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Introduction to VHF and UHF Propagation

Sporadic E | Backscatter | Troposphere Scatter | Meteor Scatter | F2 | Aurora | TEP | Lightening Scatter

The importance of understanding basic VHF and UHF propagation cannot be overemphasized. Unlike DXing on the shortwave bands, you can't just turn on the receiver, tune around, and expect to get a faraway station. On VHF and UHF the normal situation is no DX --- until some abnormal propagation occurs. Also, unlike shortwave, there are several different kinds of propagation above 30 MHz. To DX effectively, you should be able to evaluate what you pick up. This requires being able to tell one type of propagation from another. Fortunately, the propagation modes differ sufficiently from one another that by combining a little experience with the descriptions to follow, you should seldom have any difficulty assigning one definite propagation mode to each DX catch.
Why is this so important? If all you care about is the quantity of DX --- receiving each station once --- it may not matter so much. But it does matter for quality. For example, over the course of several years you may be able to pick up a given station by two, or even three different propagation modes. One will undoubtedly be much rarer than the other(s). For instance, a channel 2 TV station 1000 miles away is relatively easy to see via sporadic E, while the same station is very seldom subject to tropo propagation over such a distance. On the other hand, a channel 7 TV station will seldom cover such a distance, but when it does it will do so more readily by tropo than by sporadic E. So let's look at these mechanisms to which we owe our hobby.

Sporadic E skip (Es)

Es has a set of characteristics which, when taken all together, set it apart from all other forms of DX propagation. It builds up from low frequencies to a certain maximum usable frequency (MUF) which may vary widely from minute to minute, and opening to opening. Es always hits the lower frequencies first. It may or may not get above channel 2. Of course, if the skip is coming from a sparsely populated area, there may be no channel 2 transmitter --- so check channels 3 and 4 as well. A good opening will not stop at channel 6, but may continue upward into the FM band which begins immediately above channel 6. An extraordinary opening may even continue into the aeronautical band above 108 MHz, through the "2 meter" (144 MHz) ham band, past the heavily-populated 2-way mobile bands, even up to channel 7 --- 180 MHz! An Es opening reaching channel 7 is a rare treat; high-band (TV channels 7-13) Es may even poke a channel or two above 7. Usually, highband Es can be ruled out when the skip is not extending above channel 6 into the FM band.

In general, as Es distances shorten on the low band and FM, the opening is becoming more intense and the MUF goes up. As experienced Dxers can attest, Es at 1000 miles is much more common than Es at 600 miles. The usual minimum distance for Es is about 500 miles. In fact, a study we made of hundreds of Es receptions revealed that 950 miles is the optimum distance on channel 2, with distances lengthening slightly with each higher channel, up into the FM band.

Es occurs when patches in the E layer of the ionosphere, about 65 miles above ground, become ionized. This layer normally refracts shortwave and mediumwave signals but is transparent to VHF radiation. The cause of Es ionization is not precisely known; some researchers connect it with low pressure areas and thunderstorms. However, it has no direct connection with surface weather, and the novice's association of Es with regular clouds in the sky is completely erroneous.

For once thing, the Es patch must be at approximately the midpoint between the transmitter and receiver, far beyond the visible horizon. Sometimes the patches remain fairly stationary, but usually they move at speeds up to several hundred miles per hour, more or less in a straight line. This means that one station after another will come in, with quite a lot of interference as they overlap.

They will probably be in a rough geographical progression, but not in a straight line. One can plot the midpoints on a map, and by correlating observations with other DXers viewing at the same time from other angles, pin down the Es patch with a degree of accuracy. This can prove useful in determining probable target areas (PTAs).

Long single hops of Es can reach about 1500 miles. Double-hop or cloud-to-cloud hop Es often occurs during the summer when more than one Es patch may be active simultaneously, in different parts of the continent. The two patches, the station, and you must all be along the same line. However, not too many stations are identified by double-hop Es for several reasons:

(1) Interference: the TV and FM bands are so congested in North America that there are usually stations on the air near the double-hop path midpoint, severely interfering with further stations.

(2) The earth is a rather poor reflector of VHF signals, but this it must do at the midpoint. Double-hop Es where the midpoint is water (an ocean or Great Lake) is much more efficient.

(3) The patch with the lower MUF is the controlling factor. For this reason, there's much more double-hop Es on channel 2 than on channel 6 or FM.

