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Radio Astronomy

Build a radio telescope · Track 2: hear the Milky Way

Make your own radio telescope

You can build a telescope that hears hydrogen gas floating between the stars of our Milky Way. Its radio waves are 21 cm (about 8 inches) long, so your telescope only has to be about as accurate as a good woodworking project. This page shows every step, with pictures, for two projects: an easy horn and a bigger dish.

Hydrogen's radio waves21.1 cm8.3 inches long, 1,420 million waves a second (1420.4 MHz)
Your dish may be off by2 cmabout ¾ inch and still work (SETI League rule)
Mesh holes can be up to2.6 cm1 inch; ½-inch hardware cloth is fine

How a radio telescope works

Stars and gas clouds give off radio waves. They are a kind of light, but your eyes can't see them. They come from so far away that they arrive in straight, side-by-side lines, like rain falling straight down on a windless day.

A dish has a special curve called a parabola. Every wave that hits it bounces to the same spot, the focus. Put a small catcher there, called the feed, and it collects the waves from the whole dish at once. A bigger dish catches more waves, so it hears fainter things.

A horn works a different way: it is a big metal-lined funnel that steers the waves down into a box at its narrow end. It is easier to build, but it hears a wider patch of sky, so its view is blurrier.

Inside the feed, a short copper wire called the probe turns the waves into a tiny electric signal. An amplifier makes it stronger, a USB radio turns it into numbers, and the computer draws a graph.

to the amplifier1234
  1. Radio waves from space arrive in straight, side-by-side lines.
  2. The curved dish bounces every one of them…
  3. …to the same spot: the focus.
  4. The feed sits at the focus and catches them. A cable takes the signal to the amplifier.

Words to know

WavelengthThe distance from the top of one wave to the top of the next. Hydrogen's are 21 cm long.
Frequency, MHzHow many waves pass each second. 1 MHz is a million a second, so 1420 MHz is 1,420 million.
Hydrogen lineThe radio "colour" that hydrogen gas gives off: 1420.4 MHz. Hydrogen is the most common stuff in the universe.
ParabolaThe curve of a dish. Waves coming straight in all bounce to one point.
FocusThat one point where all the bounced waves meet.
FeedThe catcher at the focus: a metal can with a probe inside.
ProbeA short copper wire in the can. Radio waves make tiny electric currents in it.
WaveguideA metal tube or box that steers radio waves inside it, the way a hose steers water. The can is one.
Amplifier (LNA)Makes the faint signal 10,000 times stronger. LNA means low-noise amplifier: it adds very little hiss of its own.
SDRSoftware-defined radio: a radio on a USB stick. The computer does the tuning.
Bias teeLets the radio send power up the same cable the signal comes down, so the amplifier needs no extra wires.
SpectrumA graph of how strong the signal is at each frequency. Hydrogen shows up as a bump.
BeamThe patch of sky the telescope hears at once. A bigger dish has a smaller beam and a sharper view.
K (kelvin)Radio astronomers measure signal strength as a temperature. More K means more signal.
dB and dBiA short way to write big multipliers: every 10 dB is ×10, so 40 dB is ×10,000.
f/DFocal length divided by dish width. A small number is a deep bowl; a big one is a shallow bowl.
Hardware clothStiff wire mesh from the hardware store, with ½-inch squares. It reflects radio waves like solid metal.
SMAThe small screw-on plug used on the amplifier and the radio.

Safety first

  • A grown-up runs or watches every power tool: the jigsaw, drill, utility knife and soldering iron.
  • Wear safety glasses when cutting, drilling or snipping wire. Cut wire mesh is sharp, so wear work gloves and tape over the cut edges.
  • A soldering iron is hotter than 300 °C (600 °F). Keep it in its stand, and solder where the air moves.
  • Use a new, empty metal can. A can that held paint thinner has fumes that catch fire easily.
  • Never look at the Sun, even when aiming at it. Use the shadow trick in First light.
  • Stay far from power lines. A dish catches wind like a sail: tie it down or bring it in when it's windy.

Pick your project

Both use the same electronics: a SAWbird+ H1 amplifier ($49.95) and an RTL-SDR Blog V4L radio ($37.95), $87.90 in all. If this is your first build, start with the horn.

Project 1: Horn

Easier
TimeOne weekend
Made fromFoil-faced foam board, foil tape, a metal can
SizeOpening 75 × 60 cm (30 × 24 in), about 95 cm (3 ft) long with the can; fits through a door
BeamAbout 19° wide
Signal from the Sun today—

Students build this one in NRAO's DSPIRA program. It clearly shows the Milky Way's hydrogen.

