Hi and welcome to the ICDX FM/TV DX FAQ file. This FAQ is intended to demistify and explain the phenomena of FM/TV DX, the art of recieving Television and FM stations from beyond the boundaries of your town, and even your country!
The official home of the ICDX TV/FM DX FAQ is ICDX's home page at http://www.oocities.org/icdx_australia/index.html, you will find the latest version on our site.
Put quite simply, DX is receiving TV and FM stations from further away than your local stations. You may notice a station from way out of town can be viewed on a cool evening after a hot summer day. Or that a station from overseas is affecting your local station. These receptions are not entirely predictable but occur more often in summer than in winter, and with high levels of sunspots (dark spots on the sun).
Although radio (and Television) signals travel in straight lines, it is possible to bend or "refract" them. Normally the signal from your local station goes straight out the horizon and keeps right on going untill it disappears into space. But the upper atmosphere, called the "Ionosphere", can become electrically charged or "Ionised" when particles given off from the sun hit it. Ionisation usually occurs in patches or "clouds" and when the charge is strong enough it will bend signals back down to earth. The effects begin with low frequencies first and gradually move up to the higher frequencies.. This is broadly known as "Ionospheric propagation".
Closer to the ground is the Troposphere. Here signals are refracted back to the Earth by dust particles and thermal effects caused by changes in weather patterns. The troposphere refracts most frequencies from VHF to UHF though not always as much as each other.
And there are several other ways in which signals are received outside the normal coverage area of the transmitter. In the following pages we'll discuss these "modes" in more detail.
The Ionosphere is made of a number of different layers, all having different effects on signals. The ones we are concerned with are:
F2. Approximately 320Km up, this layer combines with the F1 during the night but in daylight hours the two layers separate. Signals refracted from this layer are typically very weak, suffer from smearing, ghosting, and multiple images. F2 refraction also covers the greatest distance, usually some 4,000 Km.Up to 10,000 km is not impossible, there are a number of documented receptions of the former BBC1 VHF transmitter in London as far off as Melbourne and Sydney Australia. F2 is most likely in the winter months as the high solar radiation levels in summer disperse the layer.
TransEquatorial Skip: also called TransEquatorial Spread F. This occurs chiefly around sunset when the F layer breaks up to form the F1 and our favourite, the F2 layer. Refractions occur amongst the fragments and tend favour the North- South direction rather than East-West. TE is most frequent in summer when the greater temperature changes cause faster changes in the F layers.
SpE/Sporadic E: So called because it occurs unpredictably, though mainly during the summer months and a small peak around midwinter. SpE is the product of the E layer whose altitude is around 120Km. SpE signals are usually quite strong. It's not unusual to pick up distant programmes on an indoor antenna. SpE refraction will usually cover around 800-1600Km, Brisbane to Melbourne for instance. Because SpE signals are quite strong it's quite possible to receive several stations on the same channel so to identify them can be quite a task! Additionally, SpE is quite capable of several hops. That is, a signal may arrive somewhere, then travel back out to the ionosphere for a 2nd refraction back to earth. Pictures are usually quite clear but can suffer from ghosting and multiple images. FM radio also comes through clearly, but like TV can be affected by multipath distortion, the radio equivalent of ghosting.
SpE signals typically fade very quickly and erractically, From noise (snow) to overload in seconds. Often as though the antenna were blown about in a gusty wind. This is because SpE ion clouds travel at speed of up to 400Km/h causing rapid changes in strength and polarisation.
With Ionospheric propagation, the greater the ionisation, the higher the frequency that will be received at the distant location. For example, as SpE builds up, Melbourne DX enthusiasts first see RTQ 0 Darling Downs Qld, then ABSQ 1 Southern Downs Qld, then ABV 2 Brisbane, ABTQ3 Townsville Qld.. As the Australian Ch3 is in the international FM band this is a good time to look out for FM reception. When these signals fade out, it occurs in the reverse order..
