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Re: No Stupid Questions, right? (focal point)



I really do not think the phase center is at the back of 
the helix but rather just inside the "nose" of the helix.  If
one were to make a line between the outer edge of the dish to
the center line of the helix, that would probably be as close
as one could guess other than by experimentation.  Cliff K7RR

"John Stephensen, KD6OZH" wrote:
> 
>  John Kraus, in "Antennas", indicates that two nested helices mounted on a
> common ground plane work well as a feed. He doesn't say so but the picture
> implies that the phase center is at the back of the helix.
> 
> 73,
> 
> John
> KD6OZH
> 
> ----- Original Message -----
> From: "Frederick M. Spinner" <fspinner@hotmail.com>
> To: <k0vty@juno.com>; <amsat-bb@AMSAT.Org>
> Sent: Wednesday, 09 May 2001 21:04 UTC
> Subject: Re: [amsat-bb] No Stupid Questions, right? (focal point)
> 
> > Actually I almost addressed this in the earlier post.
> > Yes, (especially) in a log perodic antenna the phase center moves
> > with frequency.  This is why the LPDA ia considered a poor feed
> > for a dish.  It'll work, but it has to be repositioned for different
> > freqs.  Also the LPDA's are relatively low gain (6 dBi) also meaning
> > that they have a large beamwidth, and tend to over illuminate the dish.
> >
> > I'm fairly sure that the axial-mode helix feed is better behaved with
> > respect to moving phase center, but I'm not totally certain how
> > well behaved.
> >
> > The LPDA's active vs. inactive areas change with freq, but I don't think
> the
> > current distrubution in an axial-mode helix changes as much-- it might be
> a
> > better choice for a "wideband" feed.  A Quadrifiliar Helix feed is another
> > possibility...
> >
> > Fred W0FMS
> >
> >
> >
> > >From: Joseph Murray <k0vty@juno.com>
> > >To: fspinner@hotmail.com, amsat-bb@AMSAT.Org
> > >Subject: Re: [amsat-bb] No Stupid Questions, right? (focal point)
> > >Date: Wed, 9 May 2001 11:24:45 CDT
> > >
> > >Hi Fred
> > >
> > >Can I ask a question relative to the phase center subject ?     When
> > >phase center is calculated , is it not calculated for a specific
> > >frequency?     Case in question is where a log periodic feed antenna is
> > >used ,  the phase center moves with frequency .    Or can a log feed be
> > >made for a single frequency.       And how about a similar case where a
> > >wide band helix feed where a dual coaxial, helix feed does not have to
> > >move with frequency .     I have been wanting to try the dual coaxial ,
> > >same hand , axial mono filar helix feed for 1.2 and 5.6 up links.
> > >
> > >Thanks
> > >
> > >Joe  K0VTY
> > >=======================================
> > >On Wed, 09 May 2001 14:17:45 -0000 "Frederick M. Spinner"
> > ><fspinner@hotmail.com> writes:
> > > > Phase center refers to the "point" at which maximum signal is
> > > > radiated in the near-field of a feed/antenna (highest current
> > > > density).
> > > > Some of the more mathematical friends of Maxwell around
> > > > here will say this is "oversimplified" but this is the point when
> > > > dealing with dish feeds/dishes.
> > > >
> > > > Focal point, of course, is the optical focal point of an object,
> > > > usually a
> > > > parabola (but a sphere has a focal "line" so other geometric objects
> > > > can be
> > > > used).
> > > >
> > > > The idea is:  Put the phase center at the focal point for max gain.
> > > >
> > > > This can be done by calculation, or even easier still, slide the
> > > > feed back and forth for maximum signal while listening to the bird.
> > > >
> > > > Fred W0FMS
> > > >
> > > >
> > > >
> > > > >From: "Howard Long" <howard@howardlong.com>
> > > > >To: "Rich Dailey, KA8OKH" <okh.npi@gte.net>, <amsat-bb@AMSAT.Org>
> > > > >Subject: RE: [amsat-bb] No Stupid Questions, right? (focal point)
> > > > >Date: Wed, 9 May 2001 08:03:02 +0100
> > > > >
> > > > >Rich
> > > > >
> > > > > > Ok then... what's the difference between "phase center"
> > > > > > and "focal point"?
> > > > >
> > > > >For us Microwave Newbies all this info is of interest.
> > > > >
> > > > >I have no idea, even when you know the phase center, which bit of
> > > > the
> > > > >coffee
> > > > >can / helix / yagi / log periodic / trapezoid should be putting
> > > > there
> > > > >anyway.
> > > > >
> > > > >Any info greatly apprciated here too!
> > > > >
> > > > >73 Howard G6LVB
> > > > >
> > > > >----
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> > > >
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