[VHFcontesting] Sizing a horn for a dish

[kb8u vhf]

The horn you have sounds like it's too big and you will not be effectively illuminating the whole dish if you try to use it.  It has too much gain so the beam width is too narrow.  If you moved the feed further away from the dish it would then be illuminating the whole dish, but then it would no longer be at the focal point so that won't work either.  There's also the issue of how uniformly the dish is illuminated.  In short, a feed horn has to be sized to illuminate the whole dish from the focal point taking into account the shape of the dish (f/D ratio).

I built a small horn to feed my dish at 10 GHz based on information from the W1GHZ microwave antenna book.  See figure 6.4-11 in http://www.w1ghz.org/antbook/chap6-4a.pdf
I think I ran the HDL_ANT program mentioned to generate the template.  It was pretty easy to do.  I later swapped it out for one of these: http://www.chaparral.net/feed-horns/offset-straight-feedhorn/ but I honestly didn't notice a huge improvement (but I did not actually make any measurements nor try to adjust anything for best performance).

A great resource is the microwave email reflector https://mailmanlists.us/mailman/listinfo/microwave

Very 73, Russ  KB8U
11/12 GHz Offset Straight Feedhorn | Chaparral Satellite Components<http://www.chaparral.net/feed-horns/offset-straight-feedhorn/>
www.chaparral.net
11/12 GHz Offset Straight Feedhorn. View our entire line of


W1GHZAntenna Book W1GHZ<http://www.w1ghz.org/antbook/chap6-4a.pdf>
www.w1ghz.org
As a simple example of a conical horn antenna, I took the NEC model for a large W2IMU dual-mode feed in Figure 6.5-2 and removed the output section, leaving only the flared section as a conical



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From: Sean Waite <waisean at gmail.com>
To: VHF Contesting <vhfcontesting at contesting.com>
Subject: [VHFcontesting] Sizing a horn for a dish
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        <CAGz_XJxAwHVFK5J8G2xBTnNLJ0+jW2C4e2iGGAkrVC44d2HBMA at mail.gmail.com>
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Hi all,

Now that I've (mostly) finished the rather significant process of cleaning
up my office and workbench and we've mostly settled into the "oh man there
are two kids now" lifestyle, I'm back to trying to get my 10 gig station
together.

I have a small horn with an attached waveguide transition. The waveguide
appears from my novice measurements to be WR90 (it's about .9" wide), and
the horn flairs out to about 7.5cm x 5cm. The seller claimed it to be about
16dBi of gain.

I have an 18" TV dish that I hope to use. I've seen W1GHZ's documents on
using bits of string and such to properly aim an offset dish, but I have no
idea if this horn is of a size to acceptably illuminate the dish. If not,
no big deal I can still use the horn by itself for short contacts, or where
having a dish would be irritating.

Does anyone have pointers to documents showing how to size this? Is this
horn a usable size? I assume horn gain changes how close or far the horn is
to the dish.

It's a whole new world up in these bands, and info is scarce. Any help is
appreciated!

73,
Sean WA1TE

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