Project 2: Dish

Harder
TimeSeveral weekends
Made fromPlywood ribs, ½-inch hardware cloth, a metal can
SizeYou choose, 2 to 20 feet across
Beam8° for a 6-foot dish; smaller for bigger dishes
Signal from the Sun todaySee the calculator

A sharper view and a stronger signal, but more cutting and measuring.

Project 1

Build a horn

This horn follows the student design from NRAO's DSPIRA program (West Virginia University): a funnel of foil-faced foam with a metal can at its narrow end. We checked every size with the radio-wave formulas: the opening gives about 18 dBi of gain (about 65 times a simple antenna) and a beam about 19° wide, matching a 2025 university build of the same design.

Shopping list

Foil-faced foam insulation board, ¾ inch thick, one 4 × 8 ft sheethardware store
Aluminium foil tape (heating-duct tape; duct tape won't work, because it isn't metal)hardware store
A new, empty 1-gallon "F-style" metal can (the rectangular kind paint thinner comes in)online or paint store
SMA female bulkhead (panel-mount) connector with a solder cuponline
A few inches of 20-gauge solid copper wirehardware store
Two short SMA cables (6 to 12 in), male on both endsonline
A small plastic box with a lid, a USB extension cable and 2 clip-on ferrite chokesonline
Some 2×2 lumber for a standhardware store
Nooelec SAWbird+ H1 amplifier$49.95
RTL-SDR Blog V4L radio$37.95

Tools

  • Tape measure and a metal ruler or straightedge
  • Marker
  • Utility knife with fresh blades
  • Drill and a ¼-inch bit (SMA connectors have ¼-inch threads)
  • Can opener and a metal file
  • Soldering iron and solder
  • Wire cutters
  • Safety glasses
  • A level, or a level app on a phone

Electronics prices from the sellers' pages on October 20, 2026; prices change.

  1. Draw the four panels

    Lay the foam board flat, foil side up. You need two wide panels and two narrow panels, each shaped like a slice of pizza with the tip cut off (a trapezoid).

    To draw one: draw a straight centre line as long as the panel is tall. At the top of the line, measure half the top width out to the left and right and make a mark each side. Do the same at the bottom with half the bottom width. Then join the marks with straight lines.

    TallHalf the topHalf the bottom
    Wide panel74.3 cm 29 1/4 in37.7 cm 14 13/16 in8.4 cm 3 5/16 in
    Narrow panel75.9 cm 29 7/8 in30.0 cm 11 13/16 in5.2 cm 2 1/16 in

    All four fit on one sheet if you turn every other panel upside down.

    You got it right when every slanted side measures 79.8 cm (31 7/16 in). If one is off by more than 1 cm (⅜ in), redraw it.

    WIDE PANEL · MAKE 2NARROW PANEL · MAKE 275.4 cm · 29 11/16 in16.8 cm · 6 5/8 in74.3 cm tall29 1/4 in60.0 cm · 23 5/8 in10.4 cm · 4 1/8 in75.9 cm tall29 7/8 inCheck: all eight slanted sides = 79.8 cm (31 7/16 in)Foil side up. The small ends fit the can.

  2. Cut out the panels

    With a grown-up, cut along your lines with a utility knife against a metal straightedge. Hold the knife straight up and down, and make several light passes instead of one hard push. Keep the foil smooth: wrinkles and tears are fine on the outside, but the foil side becomes the inside of the horn.

    You have four panels whose sizes match the table.

  3. Open the can

    Measure the inside of your can's top: it should be about 16.8 × 10.4 cm (6 5/8 × 4 1/8 in). A grown-up cuts the whole top off with a can opener. File the sharp edge smooth, then cover it with foil tape.

    Now the can is a metal box with one closed end, called the bottom. It is the waveguide: radio waves can travel inside it at 1420 MHz.

    Your can's inside is within ½ cm (¼ in) of those sizes. If yours is quite different, ask us before you drill, because the probe position depends on the can's width.

  4. Drill the probe hole

    Pick one of the can's two wide sides. Find the line down its middle. Hold a ruler inside the can against the bottom and find 6.8 cm (2 11/16 in) up from the inside floor. Mark that spot on the outside, on the middle line.

    Put a piece of tape over the spot so the drill doesn't slide. A grown-up drills a small starter hole, then a ¼-inch hole.

