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Spiral antennas w/infinite balun feed, bridge wire questions

 
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superkuh
Antenna-Theory.com Newbie


Joined: 16 Jun 2012
Posts: 2

PostPosted: Sat Jun 16, 2012 1:52 am    Post subject: Spiral antennas w/infinite balun feed, bridge wire questions Reply with quote

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
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Sat Jun 16, 2012 2:52 am    Post subject: Spiral Antennas Reply with quote

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.
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superkuh
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Joined: 16 Jun 2012
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PostPosted: Sat Jun 16, 2012 11:35 am    Post subject: Re: Spiral Antennas Reply with quote

bigSteve wrote:
On the question of the bridge wire: I think this is the center conductor of your feed cable?

Yes. I was calling the extension of the feed cable's inner conductor to the secondary cables outer shield the "bridge wire".

bigSteve wrote:
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.

Yes. I meant for the conical extension at the end of the secondary cable (gray) to represent the outer shield. The extension of the inner conductor from the feed cable (black) would pierce this through the side and the outer shield wrapped down tight to the inner conductor with bare copper wire and soldered.

I'm glad to get confirmation that the lower frequencies will be less effected by unbalanced hookups in the center of the infinite balun feed.

c/(2.2 Ghz*4) = 3.4cm
inner radius = 0.5cm

I will assume that since 3.4cm is so much larger than inner diameter (1cm) and the ~2mm inbalance in the electrical contact will not matter enough to change the design. I suppose the best way to check will be making it.
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