It has to be hard to be a kid interested in radio these days. When I was a kid, there was a lot of interesting things on shortwave. There wasn’t any cable TV (at least, not where I lived) so it was easy to hack antennas and try to pull in weak TV and broadcast stations. The TV stations were especially interesting.
It was one thing for me to build a dish antenna to pick up Star Trek from a station just barely out of range. But sometimes you’d get some really distant TV station. The world’s record is the reception of a BBC TV station in Australia (a distance of 10,800 miles). That’s extreme, but even from my childhood home near New Orleans, I’ve personally picked up TV stations from as far away as New Mexico. Have you ever wondered how that’s possible?
Radio signals behave differently depending on their frequency. The TV frequencies used in the old analog signals were VHF signals (well, the channels between 2 and 13 in the United States, anyway). In general, those signals usually travel through the air, but don’t bounce off any part of the atmosphere. So if you aren’t in a line of sight with the transmitter, you can’t see the broadcast. The other problem is that local stations tend to drown out weak distant stations. A TV DXer (ham lingo for someone trying to hear distant signals) has to wait for local stations to go silent or listen on frequencies where there are no local stations.
Basic Radio Propagation
At shortwave frequencies, distant propagation is much more common. Shortwaves travel via ground wave (short distance) and sky wave. However, parts of our atmosphere–particularly, the part about 25 to 250 miles overhead called the ionosphere–can bounce signals back to Earth (technically, the radio signals are refracted or bent; see image to the left). What makes the ionosphere special is that the air pressure is low enough that ions can travel for a long time without colliding into other atoms and turning neutral.
The ionosphere is divided into different layers and each layer has its own characteristic. The bottom layer is the D layer and tends to absorb radio signals, especially those at lower frequencies. However, the D layer also vanishes at night, which is part of why lower shortwave bands are usually dead during the day and active at night.
Above the D layer is the E layer. It also is a daytime-only layer, and at low frequencies it can absorb radio waves (although not nearly as much as the D layer). The E layer isn’t very important for shortwave frequencies, but for the TV (and FM radio) bands, it can provide E skip (see below).
If you are wondering why these layers disappear at night, it is because the lower layers are almost exclusively ionized by the energy of the sun. The E layer gets some ionization from other sources (like X-rays and meteors), but most of the ions come from the sun.
The F layer is the next part of the ionosphere, and is usually broken into the F1 and F2 layers. These layers are interesting because while the sun ionizes them, the atmospheric density is so low that ions formed during the day may not recombine all night long, so the F layer doesn’t always disappear at night–at least not all of it. The F1 layer is almost the same as the E layer and it does vanish at night. The F2 layer remains at night.
The F2 layer’s density and the frequency of the wave determines how much the radio wave is bent or refracted and this, in turn, determines how far apart the receiver and transmitter can be and still maintain contact. If the F2 layer isn’t very dense with ions, high frequency signals will not refract enough to go back to Earth and will, instead, just zoom into space. The denser the ions in the F2 layer, the higher the frequency that will refract back to Earth. People who study propagation quote the MUF (maximum usable frequency) as an indication of how dense the ionosphere is. TV signals have a pretty high frequency, so to get refraction in the F layer, the MUF must be very high.
The MUF isn’t the same everywhere on the Earth. You have to consider the MUF between two spots (say, Houston to Paris). Naturally, this changes based on the time of day and other factors like sunspots and other solar weather phenomena.
You can see a near real time map of MUF for 1800 mile paths online. You’ll probably notice that the highest numbers on the map are usually between 30 and 40 MHz–too low for TV signals. However, with enough solar activity, the MUF can rise high enough to refract even TV signals and reception over 2,000 miles is possible.
Another part of the ionosphere is the E layer and it is subject to having sporadic ionization. These ionized areas will reflect radio signals up to 1,400 miles. Sporadic ion clouds in the E layer are measured using ionosondes, and you can find maps showing where these ion clouds are.
E skip tends to come and go quickly, but can also be very strong. Sporadic E skip is thought to be responsible for the 1939 reception of an early Italian TV transmitter in England, for example. In 1957, a high-band (channel 7 to 13) signal was received via E skip in Arkansas. The transmitter was 2,300 miles away in Venezuela.
Normally, TV signals don’t bounce off the atmosphere because the MUF is too low, but certain weather conditions (temperature, density, and humidity) will cause the troposphere (the lowest layer of the atmosphere) to refract it. When a temperature inversion occurs (warm air over cool air), the troposphere can form a duct that can transport signals over a thousand miles.
Ducts have a tendency to form between the same two points and in some parts of the world, they will last for months at a time. Viewers often get accustomed to watching distance stations.
There is a special propagation mode that allows transmitters to hit receivers up to 5,000 miles away when the receiver is about the same distance from the equator as the transmitter (but on opposite sides of the equator). For example, television from Japan is sometimes received in Australia, thanks to transequitorial propagation.
There are actually two distinct times that this type of propagation occurs: afternoon to early evening and late evening. The earlier period usually doesn’t support very high frequencies. The later period tends to occur when there is high solar activity and low geomagnetic disturbance index.
When there is a meteor shower, hams use special software to communicate with other hams over long distances. This is often called Meteor scatter, but it actually relies on the ion clouds created in the E layer by the meteors. So from that perspective, this is the same as E skip, but generally of very short duration. The clouds generally only last for a matter of seconds.
The effect is greatest in the early morning hours, although with the right conditions, meteor-based propagation can happen at any time of day.
Although you don’t bounce signals directly against meteors, you can bounce a signal against the moon. The moon is about 239,000 miles away so path losses are around 240 dB. That means you have to have pretty good antennas and receivers to even attempt picking up signals bounced off the moon.
When there were fewer TV stations, it was slightly easier. In the mid 70’s, there were only two TV stations in the United States on UHF channel 68, for example, and [John Yurek] was able to pick them up via moonbounce using some homemade gear. The big dish in Arecibo has also picked up TV signals bounced off the moon. That dish, however, is a bit out of reach of most hackers as it is a 1,000 foot dish. However, radio hams frequently bounce signals off the moon with somewhat more modest antennas (see right).
Another space phenomenon that can cause distance TV reception is an aurora. Solar flares (as well as other solar weather events) take about a day to reach Earth and can create an aurora. Depending on the characteristics of the event, there may be an aurora and that can cause part of the atmosphere to reflect radio waves. However, signals propagated via aurora propagation tend to be distorted and flutter (that is, go up and down in volume rapidly). In addition, due to plasma particles having different velocities, there is a Doppler frequency shift, as well.
What about DTV?
Digital TV is subject to similar propagation effects. There are two problems. Today, you are more likely to have cable and less likely to have an external antenna well positioned for distance reception. The other problem is that the digital signals tend to degrade all at once. On an old analog signal you could squint and use the wet video processor between your ears to tease out a callsign from a snowy picture. With digital television, you probably are getting the signal or you aren’t. Sure, you might miss a few frames, but you don’t get the same kind of weak signals you got with the old system.
So TV DXing (and FM radio DXing) isn’t dead, but it isn’t as easy as it used to be. The video below shows [WD0AKX] doing some DTV DX during a band opening. If you get interested in trying yourself, there are a few good resources at the Worldwide TV FM DX Association (yes, that’s a thing).
If you are interested in propagation in general, a group of hams operate a world wide beacon system that can help you estimate what conditions are to different parts of the world. The beacons identify using Morse code, but since they broadcast on a known frequency and time, you don’t really need to be able to copy Morse in order to use the system.