[MLB-WIRELESS] Focal length of 24dBi conifer antenna

Ratbaggy wireless at smithsgully.net
Mon Jul 21 21:04:54 EST 2008

Hi Mike,

If you make a couple of simple measurements, ie width and depth, you can 
calculate the focal length.

As per this page:


However that isn't the whole story, you need to know where the "Phase 
Centre" of the feed is.  The phase centre of an antenna is the point at 
which the wave front appears to radiate from spherically.  It is the 
phase centre of the feed that needs to sit at the focus.  Depending on 
the type of feed, the phase centre could be somewhere along the axis of 
the feed or even behind it in space.  Unfortunately the position of the 
phase centre isn't easily predicted or measured.

For a given type of dish and feed, it may be better to get someone to 
measure the distance from the bottom of the dish to the back of a 
reflector or the front in the case of a cantenna type feed.


Michael Borthwick wrote:
> Can someone please advise the focal length of the 24dBi conifer antennas
> Cheers,
> Mike
> NodeBAE
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