Cantenna, Horn or Dish: Choosing a Hydrogen-Line Antenna

The question we get most after “what SDR should I buy” is “do I need a dish.” The honest answer is no, not to start, and possibly not for a long time. But the reason is more interesting than “horns are cheaper,” and it took us an embarrassingly long time to state it correctly. Including in our own LAB-006, which says collecting area drives sensitivity more than anything else in the chain. For a point source that’s right. For the Galactic hydrogen line, it needs a qualification, and this paper is it.

The three options

Cantenna. A metal can, open at one end, with a quarter-wave probe inside. It’s a short length of circular waveguide. At 1420 MHz the can’s inside diameter has to fall between about 124 and 162 mm (Bench 1): narrower than that and the can is below cutoff and won’t carry the signal at all; wider and a second mode can propagate and the pattern gets unpredictable. A 150 mm can is comfortably in range. The probe goes about 93 mm from the closed end and is about 53 mm long, and the can should be at least 280 mm deep.

On its own a cantenna has low gain and a very wide beam. Its real purpose is as a feed: you put it at the focus of a dish, where its wide pattern is what you want for illuminating the reflector. Large coffee and paint cans are the classic source. Many are close enough to 150 mm to work.

Pyramidal horn. The antenna from LAB-008. A rectangular waveguide throat flaring to a large mouth. At 17 dBi (Bench 2) the mouth is about 675 by 514 mm and the beam is very roughly 20 to 25 degrees wide. Easy to build from foam board and foil tape, predictable, very little to go wrong, and you can carry it.

Dish. A parabolic reflector with a feed, usually a cantenna or a small horn, at the focus. A 1.8 m dish at 55 percent efficiency (Bench 3) gives about 26 dBi and an 8.2 degree beam. A 3 m dish (Bench 4) gives about 30 dBi and a 4.9 degree beam. Sixmile Run’s is in this class. Satellite TV dishes are the usual source: old C-band dishes for prime-focus, smaller offset Ku-band dishes if you can live with a smaller aperture and an awkward feed geometry.

Gain, beamwidth and what they mean here

Every 3 dB of gain halves the solid angle of the beam. Going from a 17 dBi horn to a 30 dBi dish is 13 dB, so the dish’s beam covers about one twentieth of the sky area. That has two consequences, and they cut in opposite directions for the hydrogen line.

Resolution goes up. A 5 degree beam can separate features along the Galactic plane that a 25 degree beam blurs together. A horn at l = 30° is also seeing l = 20° and l = 40° at the same time. If you want to map the plane in any detail, or do the rotation curve work in LAB-013 at more than a handful of longitudes, beamwidth is the thing that limits you.

But the line doesn’t get proportionally stronger. This is the qualification. For a compact source, such as the Sun or a supernova remnant, the power you collect scales with your antenna’s effective area, so a bigger dish sees it more strongly. The Galactic hydrogen emission isn’t compact. Along the plane it fills a band many degrees wide. When a source is larger than your beam, the antenna temperature you measure approaches the source’s brightness temperature no matter how big the antenna is. A bigger dish sees a smaller patch, at about the same temperature.

What the horn loses isn’t collecting area, it’s beam filling. Its 25 degree beam is wider than the bright part of the plane, so part of the beam is on fainter gas at higher latitude and the average comes down. That’s a real loss, maybe a factor of two or three compared with a dish, but it’s not the factor of twenty the gain difference suggests. It’s why the line is reliably visible with a horn in a few minutes, and why the horn is the right first antenna.

The noise calculator has a field for “line strength at the antenna.” This is the number that changes when you change antennas. For a dish on the inner plane we use 10 K as a cautious starting value; for a horn, 5 K. The receiver behind both is identical.

The effort side

The dish row is the one that surprises people. A 3 m dish with a 5 degree beam has to be pointed to well under a degree to stay on target, and holding that in wind takes a mount that weighs more than the dish. Our pier is concrete for a reason. If you want a dish, find the mount problem’s answer first.

Feeding a dish with a cantenna

The feed has to light up the dish’s surface evenly without spilling much past the rim. That’s controlled by the dish’s focal ratio f/D. A deep dish (small f/D) needs a wide-pattern feed; a shallow one needs a narrower feed. The dish and horn calculator gives the angle the rim subtends at the focus for your dish’s f/D, which is the angle your feed has to illuminate. A bare cantenna has a fairly wide pattern and suits deep prime-focus dishes in the f/D 0.35 to 0.45 range. Shallower dishes want a feed with a choke ring or a short flare.

Measure f/D from the dish itself: diameter across the rim, and depth at the centre. The focal length is D² / (16 × depth). Don’t trust the label on a salvaged dish.

What we’d tell a friend

Start with the horn. You’ll see the line in your first week, learn where your noise comes from, and find out whether you like doing this before you start looking at classified listings for C-band dishes. If you want resolution later, the horn becomes a test antenna and the same LNA and SDR move to the dish. Nothing in the receive chain from LAB-008 is wasted.

And whatever you build, don’t use an off-the-shelf UHF Yagi. The common amateur ones are cut for the 70 cm band, around 430 MHz, and are simply not 1420 MHz antennas.

Gear links are affiliate links. A dish, sadly, is not something we can link to. If it were, Walt would have stopped mentioning the hosting bill.