Stop taking anyone’s word for it. Go and receive the evidence yourself.

00Free. Right now. On somebody else’s radio.

You do not need to own anything to start. There are receivers all over the world connected to the internet and left open to the public, called WebSDRs. You tune them from a browser. So:

1. Find out when the Space Station is next over a receiver. Any tracking site will tell you.
2. Open a public WebSDR near that receiver’s location.
3. Tune to 145.800 megahertz (MHz), mode FM (frequency modulation).
4. Listen.

You will hear the Space Station. Voice, sometimes. Data bursts, often. And you will hear it arrive and depart on a schedule that somebody published in advance.

01About $30. A photograph of the Earth, taken by a Russian satellite, delivered to your garden.

This is the first level where you own the evidence. Buy a software-defined radio dongle. They cost about $25 to $40. Hang a piece of wire outside. Then wait for a Russian weather satellite to fly over, and take its picture off the air.

What you are listening for

Russia’s Meteor-M satellites (2-3 and 2-4) transmit their images digitally, in the clear, on 137.9 MHz. Point the software at a high pass, let it run, and an image of the Earth builds up as the satellite crosses the sky. In color.

02About $30. Hear the Space Station, and measure how fast it is going.

This is the one. This is where you stop collecting demonstrations and start collecting a measurement.

Buy the cheapest handheld radio on the market. A Baofeng costs about $25 to $30 and comes with a stub antenna that is good enough. You do not need a license to listen. Not in the United States, not anywhere. A license is only needed to transmit.

Now do the part that matters

Find a pass that goes at least 30° above the horizon. Higher is better. Start listening two minutes before it rises. And watch the frequency, not the signal.

The carrier does not sit still. It arrives about 3.5 kilohertz (kHz) high, near 145.8035 MHz. As the station comes overhead it slides down through 145.800. And as it leaves, it keeps sliding, to about 3.5 kHz low. You have to retune your radio to follow it.

That slide is the Doppler shift, and it is not a curiosity. It is a measurement, and you can check it against physics in one line:

Now ask what else could do that. A light on a dome does not Doppler-shift. A projection does not Doppler-shift. A stationary transmitter on a hill does not Doppler-shift. Only a thing that is genuinely moving at 7.66 km/s relative to you does that to a radio wave, and it does it whether or not anybody believes in it.

And on a good day it will send you a photograph

Periodically the Russian Service Module transmits slow-scan television, on the same 145.800 MHz, using the callsign RS0ISS, in a mode called PD-120. You do not need a decoder box. Hold your phone next to the radio’s speaker, run a free SSTV app (Robot36 on Android, an SSTV app on iPhone, MMSSTV on Windows), and the picture builds down the screen, line by line, sent to you from orbit. The software, and what to do with it →

03About $100. The whole round Earth, live, from 35,786 km, on a repurposed WiFi antenna.

This is the one that ends the argument about photographs.

The American GOES weather satellites sit in geostationary orbit and broadcast full-disk images of the entire Earth, continuously, unencrypted, at anybody who cares to listen. It is not a press release. It is a radio transmission, and it is aimed at the ground.

Tune to 1694.1 MHz. Point the dish. And the entire sunlit face of the Earth builds on your screen, over and over, all day.

And now the part nobody points at

04A license, and some patience. Talk to the Moon and wait two and a half seconds for the answer.

This one you cannot do with a dongle, and it is worth describing anyway, because it is the cleanest measurement of the lot.

It is called EME, for Earth–Moon–Earth, and radio amateurs have been doing it since the 1950s. You point an antenna at the Moon. You transmit. The signal travels 384,000 km, scatters off the lunar surface, and comes back. And you hear your own voice, or your own morse, coming back at you out of the sky.

And there is a button for it. The free software that made amateur moonbounce practical is WSJT-X, written by Joe Taylor, K1JT, a Nobel laureate in physics who then wrote you some free software. It contains a mode called Echo, and the entire purpose of that mode is to send a tone at the Moon, listen for your own signal coming back, and measure it. It corrects for the Doppler shift on its own. Where to get it, and what the modes are →

It is not easy. The path loss is enormous, the antennas are large, and you need a license to transmit at all. But the barrier is effort and paperwork, not secrecy. Nothing about this is hidden. There are people doing it tonight, on published frequencies, and they will happily tell you how.

ANThe antenna. You cut it yourself, and the length is the argument.

Everything above this point runs through a receiver you bought. That is a fair objection, and it deserves a straight answer. A dongle is a sealed box, built by somebody else, running software written by somebody else. If you do not trust the people who make the equipment, then the equipment cannot settle anything for you.

So build the antenna yourself. It is a length of metal. There is nothing inside it to accuse of anything.

And the length is not arbitrary. It falls out of one equation, and that equation has the speed of light sitting in the middle of it.

Cut the metal to that length and it hears the Space Station. Cut it to any old length and it does not. That is a prediction, it came from physics, and you can test it on a kitchen table with a tape measure and a hacksaw.

The lengths, straight out of the one equation

Two frequencies matter here. The weather satellites sit near 137 MHz, so take 137.5 MHz as the middle of that band. The Space Station is on 145.800 MHz. Divide the speed of light by each:

Now look at the two numbers in the last column, and take them seriously for a moment.