    6.8 cm is a quarter of the wave's length inside this can (27.1 cm), which we worked out from the can's 16.8 cm width. DSPIRA's plans use 6.5 cm; both work.

    to the amplifier6.8 cm (2 11/16 in)from the closed end60 × 75 cm opening1234
    1. The can, with its closed end on the left.
    2. The copper probe, 6.8 cm from the closed end. The SMA plug is on the outside.
    3. The foil-lined horn steers the waves into the can.
    4. Radio waves come in through the big opening.

  5. Make and fit the probe

    Straighten about 8 cm (3 in) of copper wire. A grown-up solders one end into the cup on the back of the SMA connector, keeping the wire straight.

    From inside the can, push the connector out through the hole so its screw threads stick out. Tighten the nut on the outside. The wire now points straight across the can toward the other wide side.

    Trim the wire so exactly 5.25 cm (2 1/16 in) sticks into the can, measured from the inside wall. That is a quarter of a wavelength.

    The wire is straight, touches nothing, and its tip ends about halfway across the can.

  6. Tape the horn together

    Stand the panels up with the foil facing in: the two wide panels opposite each other, and the two narrow panels between them. The inside corners touch, and each end makes a rectangle.

    Reach in from the big end and run foil tape down every inside seam, so the metal lining has no gaps. Then tape the outside seams too.

    The small end is a 16.8 × 10.4 cm rectangle, and the big end is 75.4 × 60 cm.

  7. Join the can to the horn

    Put the can's open end against the small end of the horn. Line the can's wide sides up with the wide panels, so the SMA connector is on a wide side.

    Tape the inside joint with foil tape so the can's metal and the horn's foil touch all the way round. Then tape the outside.

    Shine a flashlight in the big end: you can see the probe, and no light leaks through any seam.

  8. Give it a stand and point it up

    The horn is light but floppy. Screw 2×2 wood pieces together into a frame that holds it by its corners, with the big opening facing up and the can at the bottom, high enough that the connector clears the ground.

    Lay a level (or a phone with a level app) across the opening and adjust until it reads level. Now it points straight up.

    Fix the plastic box with the amplifier and radio to the stand right beside the can, so the SMA cables can stay under 1 foot long (see Hook up the radio).

    Foam board and electronics don't like rain, so bring the horn in when you're done.

    Next: hook up the radio.

Project 2

Build a dish

A dish works when its surface stays within about a tenth of a wavelength of the perfect curve: about 2 cm (¾ in) at 1420 MHz. Wire mesh reflects like solid metal as long as its holes are smaller than about an eighth of a wavelength: about 2.6 cm (1 in). Both rules come from the SETI League (surface, mesh).

1 Choose your size

Type in the size you want. The calculator works out every number you need, and the tables update as you type. For a first dish, try 4 to 6 feet with a depth of 0.40.

Dish diameter
How wide the dish is, rim to rim. Bigger hears fainter things, but is harder to build and catches more wind.
Depth (f/D)
0.35 is a deep bowl; 0.50 is a shallow one. 0.40 is a good middle.
Ribs
The curved boards that hold the shape. More ribs keep the mesh closer to the curve. Use 8 to 12 for small dishes and 12 to 16 for big ones. The struts that hold the feed bolt to some of the rib tips, so these counts all split evenly into 3 or 4 struts.
Feed can inside diameter
Measure a round metal can (or duct, or tube) across the inside. The calculator tells you if it will work.
Rib height table
Where to draw the curve on each rib (step 2).
String to mesh
Your check that the finished dish has the right shape (step 6).
Petal table
How to cut the mesh pieces (step 7).
Feed can
Where to drill the probe hole (step 9), the collar and struts (steps 10 and 11), and distance A (step 12).

Dish and feed calculator

Rib heights (step 2) and the string check (step 6)
Mesh petals (step 7)
Feed can, collar and struts (steps 9 to 12)

How it's worked out: the curve is height = distance² ÷ (4 × focal length). Rib length is measured along that curve. Petal width is the straight gap between two ribs. Beam ≈ 70 × wavelength ÷ diameter; gain and Sun signal assume the dish uses 50% of its area. The can must let 1420 MHz travel in only one pattern (TE11 starts at λ/1.706 and TM01 at λ/1.306). The probe goes a quarter of the wave's length inside the can from the closed end. Where the focus sits inside the can uses the SETI League feed formula.

