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Re: S-Band Antenna Comparisons, etc.



Circular polarization requires a solid dish surface.    A small mesh will
work .  The Bar-B-Que does not support circular polarization.    To test
the dish surface, a simple test of rotating a Rx dipole in front of a Tx
dish antenna on the same axis as the dish will allow detection of the
polarized waves .    The polarity of the prime feed  antenna used with
the Bar-B-Que dish must be aligned with the rods making up the dish
surface.    Another thing about the Bar-BQue dish, It is truncated ,
which means that the beam radiated form that dish will have the shape of
a beavers tail.   Or canoe paddle.   

These facts can be verified in " Antennas" 2 nd by John Kraus >

I hope this helps

Joe  K0VTY



On Sun, 07 Jan 2001 15:14:10 -0600 "David M. Tipton, PhD"
<tiptond@dnrconsulting.com> writes:
>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
>> > > > > >
>> > > > > > ----
>> > > > > > Via the amsat-bb mailing list at AMSAT.ORG courtesy of 
>AMSAT-NA.
>> > > > > > To unsubscribe, send "unsubscribe amsat-bb" to 
>Majordomo@amsat.org
>> > > > > ----
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>AMSAT-NA.
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>Majordomo@amsat.org
>> > > > >
>> > >----
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>
>
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