Es is very unpredictable, but we do know this much! Es is very much a summertime phenomenon in the temperate latitudes, with peaks in June and July; very good openings also in May and August; and a sprinkling in late April and early September. It can occur on any day of the year; these are known as off-peak openings. The winter solstice also brings a minor peak in December and January, as if some of the Es' fury were "bleeding over" from the southern hemisphere where, of course, the summer peak is in progress. The winter and off-season openings are most likely in the early evening hours. During the main "season", Es may start early in the morning and continue all day, into the night, but it likes to take a breather around mid-morning and mid-afternoon, and seldom lasts much past midnight. If you don't want to miss an Es opening, try checking once or twice an hour just before ID time, or tune a VHF radio paging channel.

Es can be very strong with lots of fading and interference. But strong signals may rival those of local stations and even interfere with them. Es may build up rapidly, over the course of a few minutes, but usually it decays more slowly. Weak openings in which the MUF hovers around TV channel 2 may tantalize you as stations fade in and out.

Es is more likely in southern areas during the off-season, but northerners should not assume that subzero temperatures or snowstorms rule out any DX! Closer to the equator, Es becomes more and more a year-round, daily phenomenon. "Diurnal Es" may provide a weak, scattery signal virtually every day over an Es-distance path. Other strange things happen, such as Es reception at double-hop distances but with the signal bouncing from one path to another without touching ground in between.

Backscatter

A powerful station too close to you for Es may be getting out elsewhere on Es; if the signal comes down on an ocean, a tiny portion of it may scatter back along the same path, but come down the second time in your location. There's a lot of distortion, especially if the backscatter signal interferes with direct groundwave or tropo reception.

Tropo (tropospheric bending)

Tropo is the other major form of DX propagation; as the name implies, it's dependent on conditions in the troposphere where weather takes place. In contrast to Es, tropo is best on higher frequencies --- though there is no downward progression of "minimum usable frequency". As a rule tropo is best on UHF, very good on the high VHF band and FM, but definitely inferior on the low VHF band. However, unless you have a top-notch UHF receiving installation, it may seem to you that tropo is best on high VHF and FM bands.

Tropo occurs along temperature inversions, often associated with frontal passage. It often happens over a large, stable high pressure area ahead of a cold front, especially where there is an influx of warm air from the Gulf mixing with colder air from the north. By correlating your tropo DX with weather maps, you should eventually be able to recognize the conditions likely to produce tropo in your area. Pay special attention to areas of the same atmospheric pressure (connected by isobars).

Extremely long distances (up to 1500 miles on UHF) may apply when, as rarely happens, the front is a straight line between you and the station. Tropo is legendary along the Gulf Coast --- where it's known as Gulf tropo. This has been known to blanket the entire coast up to 250 miles inland for a week at a time. This usually happens in non-frigid portions of the winter, and in the fall and spring.

Arid high elevations and mountainous areas form an effective barrier to tropo. Thus there are no instances known of tropo across the Rocky Mountains. Colorado and New Mexico stations east of the mountains do occasionally get tropo. Gulf tropo extends as far inland as Monterrey, Mexico, and as far south as Veracruz and other points along Campeche Bay. The entire island of Cuba can make it to the US on tropo. Other Caribbean islands have never reached the US on tropo; but easterners should be on the lookout for Bermuda, which has. Eastern mountain ranges are neither high nor dry enough to block out tropo. The midwest and Great Plains are perhaps second only to the Gulf Coast as prime areas of tropo activity. Areas around the Great Lakes are also excellent.

DXers in cold northern climes may expect little if any tropo during the winter months, except during abnormal warm spells. The spring and fall months seem to be the best, when there is a fairly wide temperature variation between day and night.

Ordinary tropo builds up quickly after sunrise but tends to "burn off" during the hot afternoon hours; it may fade back in after sunset from the same area seen in the morning.

Tropo may link up with other propagation modes, making it difficult to ascertain just how the signal gets from one place to another. Transequatorial scatter reaching the latitude of the Tropic of Cancer may be spread further by simultaneous tropo; instances of Es in the 1500-1900 mile range may be explained by a tropo link-up at one or both ends.

There is no minimum distance for tropo. Depending on your equipment, you may notice tropo improvement on stations as close as 50 miles; with a reasonable setup east of the Rockies, distances in the range up to 600 miles are not uncommon. UHF distances may at times surpass 1000 miles.