Shopping list

½- or ¾-inch plywood for the ribs and the hubhardware store
½-inch galvanised hardware cloth, at least as wide as your widest petal (see the calculator)hardware store
Lots of black (UV-resistant) zip tieshardware store
½-inch PVC pipe or plastic irrigation tubing for the rim ringhardware store
Metal angle brackets and screwshardware store
3 or 4 struts (the calculator says how many and how long): 1×2 boards, ¾-inch dowels or ¾-inch PVC pipehardware store
A piece of ¾-inch plywood for the collar, 2 stainless hose clamps that fit round your can, bolts and wing nutshardware store
A round metal can (check it in the calculator), or one of the other feeds, plus an SMA female bulkhead connector and some 20-gauge copper wire
Two short SMA cables (6 to 12 in), male on both endsonline
A small plastic box with a lid, a USB extension cable and 2 clip-on ferrite chokesonline
SAWbird+ H1 + RTL-SDR Blog V4L$87.90

Tools

  • Jigsaw (a grown-up runs it) and sandpaper
  • Drill, screwdriver, ¼-inch bit
  • Tin snips for the mesh
  • Work gloves and safety glasses
  • Tape measure, marker, a protractor
  • A thin, bendy strip of wood as long as a rib (a batten), and a few small nails
  • Strong string
  • Soldering iron, wire cutters, a level
  • Hole saw (or the jigsaw) for the collar hole

  1. Draw the curve on one rib

    This first rib becomes your pattern for all the others, so take your time.

    On a plywood board a bit longer than the "Distance from the centre" column, draw a straight zero line 3 inches up from the bottom edge. Starting at the left end (the centre end), make a tick on the zero line at each distance in the table.

    At each tick, measure straight up from the zero line by that row's Rib height, and make a dot. The first dot is right on the line; the last is the highest.

    Tap a small nail into each dot. Bend the thin wood strip so it rests against all the nails, and trace along it. That smooth line is your curve.

    Your curve passes through every dot without bumps or flat spots.

    0123456centre endrim endcut off and throw away1234
    1. Zero line, 3 inches up from the bottom edge.
    2. Ticks at the distances in the table (0 = the centre).
    3. Measure up by each rib height and make a dot.
    4. Bend a thin strip through the dots and trace it.

  2. Cut all the ribs

    A grown-up cuts along the curve with a jigsaw, and you sand it smooth. Then lay this rib on the next board, trace round it, and cut. Repeat until you have as many ribs as the calculator says.

    Stack the ribs together: the curves should line up within about ⅛ inch.

  3. Fix the ribs to a hub

    Cut two plywood circles about 10 inches across: these make the hub. With a protractor, mark lines from the centre every so many degrees (the calculator gives the angle; for 8 ribs it is 45°).

    The hub fills the middle of the dish, so cut the same amount off the centre end of every rib. For a 10-inch hub that is 5 inches: the ribs then start at the 5-inch mark.

    Fix each rib on a line, curve facing up, with metal angle brackets, and sandwich them between the two circles. Every rib's centre end must sit at the same height.

    From above, the ribs look like bicycle spokes, evenly spaced.

    45°1234
    1. The hub: two plywood circles.
    2. A rib, with its curve facing up.
    3. The rim ring ties the rib tips together.
    4. One mesh petal covers the gap between two ribs.

  4. Add the rim ring

    Cut PVC pipe or irrigation tubing to the calculator's Rim ring length. Bend it into a circle and screw or zip-tie it to every rib tip. It stops the ribs from twisting or wobbling.

    Measure across the rim between opposite rib tips: every measurement equals the dish diameter, within ¼ inch.

  5. Check the shape with a string

    Pull a string tight across the rim, over the centre, from one rib tip to the opposite one. At each distance in the table, measure from the string straight down to the top of the rib. Compare with the String to mesh column. At the centre it is the full depth: —.

    Do this for every pair of opposite ribs. If a rib is too high or too low, shim it or adjust its bracket.

    Every measurement is within ¾ inch (2 cm) of the table. Closer is better.

    at the centre: the full depthmeasure down:"String to mesh"string pulled tight across the rim

  6. Cut the mesh petals

    Put on gloves and safety glasses. Flat mesh can't bend into a bowl without wrinkling, so cut it into petals, one for each gap between ribs.

    Draw a petal on the mesh with a marker: draw a centre line as long as the last "Along the petal" number. At each distance along that line, mark the Petal width (half on each side), and join the marks. Then add 1 inch extra on each side for the overlap, and cut with tin snips.

