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

[Michael Borthwick]

I guess I was just asking someone to measure the distance between the  
dipole and base of the reflector on the standard grey plastic MMDS  
downconverter so I can position my Canopy module at about the right  
distance.



On 21/07/2008, at 9:04 PM, Ratbaggy wrote:

> Hi Mike,
>
> If you make a couple of simple measurements, ie width and depth,  
> you can calculate the focal length.
>
> As per this page:
>
> http://www.analyzemath.com/parabola/parabola_focus.html
>
> 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.
>
> Dave.
>
> Michael Borthwick wrote:
>> Can someone please advise the focal length of the 24dBi conifer  
>> antennas
>>
>> Cheers,
>>
>> Mike
>> NodeBAE
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>>
>