Re: [amsat-bb] S-Band Antenna Comparisons, etc.

["David M. Tipton, PhD" <tiptond@xxxxxxxxxxxxxxxxx>]

I don't think so.  If I understand correctly, it sees it as a solid, and 
not a "Grid" at all.

Dave

At 16:44 1/7/01 -0500, you wrote:
>I only meant that the linear grid cannot efficiently support circular
>polarization.  If a circularly polarized feed is used on a solid
>reflector, it induces circular currents on that reflector.  These
>currents can't flow on a bar-b-que reflector because of its parallel
>wire structure.  Isn't that correct?
>
>Bob
>
>
>"David M. Tipton, PhD" wrote:
> >
> > Cliff, you are incorrect.  The linear polarisation is determined by the
> > "Dipole" Element in the center and not by the direction of the grid.
> >
> > The reflector has nothing to do with it.
> >
> > Dave
> > At 14:17 1/7/01 -0500, you wrote:
> > >Cliff,
> > >
> > >The bar-b-que style of dish that Jerry is using can, I believe, only
> > >support linear polarization, in the direction of the bar-b-que rods.  It
> > >could support any polarization, however, if it was lined with metal
> > >screen.  That shouldn't be hard to do, by the way.
> > >
> > >
> > >Clifford Buttschardt wrote:
> > > >
> > > > Jerry, it is not clear to me if the dish antenna is linear or were you
> > > > able to compare both with RHC?  Truthfully, I have had a devil of a 
> time
> > > > with the small dish and about ready to go to the helix regardless 
> of gain.
> > > > Cliff K7RR
> > > >
> > > > On Sun, 7 Jan 2001, Bob Snyder wrote:
> > > >
> > > > > I did some quick helix antenna runs using NEC.  These runs use an
> > > > > infinite backplane, so they are only approximations, but may be 
> useful
> > > > > for comparisons.  The 7-turn helix you describe (with 44 mm dia. i.e.
> > > > > 1.106 wavelength circumference; and 37 mm spacing, i.e. 15 degree 
> pitch)
> > > > > has 11.9 dBi gain.  The 16-turn helix (I assume it is built to 
> nominal 1
> > > > > wavelength circumference and 12 deg pitch) has 14.8 dBi gain.  The
> > > > > parabolic reflector on the 7-turner may be enhancing it's gain 
> somewhat.
> > > > >
> > > > > 73,
> > > > > Bob KD1VV
> > > > >
> > > > >
> > > > > K5OE@aol.com wrote:
> > > > > >
> > > > > > I did some antenna experiments this AM when AO-40 came over my
> > > horizon (1200
> > > > > > utc, MA=4.5).  Those window-shopping the various options may be
> > > interested in
> > > > > > these observations.  What can be determined from current
> > > observations with
> > > > > > respect to the final satellite configuration is subject to 
> considerable
> > > > > > speculation:  these observations were taken with a squint of
> > > greater than 140
> > > > > > degrees on AO-40's low gain antenna at low power.  What one can
> > > extrapolate
> > > > > > to a favorable orbit, orientation, and squint, plus the change 
> to the
> > > > > > high-gain antenna and the high power amplifier, is clearly just
> > > speculative
> > > > > > at this time.
> > > > > >
> > > > > > In a quasi-designed-experiment, I used several antenna and
> > > downconverter
> > > > > > combinations using a Drake 2880 and my Yaesu FT-100 as a 
> receiver, and
> > > > > > recorded the results enumerated below.  One unknown in my
> > > measurement system
> > > > > > is the accuracy of the meter on the Yaesu.  I have previously
> > > "calibrated" it
> > > > > > using a variable power 2 m FM signal, but only in the crudest sense
> > > (after
> > > > > > all, I am an "amateur").
> > > > > >
> > > > > > 1.  The 24 dBi dish antenna (bar-b-que style, borrowed from KK5DO),
> > > does
> > > > > > indeed offer a 12 dBi gain (2 full S-units) over my 16-turn helix
> > > (estimated
> > > > > > at 12 dBi).  This was consistent with-and-without a preamp.  Note
> > > to those
> > > > > > without automatic antenna tracking:  this antenna's beamwidth was
> > > so narrow,
> > > > > > that 12 dB of signal was lost in +/- 5 degrees of movement.
> > > > > >
> > > > > > 2.  The Conifer preamp does NOT provide the rated 10 dBi gain, but
> > > appears
> > > > > > much closer to 6 dBi (when close-coupled to the Drake).  It was
> > > difficult to
> > > > > > judge the impact on S/N ratio with this setup, but I believe it was
> > > > > > substantially improved, even though the noise floor is up 6 dB 
> as well.
> > > > > >
> > > > > > 3.  At MA=18, I could detect the beacon with a coffee can and
> > > preamp/Drake
> > > > > > combination, but it did not move the S-meter.  I could not hear the
> > > beacon
> > > > > > without the preamp.
> > > > > >
> > > > > > 4.  Using an unmodified Drake 2880, I could not detect the beacon
> > > at 121.3xx
> > > > > > MHz with both the dish and the preamp in line.  This surprised
> > > me.  If I have
> > > > > > the frequency conversion for the stock crystal wrong, please let me
> > > know.
> > > > > >
> > > > > > 5.  In comparing the 16-turn helix I have been using for a month to
> > > a much
> > > > > > smaller one built for me by my friend Oleg, RV3TH, the two were 
> fairly
> > > > > > comparable.  The interesting thing is that helix is only
> > > 7-turns!   Oleg's
> > > > > > helix employs some non-conventional metrics:  the diameter is about
> > > 44 mm and
> > > > > > the turns spacing is about 37 mm, plus it uses a 100 mm 
> semi-parabolic
> > > > > > reflector.  I will report more on this after I completely
> > > reverse-engineer it
> > > > > > :-)
> > > > > >
> > > > > > For those interested in seeing the hardware, I have put a few
> > > pictures of
> > > > > > this equipment at:
> > > > > > http://home.swbell.net/k5oe
> > > > > >
> > > > > > 73,
> > > > > > Jerry, K5OE
> > > > > >
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