New feed for LRO-H3

[andrew....@googlemail.com]

 

https://www.astronomy.me.uk/changing-over-to-brian-colemans-hydrogen-line-feed-on-lro-h2-solar-cooker-radio-telescope

 

[Alex P]

The positioning of that feed equates to it probably seeing ~ 30cm dia of the dish, but with the central 15-20cm or so blocked by the feed.

[Alex P]

If this was a solar oven, how much light would be getting to the 1/4wl piece of feed wire ?   " see " the problem ???

[Preston Ozmar]

The feed does not appear to be at the focal point of the dish.

On Thu, Mar 19, 2026 at 3:15 PM 'Alex P' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

If this was a solar oven, how much light would be getting to the 1/4wl piece of feed wire ?   " see " the problem ???

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[andrew....@googlemail.com]

So far, I’ve only got as far as mounting it – next step is to sort out focus. Mounting it was hard enough for today!

Andy

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[andrew....@googlemail.com]

I still have focus to sort out – today was about getting it to sit in centre of dish – a bigger task than I thought it would be.

 

Obstruction = (20 x 20) / (150 x 150) = 1.8% central obstruction [NB I know area has pi in the equation but pi top and bottom cancel out].

 

Would the best place for the focal point be just inside the bottom end of the feed?

 

Andy

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[b alex pettit jr]

yes.  purpose of the long extension ?

[Andrew Thornett]

Not sure - good question - ill ask Brian who made it.

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From: 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: Thursday, March 19, 2026 9:26:04 PM
To: andrew.thornett via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Re: New feed for LRO-H3

 

yes.  purpose of the long extension ?

Would the best place for the focal point be just inside the bottom end of the feed?

 

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[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

Op do 19 mrt 2026 om 22:26 schreef 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

yes.  purpose of the long extension ?

Would the best place for the focal point be just inside the bottom end of the feed?

 

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[Andrew Thornett]

Thanks Eduard - very helpful!

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From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Eduard Mol <eddiem...@gmail.com>
Sent: Friday, March 20, 2026 7:21:49 AM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: Re: [SARA] Re: New feed for LRO-H3

 

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[Marcus D. Leech]

On 2026-03-20 11:20, 'Andrew Thornett' via Society of Amateur Radio Astronomers wrote:

Thanks Eduard - very helpful!

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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.

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[Andrew Thornett]

Thanks Marcus

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From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Marcus D. Leech <patchv...@gmail.com>
Sent: Friday, March 20, 2026 4:06:59 PM
To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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