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Re: Dielectric Loading of Helical Antennas

Very interesting Dan

Which prompts me to recall that there may be a possibility ( at least in
my case ) that if I have anything touching the helical at the point of
greatest voltage node , it may kill the forward wave considerably .
My reasoning  behind this , with your help Dan,  is the fact that long
ago I recognized that if one took a sharp lead pencil and touched the
helical wire very near the phase center ( ~ one turn from the reflector )
it would kill the antenna action .    Anywhere else along the length of
the coil , the pencil has little effect.       This is similar to
touching the tip of a vertical antenna and how it will kill the vertical
antenna action .
Your Swiss cheese notion will be a good way to test in my case.

Thanks Dan

On Fri, 2 Aug 2002 01:54:39 -0400 "Dan Fitch" <hiram@cfl.rr.com> writes:
> Gunter, Pieter, et al,
> I normally just monitor and enjoy the discussions on the BB, but 
> this thread
> is intriguing.  If one considers the helix antenna to be a multiple
> wavelength long-wire antenna configured such that the strength of 
> the fields
> at each point along the wire tend to add together as the wave front 
> travels
> the length of the antenna, is there a variation in impedance along 
> the wire
> that repeats at multiples of a half wavelength, similar to a 
> conventional
> antenna with standing waves?  I wonder if the impedance along the 
> coil of
> wire (or tape) may cause the antenna to be more sensitive to the 
> effects of
> supports and dielectric interaction at high impedance points; e.g.,
> one-quarter wavelength away from the relatively low impedance 
> feed-point,
> and thereafter at multiples of one-half wavelength.  I recall seeing 
> one
> early helix being constructed with supports in the form of spokes 
> that were
> attached at one-half wavelength points.  Another method of 
> construction that
> this thought could suggest is to support the wire coil (something
> self-supporting like a "slinky") with two dielectric rods that touch 
> the
> helix only at one-half wavelength points.  If the circumference 
> were
> one-wavelength, two rods might be ideal.  The feed point would 
> necessarily
> be located at the base of the helix near one of the rods.
> Another maybe... as a construction compromise, maybe it would be 
> sufficient
> to remove (cut or grind away) the portion of a dielectric support 
> tube (PVC
> pipe) that lies under the high impedance points of a helix wound on 
> the
> tube - sort of a Swiss cheese approach.  The "hole" in that method 
> is that
> the last quarter wavelength of the helix at the far end would be
> unsupported.
> Please keep up the good discussion!  This has been one of the 
> better
> threads.
> 73 de Dan Fitch/KB4L
> kb4l@amsat.org

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