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Re: S-Band Antenna Comparisons, etc.



But there is somethim important missing here.  WHY are so manyh of us
hearing the bird and so many not???  Cliff K7RR





On Mon, 8 Jan 2001, Anthony Monteiro wrote:

> At 02:21 PM 1/7/01 -0600, you 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
> 
> Well, it is certainly true that the polarization is determined
> by the feed but most of the BBQ "dishes" I have seen are only 
> parabolic sections and not a full (round) dish so the gain 
> would be a lot lower if you didn't use the direction of the bbq rods.
> Comments?
> 73,
> Tony
> 
> >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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> >
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> 
> Tony AA2TX@amsat.org
> North Andover, MA
> 
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