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Rosliston Astronomy Group

Archive 2009-11

Cross dipole antenna

A good place to start listening for the sounds of meteors hitting the atmosphere is in the VHF and UHF bands. For example, picking up the signals of French TV stations.

A long dipole is not the best antenna for this purpose. Rather, a crossed dipole antenna would work well - these can be made easily using thick copper wire and a two inch white plastic conduit box.

Crossed dipole antenna 25081102 White conduit box 25081102
Copper wire 250811

Four copper rods are cut from the thick copper wire - it has to be stiff enough to stay straight without support. Aluminium can be used instead.

The rods are then connected to the white conduit box through the four holes at 90 degrees to each other. The holes can be filled with cork, wooden dowling or similar and drilled to a size to allow rods to be pushed through them. Another system is to wrap ends of rods in masking tape until they fit snuggly in the holes. A conduit box (see above) is recommended as this is electrically resistant. A wooden block risks becoming conductive once it gets wet at which point the antenna will not work.

The corks/dowling in the holes can be wood or cork because it does not matter if they get wet - this won’t make a circuit with an adjacent rod because of the plastic inbetween.

The length of the rods is determined by the frequency that we want to pick up. For UHF transmission @ 600MHz, the wires (rods) need to be 12.5cm in length - all four should be the same length

The calculations for this length = 300/600 = 0.5m. This the full wavelength. A dipole is in total 1/2 wavelength, with each element being 1/4 wavelength, so each leement is 12.5cm.

 

Crossed dipole diagram 250811(ii)

In a single dipole, the outer part of the coaxial cable is attached to one element and the inner wire to the other element of the dipole.

With two dipoles, such as in the crossed dipole, the same thing happens with the second dipole as well. However, there is one important difference. One element in the second dipole (but not first) is connected to the coaxial through an extra 1/4 wave phasing line - an extra bit of coaxial cable exactly 1/4 wave long.

This 1/4 wave is not geometrical legth but electrical length. You would expect 1/4 wave to be 12.5cm. But the wire slows down the transmission of the electrical impulse. The 1/4 wavelength needs be multiplied by 0.66 to get an equivalent electrical 1/4 wavelength = 7.5cm.

Therefore, the 1/4 wave phasing line should be 7.5cm long.

It is possible to add a balun (transformer to match impedance but probably not needed with 50 ohm impedence wire.)

Weather channels can also be picked up using a similar antenna, but this would need to be cut to 136MHz.

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