    Use the first petal as a pattern for the rest. Tape over the sharp cut ends.

    Hold a petal over two ribs: it reaches from the centre to the rim, and covers both ribs with an inch to spare.

    12345
    1. The point goes at the centre of the dish.
    2. The wide end goes at the rim.
    3. Measure along this middle line.
    4. At each distance, the petal is as wide as the table says (yellow line).
    5. Dashed line: 1 inch extra on each side for the overlap.

  7. Fasten the mesh

    Lay each petal over its two ribs and zip-tie it to each rib about every 4 inches, working from the centre out to the rim. Pull it snug, so it follows the curve with no bulges. Where petals overlap on a rib, tie through both layers.

    Run the string check again over the mesh: still within ¾ inch everywhere, with no gaps between petals bigger than the mesh holes.

  8. Make the feed can

    Check your can in the calculator: it must say Good. The can needs one closed end and one open end. A grown-up removes the lid and files the edge smooth.

    No metal coffee can? A heating duct with an end cap, two big food cans, or a foil-lined tube work just as well: see Other ways to make the feed below.

    Drill a ¼-inch hole in the side at the calculator's Drill the probe hole distance, measured from the inside of the closed end. Solder copper wire to the SMA connector, push it through from the inside, tighten the nut, and trim the wire so 4.6 cm (1 13/16 in) sticks into the can, pointing at its middle.

    It doesn't matter which way round the can the probe faces: hydrogen's radio waves come in every direction of wiggle (they aren't polarized), so the probe catches them the same way at any angle.

    The probe is straight, points at the middle of the can, and touches nothing.

  9. Make a collar to hold the can

    The collar is a flat plywood plate that grips the can. The struts bolt to it.

    Cut the plate from ¾-inch plywood: —. In its middle, cut a round hole just big enough for the can to slide through (a hole saw or jigsaw; a grown-up does the cutting). Drill one bolt hole for each strut, on the line from the centre toward each corner, 2 inches out from the edge of the can hole. Leave at least 1 inch of wood all the way round the can hole.

    Slide the can into the hole, open end first. Put one hose clamp around the can just above the plate and one just below it, and tighten them with a screwdriver. The plate is now trapped between the two clamps.

    Keep the collar about 2 inches above the can's open end, below the SMA connector.

    You can't push the can up or down through the collar. Loosen the clamps and it slides; that is how you'll adjust it later.

    TOP VIEWSIDE VIEWcanamp + radioopen end123456
    1. The plywood plate, with a hole just big enough for the can.
    2. A hose clamp tightened around the can.
    3. One bolt hole for each strut, 2 inches out from the can hole.
    4. One clamp above the plate and one below hold the can still. Loosen them to slide the can up or down.
    5. The box with the amplifier and radio rides on the back of the can (step 13).
    6. The open end points down at the dish.

  10. Make and fit the struts

    The struts are the legs that hold the collar over the dish. Use wood (1×2 boards or ¾-inch dowels) or ¾-inch PVC pipe. Wood and plastic are best because they don't reflect radio waves. Use the thinnest struts that stay stiff.

    You need —. Each is — long from bolt hole to bolt hole; cut them 4 inches longer and drill the holes 2 inches in from each end.

    Bolt the bottom end of each strut to a rib tip at the rim, through the rib or with a metal angle bracket. Space them evenly: —. Lift the collar over the dish, open end of the can pointing down, and bolt the top end of each strut to a corner of the collar. Use wing nuts so you can loosen them without tools.

    The can hangs over the middle of the dish and only wobbles a little when you tap it.

    Adrawn to scale: 6-ft dish, 6-inch can (one strut is hidden behind)A = from the can's open end down to the mesh centre(the calculator gives this number)12345
    1. The feed can. The focus (yellow dot) sits a little inside its open end.
    2. The collar, with a hose clamp above it and one below.
    3. A strut, from a rib tip at the rim up to the collar.
    4. The electronics box on the back of the can, hidden in the can's own shadow.
    5. The USB cable, taped along a strut and down to the computer.

  11. Set the height, and centre the can

    Height: loosen the hose clamps and slide the can until distance A, from the can's open end straight down to the mesh at the centre of the dish, is —. Hold a tape measure straight up from the centre of the mesh. Then tighten the clamps.

