[Moon] [Moon-Net] Finding the focus point in a dish

DW Harms (Dick) qtc at kpnmail.nl
Wed Sep 16 08:47:23 CEST 2020

Hi Paul and the rest of us,

I use a simple and practical way to tweak my dish which might be useful to you, and others of course.

For a prime focus dish, the best way to adjust the feed pointing to the midpoint of the dish, is to mount a small laser pen in the center of the feed. Obviously the feed alignment should make the laser beam point exactly in the middle of the dishsurface. I am using a 3D printed mound, with the laser pen in the middle, which I can shove onto the front of the feed.

The next step is top make sure you illuminate the dish correctly, not over- and  not underilluminating it. The simple method I use, is to point the dish to the sun and measure sunnoise. Next you take a metal plate of 40x40 cm and move that slowly along the edge of the dish. If you observe slight variations in the noise, your feed is looking over the edge, in plain words you are overilluminating it! This is bad, as it means part of your feed is looking at the warm earth. If you don’t see any changes, you might underilluminate and then you have to start moving your choke ring (never move the feed!). If you don’t have a choke ring, get one!
It is a bit of work, but next move the choke ring backwards, just until you start observing effect of your 40x40 plate. Then move the choke ring slightly forwards again and fix it! By the way, pay very good attention to the contacts between choke ring and feed. Corrosion or dirt can deteriorate the contact and this is of great influence to the performance!
Never move the feed itself away from the focus point though, it will distort your beam pattern! The focal point can easily be calculated and measured/adjusted.

For an offset the alignment is even more simple. Point the dish to the sun and wet the surface (a solid surface is the easiest of course...) and observe the sun projected  on the feed and you can easily optimize its position. 

In a prime focus dish, reflection on the midpoint can disturb your VSWR measurement. It may even damage your LNA, so always use an additional sequenced relay at the LNA-entrance. I know, it deteriorates the performance, but in the end you safe the LNA.
I have seen plates of absorbing material in the middle of the PF dish, to overcome this reflection and I advise to mount it (same as the size of the aperture) during measurements but maybe even forever. I don’t have it by the way, but that’s because I am lazy 😊

Hope this is useful, wish you success Paul!

73, Dick PA2DW

P.s. I will be at PI9CAM again coming Sunday in ARI-contest. Finally, we can operate it again! Hope to see you and all 😉

-----Original Message-----
From: Moon-net [mailto:moon-net-bounces at mailman.pe1itr.com]On Behalf Of Paul Andrews via Moon-net
Sent: 11 September 2020 01:27
To: Moon-Net
Subject: [Moon-Net] Finding the focus point in a dish

Hello Moon Folk,

I am amazed at how the measured VSWR of a feed changes when you put it into a dish.  The feed in open space is trying to couple all the transmitted energy into spare.  When the feed is placed in a dish, there are reflected signals from the test instrument involved that phase add and subtract in mysterious ways..

I'm now working with a new dish and a new feed.  The measured vswr can change significantly with small changes in feed position in the dish.

Life would be simple if the dish focus was found when vswr was minimum.  Or, should the feed be placed at the geometric focus point regardless of vswr?
Is there a predictable vswr event as the feed moves inside the focus and then outside the focus.

I will begin to experiment with focus vs vswr but I hope others have already blazed this trail and can provide some hints to what I will find.

73 - Paul - W2HRO

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