TV Going The Distance: Propagation

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

Ionospheric_reflectionDay_and_Night
Image by Muttley (CC BY 3.0) via Wikimedia Commons

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.

F2 Skip

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.

E Skip

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.

Tropospheric Ducting

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.

Transequitorial

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.

Meteor

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.

Moon

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.

800px-I2FZX_UHF_EME_Antenna
“I2FZX UHF EME Antenna” by Spamhog (Public Domain via Commons)

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).

Aurora

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.

38 thoughts on “TV Going The Distance: Propagation

    1. Get yourself a set-top tuner, or “cable converter box” that can tune the cable channels. Hook your outside antenna to that and tune to cable channels 52-55, and you might see something, usually on Channel 54.

  1. One summer in Amherst, MA, I picked up a TV station from Chatenooga, TN on Sporadic E.

    I just connected a UHF beam in my attic to pick up the local DTV broadcast stations. In spite of what the marketing says, ANY UHF beam will work fine to receive DTV. Thus, I was able to get a brand new, older model high gain UHF antenna for very cheap. Since I had to run the signal down to my basement and the length of my house, I added a UHF preamp, at the antenna, and powered it with a coax tap in the basement. The result is rock solid reception of all the DTV stations in the area, and a happy wife.

  2. AM broadcast band DX’ing was my favorite when I was a kid. I had no real clue why I could pick up stations hundreds or thousands of miles away late in the evening during certain seasons until years later but it was fun.

    1. And if your screenname is indicative of your geography, I’m sorry to say I could have mailed you my copy of the Technician Class License Manual, if I hadn’t already done so to another fellow Hoosier looking to get their ham license. I picked it up for free from my area Ham club, so maybe you can do the same. Google your nearest big town/small city (whether you’re in Indiana or not) along with “Amateur Radio” and see what clubs are active and when they meet. Go to a meeting and hob nob. If no one at all has a Technician Manual to lend of give, then I miss my guess.

    1. According to Wikipedia (I know, teachers would kill me for using it directly as a source):

      “Propagation is affected by time of day. During the daytime the solar wind presses this layer closer to the Earth, thereby limiting how far it can reflect radio waves. Conversely, on the night (lee) side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection, called skywave. The extent of the effect is further influenced by the season, and the amount of sunspot activity.”

      Which makes it sound like it would be better to describe it as a nighttime layer (although that wouldn’t be correct either).

      1. I love Wikipedia, but I tend to avoid quoting articles that are flagged. If the article says “this article needs additional citations for verification”. You probably shouldn’t quote it. The Wikipedia article you quoted provides only a single citation.

    2. Well, actually there is a residual E layer at night, but it isn’t always reflected (no pun intended) in all models. There is actually a little bit of the D layer, I think, too, but not enough to be significant. Here’s another Wikipedia quote:

      At night the F layer is the only layer of significant ionization present, while the ionization in the E and D layers is extremely low. During the day, the D and E layers become much more heavily ionized, as does the F layer, which develops an additional, weaker region of ionisation known as the F1 layer. The F2 layer persists by day and night and is the region mainly responsible for the refraction of radio waves.

      So yeah, I probably shouldn’t have said “daytime-only” but at night it isn’t really significant.

  3. Nice summary, Al!

    Thank you for mentioning the International Beacon Project. The direct link to the timing of the beacons is on the Beacon Schedule Page.

    Hackaday readers might note that the hardware design and 8748 firmware that controls the beacons with GPS controlled timing is all open source and available on the site. The signals are timed accurately enough that it is possible to measure the propagation delay from transmission to reception and hence the length of the path taken.

    VE3SUN,
    IARU International Beacon Project Coordinator

  4. I have only done 3 states with TV but across the US with FM in the early summer openings.
    With DTV I only use the whole rig once a year to watch the Indy 500 from 80 mi. away defeating the blackout here.
    No joke; you had to get a new TV for sure, but a new antenna? Wait; yes, a very high gain model to pick up the new weak digital signals!

    1. The DTV signal is extremely fragile in the first place, and then they went and lowered the TX power on the point that “it’s digital, the picture is better on a weak signal”.

      So now a wind gust rattling the antenna will stop the picture for five second, and someone’s two-stroke outboard motor out on the lake will prevent you from watching TV at all.

        1. DVB-T signal has just enough redundancy to fix single bit-errors coming in at a rate of 5% which translates to what, 10 dB signal to noise ratio?

          In theory. The probability of getting more than 5% errors in a single frame is of course higher and you get dropped frames all the friggin time even when the signal is relatively good.

          1. Well, if the signal is so bad that 1/20th of what you pick up is random garbage, isn’t that kind of a SNR?

            Anyways, I think I remember that DVB was being sold on the idea that it would pick up to some ridiculously low SNR such as 12 dB etc. which of course is a practical impossibility because the noise isn’t uniform or constant.