    Centre: measure from the edge of the can's open end to the bottom of each strut. All of them must be equal, within ¼ inch. If one is longer, the can leans toward the other struts: loosen the wing nuts, nudge the collar, and measure again.

    Straight: with the dish pointing straight up, lay a small level on the back of the can. It should read level, so the can looks straight down at the centre.

    Distance A matches the calculator, all the rim measurements are equal, and the back of the can is level.

  12. Mount the electronics on the back of the can

    Put the amplifier and the radio together in a small plastic box with a lid, and zip-tie or screw the box to the back (closed end) of the can. A box no wider than the can stays hidden in the can's shadow, so it blocks no extra radio waves from the dish.

    Connect them with short SMA cables, each under 1 foot (30 cm): feed connector to the amplifier, then amplifier to the radio. Only the USB cable leaves the box. Tape it along one strut, down to the rim and on to the computer.

    Drill two small holes in the bottom of the box so any rain can drain, and use a light-coloured box so the sun doesn't cook the electronics.

    Only one cable, the USB cable, runs from the feed to the computer. See Hook up the radio for why.

  13. Set it up

    The simplest start: set the dish face-up on blocks so it points straight up. Lay a level across the rim and adjust until it reads level. The sky turning overhead does the pointing for you.

    Later you can build a frame that tilts it up and down while it faces south, like our 15-foot dishes at Hobbs. Always tie the dish down: wind can flip it.

    Next: hook up the radio.

No coffee can? Other ways to make the feed

The feed only has to be a metal tube with one closed end. It must be between 4.9 and 6.4 inches across inside: narrower, and 1420 MHz can't travel down it; wider, and a second wave pattern spoils it. It must also be at least as long as the calculator's Can length, at least (about 10¾ inches for a 6-inch tube). Any of these work. Measure the inside, type it into the calculator, then use its probe and distance-A numbers just as you would for a can.

6-in duct+ end capTwo #10food cansCardboard tube+ foil tapeRolledflashingclosed end at the top · gold = probe connector

6-inch heating duct + end cap

Easiest

Our first choice. Radio astronomers in the Society of Amateur Radio Astronomers (SARA) recommend a 6-inch heating duct with an end cap as a hydrogen-line feed. Buy a 6 in × 24 in round metal duct pipe and a 6-inch round duct cap in the heating aisle of a hardware store.

  1. Snap the duct's lengthwise seam shut, and run foil tape along it.
  2. With gloves on, cut it to about 12 inches with tin snips, or leave it 24 inches long: longer works too, it's just heavier. File the cut edge and tape over it.
  3. Push the end cap onto one end, and tape the joint all round with foil tape. That's the closed end.
  4. Measure the inside (about 6 in) for the calculator. Measure the probe hole from the inside face of the cap.

Two big #10 food cans

Easy

A "#10" can is the big size that school cafeterias, restaurants and food banks use for tomatoes, beans and fruit: 6 3/16 in across and 7 in tall. One is too short, so join two.

  1. Wash both. Take the lid off the first can; its bottom stays on as the closed end. Take the lid and the bottom off the second can, so it's an open tube.
  2. Stand the tube on the first can, rims touching, and wrap foil tape round the joint, inside and outside, so the metal is joined all the way round.
  3. Now it's 14 inches long. Measure the inside (about 6 1/8 in) for the calculator.

Cardboard tube + foil tape

Medium

Use a 6-inch cardboard concrete form tube from the cement aisle. The foil lining does the work; the cardboard just holds its shape.

  1. A grown-up saws off a 12-inch piece.
  2. Line the whole inside with aluminium foil tape, running the strips along the tube and overlapping each one by ½ inch. Smooth out bubbles.
  3. Cut a cardboard circle to close one end, cover it with foil tape, and tape it in so its foil touches the lining all the way round.
  4. Mount the probe connector from the inside. Its flat flange presses on the foil lining, which connects them.

Cardboard and rain don't mix: bring it in, or paint the outside.

Rolled aluminium flashing

Medium

Aluminium roofing flashing comes in rolls 14 in × 10 ft in the roofing aisle.

  1. Wearing gloves, cut a piece the full 14 inches wide and 20 inches long. Rolled up, that makes a tube 14 inches long and 6 inches across inside (6 × 3.14 = 18.85 inches round, plus about 1 inch of overlap).
  2. Roll it up, overlapping the edges by 1 inch. Hold the seam with pop rivets every 2 inches, or with foil tape inside and out. Check the tube is 6 inches across inside all along.
  3. For the closed end, cut a circle 8 inches across. Snip 1-inch cuts all round its edge, fold those tabs down over the end of the tube, and tape or rivet them on.

