Adapted from advice by Alex Pettit.
Alex:
The Illumination of the feed is determined by the dish<>feed spacing.
For small dishes, beam forming choke rings consume too much area.
The ‘focal point’ = optimal illumination with a fixed beam-width feed.
And it needs to be measured .. first in 2cm steps, plotted, best spacing identified, and then confirmed by several 1cm distance steps through that dish<>feed spacing.
Something like the quick real time display of SDR# with its cursor amplitude readout is required ..
Precisely point to one of the two bright H-Line locations, record cold sky & max H-line, Quickly change feed location ……
& Repeat a dozen times.. PLOT Yup Its Tedious.
Andrew:
Forgive me – I thought illumination was defined separately from focal point.
Is this the same for Discovery Dish feed? Does this mean that if this feed is used on different dish with different ofcal ratio or diameter then all that needs to happen is that the length of the boom on the Discovery Dish feed should be lengthened or shortened until the dipole is at the focal point of the new dish?
Alex:
Yuppers
( its probably a combination of focal position & feed beam spillover )
Andrew:
I have purchased one of the Discovery Dish feeds and intend to try it on my 150cm solar cooker dish = so from what you say all i need to do is put it at centre and vary height to find focal point and continue to use the same reflector on top.
Alex:
YES. knowing it is 150cm dia, if you measure the Depth, I can give you a good guess.
Andrew:
Thanks!…..I have a plan…..!