Also the lower atmosphere, called the Troposphere, can refract signals in two main ways.
a) Dust particles in the air.
b) Hot air trapped above colder air, or vice versa.
Example A is known as "Troposcatter" and example B is "Temparature inversion".
Troposcatter is often used to extend the range of a VHF transmitter, and is often why a station can be recieved well outside its stated service area.
Of greater interest to DXers is Temperature Inversion. Usually referred to as "Tropo" or Tropospheric, this often happens after a hot day when the air cools quickly and traps hot air above. The layer between the hot and cool air iscalled an "Inversion Layer". It's this layer which acts in a similar way to the ionosphere. Distances involved are much shorter, usually from a few tens of miles to a few hundred. In rare cases 1000 miles is not unusual. "Tropo" can also occur during the morning when the sun starts to heat the air, trapping cooler air above and forming an inversion layer.. Tropo extends above the VHF bands, well into the UHF TV bands and can be a big nuisance for commercial radio services.
Meteor Shower: Meteors usually burn up in the upper atmosphere with no harm done. However, some enthusiasts achieve good success DXing TV signals refracted by the trail of debris. MS "Pings" last from a few seconds to a few minutes and fade slowly from a strong signal to nothing. A "ping" on the higher VHF channels will often be followed by a "ping" on the lower VHF channels. Signals on higher channels are usually stronger than those on lower channels. MS pings occur largely during the main meteor showers and refract signalsover distances up to those of SpE.
Aurora: The "Northern Lights" or Aurora Borealis and its downunder cousin Aurora Australis are another source of DX signals for the TV/FM DXer. During high solar activity a solar flare sends a burst of radiation toward the Earth. When it hits the Polar region the Aurora can become greatly intensified and refract signals in a similar way to the Ionosphere. Auroral DX occurs mainlyin the evening when you may even see the Auroral glow. Pictures look very much like F2 but with thick bars and a loud hum on sound, FM signals are accompanied by a strong buzz or hum. Frequencies up to 200 Mhz are not infrequent. The Winter and Summer equinoxes (March - April, September - October) are the best times for this mode.
Lightning Scatter: The intense ionisation produced by a lightning strike can refract VHF to low UHF signals up to 800Km. Essentially you need a storm moving across between you and the transmitter. Reception would be very short and possibly difficult to identify unless you had a good idea what stations to expect. Sheet lightning is believed to give better results than forked lightning.
Knife-Edge: This is a phenomena observed by DXers living below a hill which obscures a transmitter. At times a refraction occurs at the brow of the hill allowing the transmitter to reach the DXer below. It is currently theorised that heat rising from the top of the hill causes a mini-inversion layer as in Tropospheric propagation.
One difficulty which exists in DXing is the different systems in use throughout the world. Fortunately there are not so many as there once were. The pre-war 441 line services are long gone, as are the old French 819 line and the old English 405 line systems. Still, TVDXers have modern technology at their disposal. A scanner can provide an Australian DXer with the sound of a US or European TV station even if their TV cannot. The main difficulties with overseas signals are:
* The spacing between Sound and Video frequencies, 4.5, 5.5, 6.0, and 6.5Mhz.
* Sound may use AM or FM modulation.
* There are three main color systems; NTSC, PAL and SECAM.
* There are two main scanning rates in use: 625 lines 50Hz and 525 lines 60Hz.
* Video polarity may be positive or negative. Most systems use negative.
Countries: Sound/Mod: Scanning: Color: Video
freq. polarity.
-------------------------------------------------------------------
UK (UHF only) 6.0Mhz/FM 625/50 PAL Negative
USA/JAPAN 4.5Mhz/FM 525/60 NTSC Negative
France/Luxemberg 6.6Mhz/AM 625/50 SECAM Positive
Australia/NZ 5.5Mhz/FM 625/50 PAL Negative.
