[VHFcontesting] Sizing a horn for a dish

Sean Waite waisean at gmail.com
Sat Sep 29 16:24:13 EDT 2018

I suspected that would be a problem. It's still handy as a short range

Making horns seems to be pretty simple. It's the coax transitions that get
me, I don't really have any of the equipment to measure these for proper
tuning if I made my own. I'll have to hunt down some commercial ones.

I'm on the uW list. Lots of good info there.


Sean WA1TE

On Sat, Sep 29, 2018 at 2:32 PM kb8u vhf <kb8u_vhf at hotmail.com> wrote:

> 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
> --------------------------------------
> From: Sean Waite <waisean at gmail.com>
> To: VHF Contesting <vhfcontesting at contesting.com>
> Subject: [VHFcontesting] Sizing a horn for a dish
> Message-ID:
>         <
> CAGz_XJxAwHVFK5J8G2xBTnNLJ0+jW2C4e2iGGAkrVC44d2HBMA at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
> 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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