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<DIV><FONT face=Arial size=2>Hi Peter,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Finding the focal length of your dish is
easy......</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>f = D squared / 16 d</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Where</FONT></DIV>
<DIV><FONT face=Arial size=2>f = focal length</FONT></DIV>
<DIV><FONT face=Arial size=2>D = diameter</FONT></DIV>
<DIV><FONT face=Arial size=2>d = depth</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>To find these stretch a string diagonally from
corner to corner and tape the ends (or get someone to hold it for you).
Measure the Diameter from corner to corner. Measure depth from
the string to centre of grid.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>However you still have a problem, the "PHASE
CENTRE" of your cantenna needs to sit at the focus. The phase centre of an
antenna is the single point from which the rays appear to be
emanating. In the case of a cantenna, this point may not be the front
of the can, the probe or the back of the can but some point inside the
can.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I think you still end up with a trial and error
problem but at least you have a starting point. Googling for "phase
centre" and "open waveguide" turns up some heavy material which didn't seem to
be helpful although this link may be of some use:</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2><A
href="http://www.qsl.net/n1bwt/chap6-1a.pdf">http://www.qsl.net/n1bwt/chap6-1a.pdf</A></FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Another option may be to use a biquad or bowtie
feed. The phase centre of these is about 0.25 wavelengths behind the
loops. They are also within the realms of possibility for home
construction and may produce a better result because of their radiation
pattern.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Dave.</FONT></DIV>
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=datec01@optusnet.com.au
href="mailto:datec01@optusnet.com.au">Datec</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=melbwireless@wireless.org.au
href="mailto:melbwireless@wireless.org.au">'melbwireless'</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Sunday, June 11, 2006 2:02 PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [MLB-WIRELESS] Dish focal
Calculation</DIV>
<DIV><BR></DIV>
<DIV class=Section1>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt">Hi
everyone, <o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt"><![if !supportEmptyParas]><![endif]> <o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt">Could
someone please tell me the correct way to focus an ex galaxy
dish.<o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt">I’ve
made a cantena collector but don’t know exactly (apart from trial and error)
where is the focal point of the dish, seeing it is only part of a larger
curve.<o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt"><![if !supportEmptyParas]><![endif]> <o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt">I can
get a rough idea of where to point the dish but when I’m looking for a spot in
the (far) distance I’m not sure if it’s the setup or the bad aiming which is
causing me to not find targets. Basically I’ve got to find how to aim the dish
and also how to aim the cantena more
accurately.<o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt"><![if !supportEmptyParas]><![endif]> <o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt">Thanks
Peter<o:p></o:p></SPAN></FONT></SPAN></P>
<P class=MsoNormal><SPAN class=EmailStyle15><FONT face=Arial color=black
size=2><SPAN
style="FONT-SIZE: 10pt; FONT-FAMILY: Arial; mso-bidi-font-size: 12.0pt">Node
JGA<o:p></o:p></SPAN></FONT></SPAN></P></DIV>
<P>
<HR>
<P></P>_______________________________________________<BR>Melbwireless mailing
list<BR>Melbwireless@wireless.org.au<BR>http://wireless.org.au/mailman/listinfo/melbwireless<BR>
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