From Brian Coleman:
Hello Andrew,
Looking at your photo the feed looks much too close to the dish. You need to calculate the focus point of the dish. The phase centre of the feed is approximately at the mouth of the feed and this needs to be at
the focal point. The f/D of my 3.7m dish is 0.43 so the focal point is about 1.6m from the deepest point of the dish.
A good way to check you’ve got it right is to use the sun as a source.
Much work has been done on optimising feeds to illuminate a dish as efficiently as possible reducing spill over which on receive means the feed sees ground raising the system noise temperature and reducing the systems sensitivity. The one you have is a Dual Mode type which works very well for dishes with an f/D of around 0.45 which is a common format for dishes . A good place to start is the W1GHZ antenna handbook. https://www.w1ghz.org/antbook/conf/high_efficiency_prime_feeds.pdf and https://www.w1ghz.org/antbook/chap6-3.pdf.
You can find details of other types of feed in various chapters of the ( on-line ) book. There are simpler ones to make but depending on the f/D the dual-mode feed is technically amongst the best.
All the best,
Brian
From Eduard Mol:
Hi Andy,
Looks like you got a W2IMU type feed. I have one as well in my 3 metre dish (although it does not look as neat as yours). The main purpose of the wider section at the front is to give the antenna a narrower radiation pattern to decrease the amount of spillover noise. Indeed the focal point should be a few cm inside the opening of the feedhorn.
See also the W1GHz antenna book pages:
https://www.w1ghz.org/antbook/conf/Large_W2IMU_Feedhorns_for_Improved_G_over_T_on_Offset_Dishes.pdf
https://www.w1ghz.org/antbook/ch6_5-1.pdf
The issue with the extended “skirt” on a feed for a solar-cooker dish is that these dishes have an F/D in the “usual” range of about 0.34-0.42, which means that a high-gain feed will under-illuminate the surface more than you’d really like.
The W2IMU feed with the extended skirt is really intended for higher F/D dishes, like offset dishes, where a higher gain at the feed is required to properly illuminate the dish, because the feed is (proportionally) further away from the dish than for a prime-focus dish in the 0.34-0.42 F/D range.
I’d try it as-is at the correct focal point, taking several Sun transits to get an idea of the beam-width. You’ll probably find that the effective beam-width is wider than you’d expect for your dish. Then remove the skirt, and measure again.
Eduard