With systems comverters becoming more affordable (actually it may be the popularity of Satellite reception pushing the sales figures), viewing USA TV on a UK set is not out of the question. Additionally, many recent model TVs and VCRs have the ability to display/replay a foreign system. Some upper range models are multi-system
Of course even when two countries do use the same system they don't usually use the same channel allocations. Not so long ago a U.S. TV set may not receive all Japanese channels and a New Zealand TV may not receive all Australian channels. As most modern TV sets have continuous tuning this is no problem for the DXer.
Basically, a TV set, FM tuner, antennas, and a notepad. Of course it's worth considering your equipment more carefully as there are a number of ways you can improve your lot. Don't forget, the higher the performance of your system, the greater will be your DX achievement..
Most DXers prefer the continuous tuner of the type, which is adjusted by a knob and has a scale similar to a radio. Today these use a "Varacap" tuner which uses a special type of diode to do the tuning.
* Must have "Radio" style tuning, a scale and knob like a radio. * It's better to have a set which is good at getting a weak channel next to a strong one if possible. * "AV" or video and audio sockets are a big must.. You can plug in your VCR and record what you see. Also you can add in a Teletext decoder and DX Teletext too! * Portables are handy for DXing on the move. And you can DX in a power failure. * Color TVs are fine, long ago they where not as sensitive as b/w sets. * LCD sets are very handy but gain may be poor on lower channels * Don't go for automatic tuning, auto search, or similar. * Sets which can receive more than one system are available, although expensive and hard to find. Note that SECAM and PAL usually use the same scanning standards so a Secam signal can be displayed in b&w on a PAL TV. It's possible for a PAL set to display NTSC in b&w although the vertical hold/ frame hold may need a good twiddle to do so. * Avoid automatic/search tuning systems, pushbuttons, and mechanical tuners. These make DXing difficult or impossible. Especially search programming as it can't be programmed for a channel without a signal being present.
FM Tuners should not have a muting cct, the cutting in and out sounds uncomfortable on tape. A high performance tuner or portable radio is ideal, but any decent receiver should do the trick. It's good to have stereo tuner as you can check for stereo reception and record your receptions in stereo.
* Digital frequency displays are really good, but there must be a manual
tuning knob. Up/Down pushbuttons are awkward to say the least.
* Scanners with FM coverage are good. Especially if they have a 30Khz
or 50Khz setting. 5Khz can be used to listen for subcarrier
transmissions. You can hear these but not without interference from
the main carrier.
* Modern pocket radios can be quite good but connecting an outside
antenna is often not provided for.
* As with TV an outside antenna is an advantage. Note that a good
tuner is needed as cheap ones can suffer too much overload from
local stations.
* If it has mute circuit, see if it can be disabled or there is a
user control to turn it off.
* A scanner or imported set covering the European "OIRT" FM band
(76-96Mhz) is really handy if you live where DX is possible from
such countries as Poland and Japan for example. Many scanners cover
this band, though not all in wideband mode.
* Separate UHF and VHF antennas provide much higher gain on each band
than combined antennas. Use a separate cable for each.
* Two short antennas in a "stacked array" are much easier to point at
a station,give greater gain, and are less cumbersome than a single
antenna.
* Make sure you include a Rotator in the installation. It will allow
you to turn your antenna to point toward sources of DX signals.
* Ideally a horizontal polarised antenna AND a vertically polarised
antenna are the way to go. But finances, space, and the aesthetics
of your street are often against the DXer. Dual polarisation
antennas are near impossible to get.. Note: In North America
all TV stations are horizontally polarized.
* Even a meagre antenna will give good results and indoor antennas can
bring in DX during the stronger SpE and Tropo reception. A "Log
Periodic" antenna is better than the typical suburban antenna, if
you can get one, the "Color Crossfire" is a great choice. A yagi
for each channel is grand but you need plenty of room.
* Good "front to back ratio", this is the ability of an antenna to
reject interfering signals coming from directly behind. The more
the better. In UHF antennas a "backscreen" or reflector array is
better than a single rod. Corner reflector antennas are very good
for this.
