antenna-theory.com

[superkuh]

Hi, first of all I want to thank you (author of antenna-theory.com) for the easy to understand articles. They've been a great help.

On the spiral antenna and infinite balun pages you describe in text, photos, and schematic diagrams the way to hook up the inner conductor of the feed coax to the outer conductor/shield of the secondary spiral arm. The photo scale makes it a bit hard to tell where the outer conductor of the feed cable ends, but it looks like there is not much precision in the 'balance' of the lengths of the spiral arm outer conductors.

How important is it to keep symmetry in hooking up the bridge wire to the secondary spiral's outer conductor? Can I just have an extension of the feed coax's inner conductor like shown in the 0.5cm inner radius gap below,



Additionally, does it matter if the bridge wire is significantly thinner than the spiral arm conductor widths (1mm vs 5mm)?

More details about what I'm trying to do are over at, http://superkuh.com/spiralantenna.html

[bigSteve]

Hello Superkuh,

In regards to hooking the inner cable of the feed cable to the outer conductor of the second cable, it is important - but for higher frequencies. If you want to extend your bandwidth (in the higher range, not the lower range) - then symmetry in the feed and the more exactness/tightness of the sprial near the feed becomes important. However, if the wavelength of interest is much larger than the dimensions of the feed, then it won't matter too much.

On the question of the bridge wire: I think this is the center conductor of your feed cable? Generally it doesn't matter but should be kept short, in general. Again this comes into play at the higher frequencies. But the ratio of the diameters shouldn't matter much.

Finally, in your picture - does the center conductor of the feed cable contact the outershield of the second cable? It's not clear but it should.

[superkuh]