A Simple Dipole for Shortwave
by Phil Beastall
The Sun and the interaction between Io and Jupiter's magnetosphere may be observed at shortwave frequencies. You will need a receiver that will tune somewhere between 20 and 30 MHz and a suitable antenna.
If space is very limited, you may want to experiment with a loop antenna, however you are likely to find a dipole much more satisfactory and just as straightforward to construct.
The dipole consists of two lengths of wire connected to the end of a feeder. You can use either ribbon feeder or coaxial cable, this should have an impedance rating of 50 ohms. One length of wire is connected to each side of the ribbon feeder or, if using coaxial cable, one to the central conductor and one to the outer braid.
The lengths of wire are brought together at an insulator in the centre, and are connected to an insulator at each end so that they may be attached to supports of some kind.
Some people recommend using ceramic insulators, however you may prefer to make some out of scrap plastic. The supports can be anything, preferable something that is there already - you probably place suitable materials in the waste bin on a regular basis.
The copper wire can be of any form that is strong enough to support itself and the feed, and is sufficiently weather-proof. One possibility, if you are not wanting to visit an electronics supplier, is to buy a roll of the twin flex used for wiring doorbells, making sure that it is the sort that is easy to split down the middle into two single wires - you will need just under 4 metres of this. You will probably find that the same shop sells coaxial cable. TV coax and bell wire are available in most DIY stores.
The two lengths of wire together should be half the wavelength of the radio waves you wish to receive. 21 MHz is often quoted as a good frequency for these purposes, and this has a wavelength of approximately 14.3 metres. Accordingly you will need an antenna with a total length of half of this, so each length of wire must be one quarter of this - that is 3.58 metres long. Now, as you will be tuning around, this length does not need to be totally precise.
You will find that this antenna is good for general shortwave reception, and you should find tuning around the various stations an interesting side issue. Try this at night when the ionosphere is more reflective and you will get stronger signals from more distant stations - you may be surprised at what you hear.
If you want to branch out into multiple bands, you may wish to add loading coils to the design to make it more efficient, but that is superfluous for the current project.
Safety Note - due care should be exercised when erecting any antenna - precautions should be taken against lightening strike.