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Re: 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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