137.5 MHz is the lower frequency, and it needs the longer element. Divide one leg by the other: 54.5 ÷ 51.4 = 1.060. Now divide the frequencies the other way round: 145.8 ÷ 137.5 = 1.060. The same number.

Length falls as frequency rises, in lockstep, because the two of them multiply to a fixed quantity. The fixed quantity is the speed of light. You did not have to take anybody's word for that. It is in the ratio of two pieces of metal on your workbench.

Build 1 — the V-dipole, for the weather satellites

This is the antenna for Level 1, and it is the easiest real antenna there is. Two metal rods and a terminal block.

Mount it outdoors with a clear view of the sky. The satellite passes overhead from pole to pole, which is why the V lies north to south: the antenna is aimed at the strip of sky the satellite will walk across.

Why 53.4 cm, when the table above says 54.5? That is the trim. The published figure comes from the people who sell the dipole kit most readers will own, and it works. If you cut 54 cm instead, it will also work. This is a receiving antenna.

Build 2 — the half-wave dipole, for the Space Station

Same antenna, different length, and straight instead of bent. This is the one for Level 2.

Why 48.9 cm, when a quarter of the wavelength is 51.4? This is the same trim as before, and it has a name: the velocity factor. A radio wave travels a little slower along a wire than it does through empty space, at roughly 95 percent of the speed. So the metal needs to be about 95 percent as long.

Hams have carried this around as a shortcut for a century: the total length of a half-wave dipole, in feet, is 468 divided by the frequency in MHz. Run it: 468 ÷ 145.8 = 3.21 feet = 97.8 cm. The same answer the physics gave. The shortcut is the physics, with the 95 percent already folded in.

For satellites, mount it horizontally. For repeaters and other signals from the ground, mount it vertically instead, because almost everything transmitted from the ground is vertically polarized.

Build 3 — the folded dipole, which is a different animal

A folded dipole is the same half-wave length, but the conductor runs out to the tip and then comes back, making a long thin loop. It looks like a small change. It is not.

Feeding a loop instead of a rod changes the impedance the antenna presents to the cable, and it changes it by a factor of four. A plain dipole in free space is near 73 ohms. Fold it and you are near 300 ohms.

So a folded dipole is not a drop-in for 50 ohm coax. You feed it with a 4:1 balun, or with 300 ohm twin-lead, which is the ribbon cable that used to run to a rooftop television aerial. In exchange you get an antenna with a wider useful bandwidth, and one that is mechanically stronger, because it is a closed loop rather than two rods hanging off a block.

That bandwidth is the reason it is worth knowing about. A plain dipole is sharp. A folded dipole covers more of the band without retuning.

Build 4 — the J-pole, in copper water pipe

This is the one that looks like a real antenna, and it is the one worth building if you build only one. It is copper plumbing. A hardware store has every part.

A J-pole is a half-wave element fed from its end rather than its middle. The end of a half-wave element is a very high impedance point, in the thousands of ohms, and 50 ohm coax cannot feed that. So the short arm of the J, the quarter-wave stub, is a transformer. It steps that impedance down to something the coax can drive. Tap it at the right height and you land on 50 ohms.

The payoff is that a J-pole needs no ground plane and no radials. It is self-contained. You put it on a mast and walk away.

Cut it from half-inch copper water pipe. You need one T-fitting, one elbow, a propane torch, flux and solder. Center conductor of the coax to the long element, braid to the stub. Wind four or five turns of the coax into a coil just below the antenna, which stops the feedline itself from radiating.

An aside on tuning, which matters less than you think

Antenna guides are full of talk about standing wave ratio (SWR). SWR measures how much of the power you send up the cable comes straight back down it, rejected by the antenna. A perfect match is 1:1. Below 2:1 is fine.

Here is the part the guides tend to bury. SWR is a transmitting problem. If you are listening, a mediocre match costs you a little signal and nothing else. Every antenna on this page will hear the Space Station without you ever owning a meter.

It matters the moment you transmit, which needs a license anyway. Power that will not go into the antenna has to go somewhere, and where it goes is back into your radio. On the J-pole, tuning is one adjustment: slide the feed tap up and down the stub, a few millimeters at a time, until the reading bottoms out. On a dipole, trim the tips.

SWThe software. What it is, where to get it, and what to do with it.

Everything on this page is free, and all of it is open source. Nothing here costs money, phones home, or asks you to make an account. Install it, point it at the sky, and it works.

SatDump — the one that does nearly everything

An SSTV decoder — for the picture from the Space Station

For Level 2, when the ISS transmits slow-scan television, you need something that turns the warbling audio into an image. You do not need a cable, an interface or a soundcard adapter. You hold your phone next to the radio’s speaker.

WSJT-X — for bouncing signals off the Moon

The odds and ends

05What all of this is for

None of these experiments is difficult. None of them is expensive. And not one of them requires you to believe a word that anybody, including this site, has told you.

That is the whole point. The argument about the shape of the Earth is usually conducted as a fight about whose photographs to trust, and that is a fight nobody wins, because a photograph can always be dismissed. So stop having it. Buy a twenty-five-dollar radio. Point it at the sky. And find out what is up there for yourself.