Buy one: Discovery Dish hydrogen feed

$125

KrakenRF's hydrogen-line feed has the amplifier and filter built in, so you don't need the SAWbird. It comes with 6 m (20 ft) of thick LMR-240-type cable, and it fits other dishes with a 3D-printed adapter.

It works best on dishes about 1 m (3 ft) across: on a 1 m Wi-Fi grid dish it beat a standard feed. For bigger dishes, SARA members suggest a can-style feed instead.

Advanced: dipole with a CD reflector

Hard

For small, deep dishes (60–90 cm), the German Astropeiler team used an 89 mm dipole made of semi-rigid cable with a balun, and an old CD as its reflector. It measured 5 to 7 dBi (their write-up). The balun makes it tricky: build this one with help from a radio amateur.

These won't work

ThingWhy not
Soup or vegetable cans, 46-oz juice cans (4¼ in), 4-inch ductToo narrow: under 4.9 inches inside, 1420 MHz can't travel down the tube.
A round 1-gallon paint can (6.6 in outside), 7- or 8-inch ductToo wide: over 6.4 inches inside, a second wave pattern gets in and spoils the feed.
A bare plastic or cardboard tubeRadio waves pass straight through plastic and cardboard. It needs a metal lining.
Duct tape instead of foil tapeDuct tape is cloth and plastic, not metal, so it doesn't join the metal pieces electrically.

Both projects

Hook up the radio

Feedhorn or canSAWbird+ H1amplifierRTL-SDR V4LradioComputerSDR# softwareSMASMADashed box: amplifier + radio in one box at the feedUSB4.5 V power travels back up the same cable (the bias tee)
Feedhorn or canSAWbird+ H1amplifierRTL-SDR V4LradioComputerSDR# softwareSMA, under 1 ftSMA, under 1 ftUSB, to the computerone boxat the feed4.5 Vpowergoes backup

  1. Connect the parts in the right order

    Connect your feed to the amplifier's RF Input with a short SMA cable. Connect the amplifier's RF Output to the RTL-SDR radio with the other short SMA cable. Plug the radio into a USB cable that runs to the computer.

    Keep the radio at the feed, too. Put the amplifier and the radio together in one plastic box right at the feed (on the back of the can for a dish, or on the stand beside the can for a horn), and keep each SMA cable under 1 foot (30 cm). Then the only long cable is the USB cable.

    The order is feed, amplifier (RF Input toward the feed), radio, then USB to the computer.

  2. Why short cables? Keeping interference out

    A long, thin coax cable acts like an extra antenna. It picks up interference, radio noise from phones, Wi-Fi, LED lights and computers, and that noise can bury the faint hydrogen signal. Thin cable also loses signal: at 1420 MHz, 20 feet of the thin RG-174 or RG-316 cable often sold with SMA plugs loses about 6.5 dB, more than three-quarters of the signal. The same length of thick LMR-240 loses about 1.9 dB.

    The USB cable carries numbers, not radio waves, so it doesn't add hiss. A plain USB 2.0 cable can be up to 16 feet (5 m) long. For longer runs use active USB extension cables (they have a tiny booster inside), up to about 98 feet (30 m) in all.

    More ways to keep the noise down:

    • Snap a clip-on ferrite choke onto the USB cable at each end. Computers leak noise down their cables, and the choke blocks it.
    • Run the laptop on its battery while you observe. Phone and laptop chargers are some of the noisiest things in a house.
    • Set the telescope up away from the house, Wi-Fi routers and LED lights, and keep your phone in your pocket or in airplane mode.
    • If the radio really must sit far from the feed, use thick, double-shielded coax such as LMR-240, never thin cable.

    With the radio at the feed and the chokes on, the thin spikes on the spectrum are fewer and smaller.

  3. Install the software

    On Windows, use the free program SDR# ("SDR sharp"). Follow the RTL-SDR quick-start guide step by step; it has pictures. The V4L radio needs the RTL-SDR Blog drivers, and SDR#'s install-rtlsdr.bat fetches them for you. On a Mac or Linux computer, the free program SDR++ works too.

    SDR# opens, you choose "RTL-SDR USB", press play, and a moving graph of noise appears.

