If you've picked up a small satellite-TV offset dish for 10GHz (3cm band) work, then you might want to consider building your own feedhorn. The G3PHO dual-mode horn is a feed you build almost entirely from off-the-shelf copper plumbing fittings, no lathe required.
The design was published by Peter Day, G3PHO, in the mid-1990s, based on the classic W2IMU dual-mode horn (Dick Turrin's original work) and a modification popularised by WA5VJB. Paul Wade, W1GHZ, later ran the geometry through NEC2 simulation and refined one key dimension. The result is a cheap, repeatable feed that's still recommended on UK Microwave Group forums today, decades after it was first described.
Why dual-mode horn?
A plain open waveguide or probe feed tends to illuminate a dish unevenly — the E-plane and H-plane beamwidths differ, the phase centre shifts between planes, and energy spills past the edge of the reflector, picking up ground noise. A dual-mode horn deliberately excites a second waveguide mode in its flared section and dimensions the flare so the two modes largely cancel at the mouth. The payoff is a horn with matched E- and H-plane patterns, a common phase centre, and sidelobes better than -30dB — which translates directly into higher dish efficiency (60%+ is realistic) and a quieter feed.
Which dishes will this suit?
Dual-mode horns of this type are matched to a fairly narrow slice of dish geometry: an f/D — focal length divided by aperture diameter — of around 0.5 to 0.6. The 47mm mouth built here is optimised for the tighter end of that range, around f/D 0.6.
The classic 60cm offset Sky dish is increasingly hard to find secondhand, but it's worth knowing that f/D isn't unique to it — it's a pretty ordinary figure for small offset Ku-band satellite dishes in general. Domestic DBS-type dishes, whatever the brand (Sky, Freesat, DirecTV, and generic Triax/Winegard-style offset dishes sold for general satellite reception), commonly come out around f/D 0.55–0.65. So a secondhand offset dish of more or less any make, in roughly that range, should illuminate reasonably well — it doesn't have to be the specific dish this feed was originally designed for.
To check a candidate dish: F is the distance from the vertex (the lowest point of the reflector surface) out to the focus (where the TV LNB face normally sits), and D is the aperture — the straight-line width across the mouth of the dish, not the slant height. Divide F by D. If there's no spec label near the mount or in the listing, this is easy enough to measure directly with the feed arm still fitted. Bear in mind offset dishes are asymmetric, so this is a practical approximation rather than an exact figure — but it's enough to tell you whether a given dish is in the right ballpark before you commit an evening to building a feed for it.
This Parabola Calculator can quickly help you check to see if your dish is the correct f/D:
[parabola_calculator title="Check Your Dish f/D"]
If you want to go further and tailor the horn dimensions precisely to a specific dish, the free Parabola Calculator tool (the same code behind W1GHZ's HDL_ANT) will work out feed dimensions once you know your dish's geometry. A dish well outside 0.5–0.6 — a deep dish down around 0.35, say, or an unusually flat one above 0.8 — will want a different feed design entirely, not just a rescaled version of this one.
What you'll need
- One 42mm x 22mm copper pipe reducer fitting (this becomes the horn’s flared throat)
- One 42mm straight coupler (this becomes the wider mouth section)
- For coaxial version, a panel-mount SMA connector (rated to at least 12GHz), for a coax-fed probe launch into the throat
- A small hose clamp (jubilee clip)
- Pipe cutter or fine hacksaw, and a file
- A directional coupler or VSWR/reflection bridge for tuning
- Alternatively, skip the SMA connector and use a length of 22mm o.d. copper pipe plus a forming tool, if you’d rather terminate in a WG16 waveguide flange
Building the horn
One gotcha worth flagging before you go shopping: UK plumbing fittings are sized by the pipe's outer diameter, and a reducer's stated size describes the two pipe diameters it accepts, not necessarily an internal bore matching those numbers exactly. Take the spec sheet with you, or check the fitting in person, so you don't come home with the wrong reducer.Crucially, use solderless (push-fit) fittings. Any solder inside the horn's RF path adds loss you don't want.