Often overlooked, cable can be a big problem in an antenna
installation. 75 ohm Coax(Coaxial cable) has some advantages over
300ohm flat ribbon cable and vice versa. The wrong cable adds
LOSS, loss is the opposite of gain, like gain it's measured in db.
The lower the number of db the better. Never connect ribbon and
coax directly together, this upsets the tuning of your antenna
system. Always connect via a balun transformer.
* Ribbon cable is not used for UHF, it is extremely lossy.
* Coax does not pick up stray signals and noise along its length.
* Although coax is more expensive for low loss it is better at higher
frequencies than ribbon.
* Coax is easier to install and not affected by adjacent metalwork.
* Make sure the coax is a good "low loss" type. Air-spaced is more
expensive but not necessarily the best. Some solid core types are
actually better.
* Make sure that baluns (a balun is a transformer which allows a ribbon
cable to be connected to a coax socket) are used where coax is
connected to a 300ohm antenna. Another example is where coax is run
from the antenna but the TV only has a connection for ribbon cable.
* Don't have any joins in coax. Even a plug and socket causes loss.
* Avoid splitters and combiners, you need every db of gain you can save.
Switches are ok, loss is very low and they're so convenient..
Once you start seeing DX receptions you'll want to record them for posterity and as a proof of your DXing. There are a few ways to do it too, you can whip out the camera, record the sound on a cassette, hook up the vcr, or even the high tech approach using a computer and digitiser. One important point, please try to announce/title all photos/recordings, DXers often exchange recordings and it can be difficult to match the contents list up. Also, please keep an accurate list of contents with your recordings. Let's look at how to make recordings of DX.
Camera: Taking a photo of your DX reception is not as easy as using an instamatic. Unfortunately the single speed shutter of modern instamatic cameras is simply too fast. At least one DXer has success using small closeup lenses on an old 126 Instamatic. The lenses are hard to get though, and, being for a Hasselblad dual lens camera, can be very expensive. Here's the way to get those offscreen photos looking nice.
Camera: You need a 35mm SLR (Single Lens Reflex) camera, it must have manual
exposure and shutter speed control. DX coding is fine as long as manual
control is available. A cheap one will do ok..
Film: Use ASA 200 (e.g Kodak Gold 200), either color or b&w as you prefer.
Exposure: Set the shutter speed to 1/25th of a second or slower and the F
stop (light setting) to F 5.6
Shooting: The TV screen should fill the viewfinder (autolabs will expose the
print incorrectly if the screen is surrounded by dark or black).
With small screens you will most likely need closeup lenses, purchase
+4 (4 dioptre) or 2 of the +2 (2 dioptre) types. Set the F stop to 3 to
compensate for light loss. An alternative is to use an extension tube
between camera and lens. Don't change the F setting. There is no light
loss in this case.
Tip: If using color film to shoot a b&w screen you may need a yellow filter to prevent the screen appearing blue.
If nothing else you can always record the sound on a cassette. Almost any cassette recorder will do. One important point though, if you don't have a direct connection, use a "Condenser" or electret microphone (these require a small battery) to avoid picking up hum from the vertical scan coils of your TV. Cassette radios are great for FM DX recording. Please announce all recordings, it's much easier for others to know what they are hearing.
Using a Videocassette recorder to record DX signals is a good idea but keep in mind that few early machines are much good at recording weak fuzzy pictures or several signals on the same channel. Likewise few will record foreign TV systems. Overall, anything you can catch on tape is worthwhile and the stronger SpE signals can give excellent results. Some VCRs will synchronise to the mains frequency if the off air signal is too weak, these give good results on poor signals. The author's ageing VCR doesn't like much snow on the picture at all. Try to get a VCR which doesn't have a video mute (goes to a black or blue screen when there is no signal). Try to announce all receptions on the tape, use the mic socket if your vcr has one. Alternatively use a small amplifier connected to the audio in socket. If you can't record an announcement, then please leave a short gap, record black by switching to "AUX" or AV IN without connecting anything to the video socket. If at all possible, please title your recordings. You can achieve better recordings by hooking your VCRs video/audio input sockets to the audio/video output sockets on a TV set rather than using the VCRs tuner. You can record FM DX on a VCR using the audio in socket, but some older VCRS won't work correctly without a video signal.
