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Home aerials, doofers and thingamabobs

In previous blogs I discussed what a radio wave is and what radio astronomy aerials are. So you have identified a site for your multi billion pound listening station but there is a problem with planning permission. What to do? Why not set up a home aerial? With just a few pounds and a little DIY it is achievable.

A meteor emits a plasma trail which gives off radio waves. This will be the subject of a later blog. For now lets look at how we can build a home aerial to detect the trail. The advantage of using a radio detector as opposed to optical detector, in the form of telescope or binoculars, is that you can follow the meteor in daylight and through cloud cover. This technique is called meteor scatter.

Home aerial

Picture from Sky at night magazine.

Based near Dijon, France is the Graves radio transmitter, transmitting on 143.05 MHz. We can build an aerial to receive this signal, to which we can add a receiver and software to analyse the data.

One type of aerial is based on the G4CQM MetScat design. The main parts are made from 15mm copper pipe. These are used to make a reflector element at the back of the aerial, a director element at the front of the aerial and a dipole in the middle. The dipole is attached to the receiver via a length of coaxial cable. Military grade RG-58 cable suffers less signal loss.

The reflector and director elements are not connected to anything but can multiply the sensitivity of the dipole by a factor of 3, provided you get the measurements right. Getting the measurements wrong can reduce the effectiveness of the signal. The central backbone of the aerial is made from wood, which is non conducting.

The boom keeps the elements parallel and at the right spacing, and provides a mounting point for the aerial. The boom needs to be at least 550mm long.

 

Below is a diagram of the construction.

Picture from Sky at night magazine

 

 

Use standard electrical fittings to attach the elements to the boom and to provide a waterproof enclosure to protect the connection for the coaxial cable. Mount the aerial 3 metres above ground level and within 10 metres of your receiver if possible. Think about weather conditions, especially high winds when it comes to mounting the aerial. Dijon lies to the south east so a clear view would be beneficial. Tilt the aerial up at an angle of 10 degrees. Whilst most aerials are mounted vertical, an horizontal mount can reduce background noise.

Finally you will need a connector. Use a SMA-type coaxial plug, being careful not to reverse the polarity. Alternatively use a BNC-type plug plus an adaptor.

You now have an aerial capable of detecting that back scatter I mentioned above. Of course there are a few more steps to get it up and running . You will need a couple of doofers and a thingamabob to plug in to the system, to connect it to your computer, which I will cover in later blogs.

Steve

 

 

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