I am also building a detector using Geiger–Müller Tubes as a test unit to use as a standard to measure the performance of the Fluorescent Tube Detector against. It will also help resolve some issues identified when using Fluorescent Tubes to fine tune the final design...

Below is what I have done so far:

Electromagnetic Cascade Induction

I am currently investigating an effect called a Electromagnetic Cascade or Particle shower as a means of increasing the aperture of a Cosmic Ray (Muon) Detector.

In a 1964 publication on the historical understanding of the nature of the cosmic radiation, the scientist Bruno Rossi describes an experiment where cosmic rays could penetrate dense materials. Finding that cosmic radiation at sea level could penetrate over 1m of lead. In these same experiments he was also surprised to record a higher rate of detection, as many as 35 per hour as the thickness of lead increased peaking at 1.5cm and then falling slowly.

When the upper part of the lead shielding was removed the rate fell to about 2 detections per hour. This effect has now been confirmed to be a direct result a Particle showers induced within the lead as the result of a high energy muon passing through triggering an avalanche of electrons, positrons and gamma rays.

A demonstration of cosmic ray induced electromagnetic cascade was also shown to work by Peter Dunne, Preston College, Preston, Lancashire, UK

Cosmic Ray effects on Earths' climate

From the available research their is no convincing evidence that confirm cosmic rays are a “major factor” in determining cloud cover. The ionising of air by cosmic rays will impart an electric charge to aerosols, which in theory could encourage them to clump together to form particles large enough to form cloud droplets, called "cloud condensation nuclei".

However, the majority of physicist who research this area say such clumping has yet shown to occur. Even if it does, it seems far-fetched to expect any great effect on the amount of clouds in the atmosphere. Most of the atmosphere, even relatively clean marine air, has plenty of cloud condensation nuclei already.

It is also not even clear whether the satellite measurements of changes in cloudiness are correct or how these changes have affected temperature, as it is unknown if clouds cover may mitigate global warming or amplify it.

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For the record, the Administrator of this website agrees with the findings of the IPCC assessment of the scientific, technical and socio-economic information relevant for the understanding of the risk of human-induced climate change.

CWN Projects

Projects I have been involved at Air Stream Wireless can be seen on their website, some of these sites include:

• Highgate Park
• Carrick Hill
• Bedford Park
• MOB
• O'Halloran Hill
• Mawson Centre
• Melrose Park
• North Terrace
• Northfield
• Skye1,2 & 3
• Pasadena
• Elizabeth Water Tower
• Parkside
• Ridleyton

My own home wireless set-up

I run a OpenWRT customised router connected to Air-Stream network using a 25dbi grid dish antenna, the router provides multihomed services with a firewall between the Air-Stream network, my local network. It also runs a VPN between my local network and my work network which is also connected to Air Stream Wireless, providing easy access to work files, email and other services.

Another web server, which is located in the shed with the router for sharing files and information with other Air Stream Wireless members over the network.

What is a Community Wireless Network (CWN)

CWN's are a true phenomena of the 21st century and is now found in thousands of countries around the world. Although there are differences, between countries and groups most have common characteristics:

1) They are non-commercial entities established and maintained by groups of individuals.

2) They use wireless LAN to form a network that:

• Spans across property boundaries and/or public spaces
• Allow TCP/IP network devices and computers to communicate and share/stream data

3) They grow by:

• Interconnecting smaller networks together to form larger networks and so overcome topographical boundaries.
• Developing a group identity designed to focus efforts and facilitates cooperation between people of different backgrounds, skills and interests.

4) Their popularity has grown due to:

• The relative low cost of wireless LAN equipment.
• The ability of individuals to connect a network across property boundaries without need of a commercial carrier or special licence.
• Public familiarity with wireless and networking/internet applications.

5) They use one or more combinations of network models:

• WLAN – Wireless local area network
• Mesh - Self organising adhoc wireless network
• WAN – Wide Area Networks using wireless for user connections and backhaul
• Hybrid – Any combination of the above including wire or fibre, in networks there are really no boundaries.

Line Of Sight (LOS), a motivator for shared networks
One of the major hurdles faced by many people setting up a wireless network over distance is the problem of establishing good LOS between two sites. Without LOS it is highly unlikely or impossible to establish a reliable network over a few hundred metres. This is because of the high frequency and low power used by most standard wireless LAN devices 802.11(a/b/g) has difficulty passing through a solid object without a significant reduction and dissipation of the signal.

With good LOS however, it is not uncommon to see links over 10kms sustained at relatively high data rates using low cost off-the-shelf equipment. But in the real world good LOS between different locations is often rare for an individual working on their own and this is the main reason many CWN groups have formed. By coordinating a group people who can share and combine their networks between those with good LOS and others without, they can overcome many of the topographical barriers that an individual would find difficult on their own.

Network Systems
To achieve the aim of building a large network and joining smaller networks together there becomes a need to develop some form of network management to allow significant numbers of nodes and users to be joined together. This is because a simple Layer 2 WLAN would not be effective and would soon congest, just as a basic wired LAN, as they do not have the ability to route or shape traffic if network congestion or failures occur.

As a result two main routing protocols have become predominantly used by CWN Groups. These are either self organizing routing systems, often termed Mesh using OLSR or Autonomous Systems similar to that used by the Internet typically BGP or in some cases OSPF.