Worldwide TV-FM DX Association

 

DX'ing FAQ  

 

 

Sunset over Phoenix antenna site

 

 

 

 

Originally written by Dave Austin

What is DX?
"DX" is radio jargon for "distant reception."  If you receive a signal from an unusually long distance, it is DX.
 
Where did the term "DX" come from?
Telegraphers over a century ago invented many abbreviations for common words or phrases.  If one didn't have to tap out all the Morse characters in a word or phrase, messages could be transmitted faster.  Amateur radio operators (hams) to this day frequently use such abbreviations.  Many ham abbreviations have become standard radio parlance; their definitions are widely understood and even transcend different languages.  Some other common ham abbreviations are:  QRM (interference), QSL (verification of reception), QTH (location), and QRP (reduced power). 
 
How does DX happen?
Broadly speaking, TV and FM DX takes place when normal atmospheric conditions change to permit distant reception. Disturbances in layers of the upper atmosphere allow TV and FM band signals to travel through "ducts" for several hundred miles.  Sunrise, sunset, and some weather patterns can set up temperature inversions that bend signals back to earth 100-200 miles beyond their source.  On rare occasions the ionospheric layers hundreds of miles up become dense enough to reflect TV and FM signals back to earth--500 to several thousand miles away from their source.  This last kind of DX mechanism is commonplace at lower frequencies: it makes worldwide shortwave broadcasts possible, and causes AM stations to come in farther at night.   But that's not "DX" because it's not unusual.  
 
So is this what they're talking about when local stations sometimes announce that atmospheric conditions are interfering with their signal?
Very likely.  The lower VHF channels (2 through 6) are more susceptible to sporadic interference from distant stations.  When I used to live near a TV station on channel 3, every summer when the "skip" kicked in they would run a crawler along the bottom of the screen explaining that the interference was due to atmospheric conditions beyond their control.
 
What's "skip"?
It means a signal-- that ordinarily would just go off into space and be forgotten--has bounced off the ionosphere (a layer of charged particles several hundred miles above the earth) and back to earth several hundreds or thousands of miles away. 
 
Is TV DX the same as getting out-of-town TV stations?
No. Typically, a TV signal is viewed throughout an area in a 25-100 mile radius of the station.  That means that every day you can count on watching your favorite programs on a certain group of stations in your antenna's range.  TV DX, on the other hand, is unusual.  You can tell it's a distant station by commercials, news, and of course the station identification (ID).
 
But don't you need special equipment?
Not always. Skip can be received adequately with rabbit ears. For some DX it is helpful to have the most sensitive antenna possible and to amplify the signal using a preamp. Sometimes DX can only be received with sophisticated equipment, but your ordinary TV antenna and set are sufficient for at least some DX'ing.
 
Does it require a special TV set?
Not necessarily.  Generally speaking, the newer "electronic" tuners (the ones with digital channel display that automatically fine-tune and block out weak signals) are not as desirable for DX as the old-fashioned click-in-place dials with a manual fine-tuning knob. Serious DXers can buy ultra-sensitive tuners that detect DX invisible to those with more typical equipment.
 
What about antennas?
Generally speaking, the more sophisticated antenna you have, the more DX you will see. Also, an outdoor antenna is almost always preferable to an indoor antenna. (A rotor is essential so you can point your antenna in the direction of the DX or minimize a local signal that is "interfering" with the DX!) However, it is entirely possible to see decent skip with rabbit ears in a basement! This is because the signal is coming down to you from the atmosphere rather than across the terrain.
 
Is TV DX predictable?
Only in the broadest sense, and even then there's no guarantees.  Generally, summer is the best time for TV DX.  DXers have also been able to chart activity over many years and find most probable times for DX based on past performance.  Important variables include weather systems, stable temperatures, sunspot activity, time of year, time of day, latitude, and path geometry.
 
How long does it last?
Generally, it lasts as long as the conditions creating it last. This can be anywhere from a few seconds to several days. Some kinds of "tropo" (tropospheric bending/ducting) can last a few days near coastal areas. I once saw channel 9 from Orlando in Myrtle Beach, South Carolina for a week. My wife's girlfriend from Tunisia used to watch Sicilian TV from Palermo all summer, but only in the summer.
 