          2. Well there is dB, dBV, dBP, dBM, dBW and if you read long enough dBMop.

            So I will assume you are talking about about a voltage induced into an antenna (which requires power to do). In that case 5% or one in twenty is about 13 dB down or -13dB

            In the analog days -20dB was good -17dB was acceptable and anything less needed to be fixed.

            BER isn’t the same thing, especially when you have signal compression which is like the compression of old days perhaps.

            The more you compress a signal (be it digital or analog) the more data transfer you need for error correction. There comes a point in digital error correction that the receiving end can’t make use of the error correction data as it also has errors. At some point, time becomes an issue as well because you have to correct the error correcting info and the corrected error correcting info has errors because it uses the same carrier.

            Remember this is pre-emptive. It’s not a two way communication so you can’t select which data you want clarified. You have to use the built in redundancy. So you have a sharp drop off of transmission quality at a critical bit error rate BER.

            Combine this with a required frame rate and then the other problem of time contributes to the issue.

          3. You can’t just assume 10dB at 5% BER, you really need to have the EB/No vs. BER curves for your particular hardware. Also, you need to understand the performance with isolated errors vs. burst errors, they are completely different.

        2. Apparently 8VSB is more spectrally and power-efficient, more resistant to transient noises like the forementioned two-stroke engine (the magneto is a spark gap emitter), and gives you a higher data rate, while COFDM deals better with multipath errors at the expense of lower payoad capacity, higher peak power and larger channel separation needed.

          So the former is useful in rural areas and the latter is better in cities.

          1. I would think that DSL has to deal with reflections from impedance mismatches in the lines, which is similiar to a multipath situation where OFDM is good.

            And in wifi is not similiar to television, because you can re-request packets. It isn’t disturbed much by rare random events like that, whereas television becomes annoying to watch because the loss of a single keyframe results in visible glitches for seconds, and the probability of corrupting the CC data in a stream during a program approaches 1.

            Seriously. Our public broadcaster insists on using the DVB closed caption streams instead of burning subtitles in the picture, which results in every foreign film/documentary/show being unwatchable because the subtitles will fail without fail at some point during an hour-long program and dissapear for a minute or so.

  5. As a ham radio guy, I approve of this post, although a bit simplified.

    I do a lot of Sporadic E contacts on the 6m band and uses the russian anlogue TV carriers as “early warning system” for openings

  6. The oddest propagation I have ever experienced in when a radio station on 103.3 out of Lincoln Nebraska captured the FM receiver in my car radio preventing me from listening to the local 103.3 station for the better part of a Saturday afternoon. The Nebraska station was 200 miles away. When I tried cycling the power the local station (100,000 Watts ERP) didn’t have enough shit in its pants to capture the receiver before the Nebraska station did. The path was about the same is is for 2 Meter amateur radio when we experience Tropospheric propagation here NW SW. Interesting stuff, but annoying sometimes. Whenever analog TV channel 2 experienced interfered I’d reach over to have the scanner scan low band UHF to hear distant land mobile radio and 6M hams.

  7. RF reception breaks my heart anymore. Like really makes me sad.

    I remember slowly tuning the shortwave radio that was built into a boombox I had as a kid and coming across so many interesting things – now most of them are gone or replaced by satellite coms. There are few old numbers stations out there and some time sync stations, but most of the cool stuff is long gone.

    I used to love to listen to local railroad operations on my scanner, from dispatchers and trains themselves to the roadside “hotbox” and “dragging” equipment detectors that would self report in their robotic voices. – Now all the railroad tracks have been taken up in my town and the distance ones I used to be able to get have switched to either sat-coms or digital mode radios that are encrypted with at least rolling inversion.

    Same thing with the local police – “to protect the innocent” and “officer safety” they have all switched to Kenwood digital radios with heavy encryption . Along with all the hospitals and EMS ambulance crews… DSD+ will show radio numbers and the like, but the voice component is encrypted for sure.

    About the only thing left is AM air band and ADS-B, those bring me a little joy. Sniffing neighboring outdoor wireless weather stations was fun for about a day, now its kind of ho-hum.

    Ham radio is terribly boring in and of itself, as all the local clubs are very much the 70-year old guys with “get the hell of my lawn” attitudes and they lovvvvveeee to treat newcomers with complete disdain because you haven’t spent your life savings on a “proper” ham shack and an antenna farm that would make the NSA jealous. And “how dare you pollute MY airwaves with your crappy sounding starter rig” – For a dying hobby, the guys that are left don’t seem to be helping bring in newcomers. And the guys that administer testing and are decent people, all seem to come from out of town to do so and operate primarily in modes you can’t attain with a just tech license. So you pretty much have to dive in completely to get beyond the local guys that have their little clique going.

    Sigint is interesting to me, but I don’t want to be breaking laws and stuff and the rest of what’s there isn’t all that interesting in the end.

  8. Hello WW. I just logged in to the FCC amateur radio test & got my call sign. Now I need stuff to tickle the eather. I don’t know about being boring. Maybe I will be, but if you get your hands ticket I’ll bet we can get the radio hornets out. Buzzzz. CQ trouble, CQ angry, CQ dangerous politics. AL6C K

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