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Re: S-Band Antenna Comparisons, etc.



Hi Dave

We are both within the same realm, however my .02 cents was more at the
beginning of the fray.   grin.    Before one ever gets to think about
pointing the dish at something like the moon or satellite there comes 
the mounting of components and aligning everything on a single axis and
proper distances from the dish surface.    If the feed antenna has any
length then the axis of that antenna must also be aligned with the axis
of the dish.    I wrestled with several methods to build a jig to provide
a good reference and still allow the feed antenna to be moved and
measured for position.    If one succeeds in the above then pointing the
dish at something will follow.

There are several methods , each with it's own problems to be found out. 
 
The method of using the square tubing to self align two directions seemed
to suit my thought process.

I have a lot of fun building dish antennas for satellite activity .    I
have learned a bunch .    I suppose I can also say with out stretching
anything that I'll hopefully learn more  ( if the snow would melt and It
would get a little warmer).
Lessons learned :
Working with a dish as far as aligning and tuning , point it straight up
.
If using a mono filar helix feed , never use less than three turns.  
terrible side lobes with less than 3 turns.
Change helix pitch to control the beam width of the helix to match a
-10db down power lever at the edge of the dish.    Every formula I have
ever seen for a Helix is always the max gain verity.     In the dish
helix feed case it is best to have the correct illumination of the dish. 
  Over illumination causes unnecessary side lobe reception.   
Insufficent illumination is wasting the dish surface and less gain.    
Use  the sun noise to indicate if the antenna is acceptable .     2db sun
noise versus cold sky measured using a IF receiver with no AGC ( Max RF
gain ) and a tuned audio RMS Volt meter  ( HP-330 analyzer ) on the
speaker leads will copy 2.4 gHz .
In order to select the diameter of a dish I determine a dish beam width
needed that will allow easy manual pointing ( minimum tracking effort  )
.    Normally the beam width is 15-17 degrees.    Up link power is the
other adjustment that needs to match the beam width chosen.      A
variable up link power is helpful.

I like things simple easy to control.     I use the analog sats.

Have fun

Joe  K0VTY



On Wed, 10 Jan 2001 17:07:55 -0500 David Reinhart
<wa6ilt@bellatlantic.net> writes:
>Joe,
>
>Are we talking about the same thing here?  Your comments seem to be 
>about
>aligning the feedpoint properly to the dish.  The area I was 
>commenting about
>was how to make sure you are pointing the antennas accurately at the
>satellite.
>
>Dave Reinhart
>
>Joseph V Murray wrote:
>
>> Hi Dave
>>
>> I have found if you are working  with dish antennas it is a  good
>> investment  to have two pieces of square aluminum tube.     Making a
>> cross shaped jig out of the two square tubes fastened perpendicular, 
> one
>> can place one tube across the dish face and have a method to line up 
>dish
>> pointing , reference for measuring the height of the dish ( used to
>> determine F/D ration of the dish )  and centering of the focal point 
>and
>> feed antenna on the dish .   Trying to make sure that a feed antenna 
>is
>> indeed pointing at the center of the dish with a good axial 
>alignment
>> from a focal point that is also centered in the middle of the dish 
>is
>> difficult without a jig.   A buck and a half angle gauge resting on 
>the
>> square axial tube will measure elevation angle very nicely.
>>
>> Have fun
>>
>> Joe  K0VTY
>> ========================
>> On Mon, 08 Jan 2001 19:06:56 -0500 David Reinhart
>> <wa6ilt@BellAtlantic.net> writes:
>> >A few years back there was an article in the AMSAT Journal about 
>using
>> >a device
>> >called a "smart level" for accurate pointing of antennas.  The 
>widget
>> >is a
>> >hardware store device that gives a machine-readable output of 
>angle.
>> >I don't
>> >remember if it was analog or digital.  Now that we're faced with 
>more
>> >accurate
>> >pointing requirements it looks like time to dig through the back
>> >issues.
>> >
>> >Dave Reinhart
>> >
>> >"Edward R. Cole" wrote:
>> >
>> >> >From: K5OE@aol.com
>> >> >My comments about the sharpness of the pattern though, are quite
>> >serious:
>> >> >half the gain of this large antenna is lost if accurate tracking 
>is
>> >not
>> >> >maintained within a few degrees.  I've never aimed a dish like 
>this
>> >at a
>> >> >satellite before and I was surprised by how sensitive the 
>pointing
>> >is--this
>> >> >is not like waving your Arrow Antenna at AO-27!
>> >> >73,
>> >> >Jerry, K5OE
>> >>
>> >> Wait till you try tracking at 10 GHz with >35 dBi gain!!!  That 
>is a
>> >beam
>> >> width of ~3-4 degrees!
>> >>
>> >> I'm not sure that ordinary antenna rotators will be accurate or 
>have
>> >fine
>> >> enough movement for microwave operation.  My 2m eme array has 19 
>dBi
>> >gain
>> >> and the Ham-III rotator is controlled with a HB speed control to
>> >slow down
>> >> movement.  Otherwise I can get only +/- 5 degree positioning!
>> >Elevation is
>> >> easy since I am using a satellite actuator-motor.
>> >>
>> >> Ed
>> >>
>> >> ----
>> >> 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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>>
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