[MLB-WIRELESS] Dish focal Calculation

Ratbaggy wireless at smithsgully.net
Sun Jun 11 16:28:46 EST 2006

Hi Peter,

Finding the focal length of your dish is easy......

f = D squared / 16 d

f = focal length
D = diameter
d = depth

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.

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.

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:


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.

  ----- Original Message ----- 
  From: Datec 
  To: 'melbwireless' 
  Sent: Sunday, June 11, 2006 2:02 PM
  Subject: [MLB-WIRELESS] Dish focal Calculation

  Hi everyone, 


  Could someone please tell me the correct way to focus an ex galaxy dish.

  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.


  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.


  Thanks Peter

  Node JGA


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