Today the computer allows you to capture not only a still picture and store it on a computer disk, but also capture moving video and sound. Indeed a PC equipped with a video digitiser such as Creative(tm) VideoBlaster(tm) could be used to capture DX video. Most computers have a sound card and the supplied software allows you to record 16 bit stereo sound at CD sampling rate on the hard disk. One problem is that computers are liable to cause significant interference. As long as interference can be minimised (keep the computer well away from the TV) this will work well. The card will tend to lose sync on weak signals, usually capturing only a partial frame or nothing at all. Generally you should be able to capture a moderately snowy picture, but expect difficulty with more than a modicum of snow. The author is aware of one DXer who has transferred his DX recordings to CD.
Although it's great fun to see a channel from some other place, and the further the better as DXers see it, you really should keep a record of what you receive. Most DXer's not only like to look back and reflect, but they often exchange info, such as recordings and logs. You can do quite a bit of record keeping in this hobby..
My favourite method is to use a notebook to jot down what I catch, then, once a month, I sit at my computer and transcribe it all into a wordprocessor. Now all that isn't necessary, after all there is nothing wrong with writing it all out in a multi column financial book. There are a wide range of PC Logging programs, although not all are well suited to TV. Most DXers I know seem to prefer a wordprocessor and set it out the way they like. Me too!
A good way to arrange your log is to make sure you note where a reception comes from, ie TV FM AM etc. Also the Town/Country as other DXers may not be familiar with the callsign.
Finally, here's an extract of a Melbourne DX'er's log showing a popular way of setting it out: (Note the use of 24 hour format to avoid confusion).
Example Log:
October 1996
01-06/10/1996 Nothing
07/10/1996 (FM) 22:45-23:32 94.1Mhz 2ABC-RN JERILDERIE NSW AUS
22:45-23:22 102.5Mhz 2MOR-FM DENILIQUIN NSW AUS
08-12/10/1996 Nothing
13/10/1996 (TV) 10:10-10:45 ABMN 0 WAGGA NSW AUS
12:45-12:55 SES 8 MT GAMBIER SA AUS
(FM) 10:10-13:20 103.3Mhz 3JJJ-FM ALBURY NSW AUS
13:10-13:20 104.1Mhz 3ABC-FM ALBURY NSW AUS
10:10-13:20 106.5Mhz 3MRR-FM ALBURY NSW AUS
(TV) 14:30-14:33 AMV 11 ALBURY NSW AUS
The following books were my references in the creation of this FAQ:
* Long Distance Television Reception (TV-DX) for the Enthusiast, BP52
Roger W. Bunney Bernard Babani (Publishing) LTD UK.
* Guide to World-wide Television Test Cards 2nd Ed.
Keith Hamer and Garry Smith, HS Publications.
The following publications are well worth reading for further info and up to date news..
* Television, Magazine, (C) Reed Business Publishing, Roger Bunney's
"Long Distance Television" column. Mostly Satellite news now, but
still some DX news.
* Practical Wireless, Magazine, (C) Reed Business Publishing, Ron Ham's
DX column.
ICDX: Contact ICDX Australlia, ICDX
Before I wind it all up I'd just like to say thanks for reading this, andhopefully you've learned a little about TVDX. The main aim of this FAQ is to give you some idea of what FM/TV DX is and how to do it. Please don't hesitate to e-mail your suggestions to ICDX Australia, likewise if you spot any errors or omissions please e-mail. Thanks again and please be sure to check the ICDX site regularly for updates.
Cheers and 73 from Wenlock Burton.