Re: [amsat-bb] helix location vs focus

[K5OE@xxxxxxx]

Greg, et al,
I've responded to this question several times on the BB over the last two years and I documented it in the paper presented at the AMSAT-NA last fall in Atlanta.  I have tested numerous helices using various methods.  The most dramatic method I found was to use a low-level signal source with a "feeler" antenna attached to it and simply run the antenna along the wire of the helix.  The loudest signal is a good indication of the focus.  I have consistantly found the focus at the end of the first turn, measured from the reflector.  This testing was all on simple-reflector designs.  I have not re-done the testing with the helix inside a cup, but would be surprised if there was much of a change as I have not observed an input impedance change when a cup is installed.

See http://members.aol.com/k5oe
- select "MMDS" for the article on helix design
- select "S-Band Toys" for the article on signal sources

73,
Jerry, K5OE/G

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