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



 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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