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Re: S-Band Antenna Comparisons, etc.



Hmmmm?? Is the spacing of the framework that bad? Usually the design is such
that it approximates a full metal back but is ribbed to lighten it and
reduce (!) wind resistance.... Something about waveguide beyond cutoff?????

John Wilcox  NS1Z
871 Route 120
Rumford, ME  04276-3836
207-364-2246
207-364-2505 (fax)
FN44RN


----- Original Message -----
From: "Bob Snyder" <kd1vv@mediaone.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, January 07, 2001 19:17 PM
Subject: Re: [amsat-bb] S-Band Antenna Comparisons, etc.


> 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
> > > >
> > > > ----
> > > > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> > > > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> > > ----
> > > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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> > >
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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>

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