I have been looking into the options for receiving amateur signals on the 600m band (500 kHz). Since I live in an apartment size does matter, so looking for a small sized antenna I looked around and found this article from VK2ZAY , showing a simple 'frame antenna' made with a cardboard box.
Being close to all kinds of noise sources I think the loop is probably the best option for a receiving antenna on such a low frequency.
Since I intend to use the antenna indoors the support material does not matter much.
The box measures are on the small side - 35x40 cm (14x16 inches if you like) - but as a proof of concept I have put 25 windings with taps at 15 and 20 windings, as well as a single winding coupling loop.
Now I will be looking in my junk box to find a suitable variable capacitor to complete the project (this may take some time).
If this works out to my satisfaction I will try out frame antennas for 80m, 160m and 2.2km as well, at some time.
Since there seems to be an experimental license available here in The Netherlands I may even try to put out a 600m signal, yes, I am considering joining the experimental group. The decision, however, has to be made soon before the opportunity expires.
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