Does it help if you're near water? What about mountains?
If water is between you and the signal, then water can be helpful, whether the reception is DX or normal, everyday reception.  Large bodies of water can refract (bend) signals, making long-haul DX possible.  Mountains have the opposite effect and tend to block out or reduce normal reception and do make some kinds of DX more difficult.  However, skip is entirely possible even if you are completely surrounded by mountains.  (Remember skip comes from high angles off the ionosphere several hundred miles in altitude.)  I once saw skip in Green Bank, West Virginia, where the National Radio Astronomy Observatory deep-space dishes are located. 
 
What's the point of watching distant TV when you can just get cable or a satellite dish?
If you are relying on DX for your daily TV reception, you're far better off getting cable or a dish. DX is an abnormal and temporary occurrence, not a source of regular TV or FM.  That's right, you can't count on DX modes to watch a non-blocked football game from another city.  However, your interest in DX may motivate you to acquire such superlative equipment that a distant TV market becomes commonplace reception for you.  For example, it would be reasonable for a person in Washington, DC or New York to set up a system optimal for DX'ing that would also allow daily, fair/good-quality reception of TV stations in Philadelphia.  DXers are fascinated by what and how far they can pick up.  
 
Is the picture clear?
Some DX comes in as clearly as a local station. Often, though, the DX signal is weak, laden with interference, fades in and out, is only video or audio, or all of the above. The harder the DX is to pin down, the more it becomes a sport.
 
Can I get these stations on satellite? Cable?
Satellite TV operates on a completely different set of frequencies and is not affected by the same kind of atmospheric conditions that create TV DX.  Your cable company's master antenna ("head end") may receive DX just like your rooftop antenna, especially if the regular station on that channel is quite far away or isn't on the air yet.  This can make for some fascinating and unusual DX!  Stronger local stations are rarely affected by DX interference on cable. (And sometimes a local station feeds its signal directly to the cable company, which wouldn't be susceptible to DX at all.)
 
Are all those antennas on your house going to interfere with my TV reception?
No.  TV antennas are for receiving only, not for transmitting.  Interference is only possible if you're transmitting something, not receiving signals.
 
Does cable ever interfere with TV DX?
It can.  Improperly installed and maintained cable lines can radiate signals unintentionally.  It can even behave like a DX signal.  If your regular TV stations are being interfered with by cable leakage, contact your local cable company.  (I don't think cable companies are under any obligation to eliminate your interference so you can DX.) 
 
Can you pick up stations from other countries?
In North America, DX signals from the United States, Canada and Mexico can cross the border and be received hundreds of miles inside the country, depending on what kind of DX it is.  Of course, people living near the border can often receive foreign stations on a daily basis anyway.  TV technical standards are the same throughout North America, so everyday reception as well as DX is possible using your present TV set.  "F2 skip" makes trans-oceanic reception conceivable, though it is rare.  Also, TV technical specifications are different in many countries outside North America.  The fundamental theory of TV reception is the same all over the world, but there are differences in frequencies, picture resolution, audio transmission, and electrical cycles, so you would need a TV set compatible with those specifications. (There is such a thing as a worldwide TV set, but they are quite expensive and usually purchased only by people who foresee moving back and forth, such as military and diplomatic personnel.)  
 
Can I also DX FM radio?
Yes!  The FM band is affected by the same kinds of atmospheric phenomena as TV.  The FM band (88 - 108 MHz) is situated just above TV channel 6 on the spectrum.  In fact, if you tune your radio to the very low end of the FM band you can hear the audio portion of channel 6 if you have a nearby station on that channel. I once heard WPVI-6 in Philadelphia while in Shenandoah National Park, a distance of over 200 miles, using my car radio.
 
How can I find out more about TV and FM DX?
This FAQ is intended to be a quick introduction to the subject.  For more technical information, check out our Treasury of Technical Articles.  Of course, the best way is to join our club!  Click here to send e-mail to the WTFDA staff.  Mike Bugaj will personally answer your questions or forward them to the right person. 
A typical suburban TV antenna
View off the Front Range over Denver.
A CATV headend in Delaware

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photos (top to bottom): sunset over Phoenix TV station towers; typical "suburban" all-channel TV antenna; view east off the Colorado Front Range from Denver's antenna farm; cable TV system headend somewhere in Delaware  (all pix by Tim McVey)