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



I haven't tried to determine where the phase center is, but if it is near
the nose then nested helices won't work much better than log periodic feeds.

73,

John
KD6OZH

----- Original Message -----
From: "Cliff Buttschardt" <cbuttsch@slonet.org>
To: "John Stephensen, KD6OZH" <kd6ozh@AMSAT.Org>
Cc: <k0vty@juno.com>; <amsat-bb@AMSAT.Org>
Sent: Thursday, 10 May 2001 00:31 UTC
Subject: Re: [amsat-bb] 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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