Some Thoughts On Aerials/Antennas
Using conventional wire antennas at vlf and lf can be a bit awkward. A quarter-wave antenna for 60 KHz, for example, would be around 5 kilometres long!
If you intend taking your vlf-ready receiver out into the countryside (which is recommended), you could try putting the top of it adjacent to a non-electrified wire fence…if the landowner doesn’t mind. In fact, any electrically isolated, large metal body is worth testing out as a parasitic antenna. And not just at vlf.
Loop antennas can be made using hula hoops, custom wooden frame supports, multi-conductor cable, old drawers, or even cardboard boxes. Loops are directional, receiving signals ‘end-on’. As they are a tuned form of parasitic antenna, loops will help to compensate for the untuned RF input stage in the Sangean ATS-803A (and many other receivers). Loop antennas pick up mostly the magnetic element of a signal. This is useful, in that the electric fields of local noise sources are attenuated.
Caution:-If connecting a loop, or any conductor across the terminals of the external antenna input, use a series coupling capacitor. Try 200pF for 100 to 500KHz; 470pF for 30KHz to 100KHz.
A good general introduction to loops is at www.frontiernet.net/~jadale/Loop.htm
An inductively-coupled loop:- www.hard-core-dx.com/nordicdx/antenna/loop/loop5.html
A cupboard door as a loop! www.hard-core-dx.com/nordicdx/antenna/loop/loop5.html
A big loop for 136KHz:- http://web.ukonline.co.uk/g3ldo/loop.htm
How to build a loop with tuned coupling into the external antenna socket:- www.stormwise.com/vlf1574loopantenna.htm
©2005 John Marsh