  4. Turn on power for the amplifier

    The amplifier gets its power from the radio, up the same cable, through the bias tee. In SDR#, open the RTL-SDR settings (the gear button) and tick Offset Tuning. With the RTL-SDR Blog drivers, that box switches on the radio's 4.5-volt power (RTL-SDR Blog guide). Only switch it on when the amplifier is plugged in.

    A tiny red light glows through the hole next to the radio's antenna plug.

  5. Tune to hydrogen

    Set the frequency to 1,420.405 MHz and the sample rate to 2.4 MSPS. Turn off both automatic gain boxes (RTL AGC and Tuner AGC), and set the gain slider to a fixed spot about halfway. Turn up the averaging, so the wiggly noise line smooths out.

    Untick Offset Tuning: the noise line drops a lot. Tick it again: it jumps back up. That jump proves your amplifier is working.

First light: your first observations

"First light" is what astronomers call a new telescope's first look at the sky.

  1. Catch the Sun

    The Sun is the brightest radio object in the sky, which makes it the easiest test. It is highest, due south, at — today.

    Aim with the shadow trick; never look at the Sun. For a dish, move it until the can's shadow falls right in the centre of the dish. For a horn, move it until all four inside walls are evenly lit, with no shadow on any side.

    Watch the noise line: it should rise. Today the Sun adds — to the horn (for a dish, see "Sun signal today" in the calculator). The amplifier's own hiss is about 59 K, so the rise is easy to see. Then keep the telescope still: the Earth turns, the Sun drifts out of the beam, and the line falls again. For the horn's 19° beam that takes over an hour.

    Dish tip, focusing: while the Sun is in the beam, slide the can ½ inch up, then ½ inch down, keeping the dish aimed with the shadow trick. Leave the can where the Sun's signal is strongest.

    The line rises when the Sun is in the beam and falls when it drifts out. That's your first radio astronomy observation!

  2. Find the Milky Way's hydrogen

    Point the telescope straight up. The next time the Milky Way passes straight over Chippewa Falls is —.

    Average the spectrum for a minute or more. A wide, gentle bump rises near 1420.4 MHz. That is hydrogen gas in our galaxy, thousands of light-years away.

    The bump isn't exactly at 1420.405 MHz because the gas clouds move toward or away from us. That shifts the frequency, just as a siren sounds higher coming toward you and lower going away (the Doppler effect). Astronomers use those shifts to measure how the galaxy spins.

    Save a picture of the spectrum when the Milky Way is overhead and another hours later. The bump is in one and gone, or smaller, in the other.

  3. Record it and share it

    The free programs Virgo and ezRA save your spectra and draw proper graphs. More steps are on the Build a radio telescope page.

    Send us your first spectrum, or bring your telescope to a work day and test it next to our 15-foot dishes.

    Work days Contact us

If it doesn't work

What you seeWhat to try
The noise line doesn't jump when you tick Offset TuningCheck that the red light is on. Check that the cable from the radio goes to the amplifier's RF Output, not its input. Make sure the RTL-SDR Blog drivers are installed.
Nothing changes when you aim at the SunMake sure the probe isn't touching the can and its solder joint is solid. Check that the automatic gain is off. Check that the feed is connected to the amplifier's RF Input.
Tall, thin spikesThat's interference from phones, Wi-Fi, LED lights or computers. Move the telescope away from them, put ferrite chokes on the USB cable, run the laptop on battery, and keep the SMA cables short. Hydrogen is a wide, gentle bump, never a thin spike.
The graph looks flattened at the top, or jumps around wildlyThe gain is too high. Turn the gain slider down a few steps.
No hydrogen bumpCheck the time: the Milky Way must be in the beam. Average for longer. Check the frequency is 1,420.405 MHz.
Your can is a different sizeAsk us before drilling: the probe position depends on the can's width.

Sources: horn design from NRAO's DSPIRA program (WVU RAIL, NRAO) and its 2025 replication (arXiv:2512.19262: 75 × 60 cm opening, 18.5 dBi, 18–21° beam, 0.251 m² effective area). Our horn sizes were recomputed for 1420.405752 MHz: optimum-horn lengths 69.8 cm (H-plane) and 70.5 cm (E-plane), gain 18.1 dBi at 51% efficiency, can cutoff 892 MHz, next mode 1441 MHz. Amplifier: Nooelec (40 dB, 0.8 dB noise figure, about 122 mA, 3.3–5 V). Radio and bias tee: RTL-SDR Blog V4L guide (4.5 V, up to 180 mA). Dish rules: SETI League.