At 06:37 AM 2005-12-14, Thom R LaCosta wrote:
What I was hoping to do was have a helix that acted like a w3edp,
ie, a multi-band antenna. So, if the edp is 87 to 89 feet, would
that mean that a helix might be 174 to 178 feet of wire wound on the form?
Thom,
The comments you have already received have been excellent.
Helical antennas are a great source of confusion to many hams and
SWLs. The main thing to keep in mind is that winding an antenna wire
onto a relatively small form changes almost every characteristic of
the antenna. The harmonic resonances will normally not occur as they
would if the wire were straight, the directivity and pattern will
usually be different, the feed impedance will be different (usually
lower), and the bandwidth and efficiency will be lower (much lower if
the length of the helix is much smaller than the straight wire version).
To a very rough approximation, a small diameter helix (aka "normal
mode") acts much like a straight wire that is the length of the helix
form, but with a lumped loading coil in the middle. Hmmm, sounds
like a mobile antenna...
You can't fool mother nature by winding a 10-wavelength rhombic onto
four 0.05 wavelength long coil forms. It would be great if you could! 8-)
BTW, the latest version of EZNEC (4.0) has a nice feature that makes
it easy to create models of helical antennas (subject to limits on
the maximum number of segments.) I can model a helical design if you like.
Keep clam,
Terry N6RY
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