Tropo ducting is a condition which seems to behave rather like "skip", in that a nearer station in the same direction, on the same channel, may not necessarily block out a more distant one. The signal is actually ducted between air masses at different heights. As a result, the duct may pass over a closer station. Ducts are often frequency selective and may, for example, "carry" a few UHF channels and not affect others. Ducting may appear at any time of the day or night, and is the cause of most tropo over 400 miles. A duct may appear and vanish in little over an hour, or last for days. Tropo is the "steadiest" of any propagation; it seldom has rapid fading, but may fade slowly in and out. Weak tropo in the range slightly beyond that normally received is often called extended groundwave.

Meteor Scatter (MS)

MS is the one mode of propagation that is somewhat predictable. We've all seen "shooting stars"; these bright trails in the sky are also capable of reflecting VHF signals (even in the daytime when they cannot be seen). Astronomical studies abound showing in which days of the year one can expect the greatest frequency of "random" meteors (as the earth sweeps through meteor debris in its orbital path); and on certain dates meteor showers occur year after year, peaking within a couple of days of the same date. Consult almanacs or astronomical magazines for dates of predicted shower peaks.

Like Es, MS affects the lowest channels most, but more often than Es, MS can be noted on FM, channel 7, and above. Signal strengths are seldom great enough to produce MS observable with an indoor antenna. Some external antenna is a must, and the bigger, the better. Above FM you'll need to squeeze every dB (decibel, a unit of signal strength) possible out of the system (such as by amplification and stacking antennas). As the radiant point crosses the sky, the most favorable directions of MS DX change with it. Tables can give you a good idea of where to point your antenna during a shower at any given time. Then, you can actually plan what stations you want to try for, by finding out their schedules, and setting up for their direction. Dxing MS is a great way to fill in the gaps left by Es and tropo Dxing --- often in the 400-800 mile range --- and to hear/see new states at any distance up to 1400 miles.

Semi-local and tropo stations are anathema to MS DX'ers. For this reason, it may be reasonable to limit your MS DX'ing to the wee hours --- westerly stations lte at night, after nearby locals are off the air, or easterly stations in the morning before the locals return.

The true peak of a shower is determined by when your part of the world sweeps through the heaviest concentration of meteorites. But this time is skewed by the fact that around 6 a.m. local means the time your part of the world is meeting the debris head-on; the speed of the earth's rotation is added to the speed of the earth's revolution. Thus, other things being equal, MS peaks at 6 a.m. and reaches a low point at 6 p.m., when the reverse occurs. The combined rotation and revolution speed means the meteors come in at greater speeds, burn brighter, and produce more ionization --- and more DX, on the average.

MS is for the pure DX'er; it's impossible to watch or listen to a program by this propagation, for it may last from a split second to a little more than a minute. An individual "burst" may be on the order of a second or so, but sometimes larger meteors, or swarms of small ones may overlap, producing a very "choppy" assortment of signals. Swarms may cause bursts many seconds long.

Fortunately for the MS DX'er, the diurnal 6 a.m. peak coincides with the time of day when many TV stations are running continuous test patterns with their call letters displayed. This makes identification possible even in a split second. A video tape recorder or even a movie camera can be very helpful in the later identification of short bursts.

FM DX'ers often note MS overriding stations normally received with a weak signal. This DX should NOT be considered a nuisance! On TV one may occasionally identify more than one station during a single "long" burst, by rapidly flipping channels. Naturally, your TV receiver must be quite stable, with all the controls pre-set. But on FM one can often log several stations during one burst. The first rule is never stay on a frequency where the MS burst brings music; hardly ever will music provide any way to ID the station. Keep tuning until you hear talk. Again, a tape recorder can be helpful.

On the 88-92 MHz band, which is a bit more subject to MS than the higher channels, one can occasionally make ID's by paralleling different frequencies with the same programming. Since FM receivers are more sensitive than TV's, you can also choose a good clear frequency, sit on it, and hear dozens of "pings" --- occasionally lengthening into a burst with some identifiable information.

F2 Skip

Don't hold your breath for this one, but be aware of its potentialities. This is the familiar long-haul skip which bounces shortwave signals around the world in hops of roughly 2000 miles each. During sunspot peaks F2 may extend up to 60 MHz, opening American channel 2 to DX, plus a number of European, African and Oceanic TV stations operating at these frequencies and below. F2 activity is greatest in years of peak sunspot activity. As sunspot activity increases, F2 MUF also rise. F2 at VHF frequencies is not probable in years of low solar activity. If you want to keep track of F2 conditions, which tend to peak in the spring and fall, monitor on a 30-50 MHz radio. Paging stations beyond 1600 miles or so on 35 and 43 MHz will give you a warning when F2 is "up".

Note that some foreign TV audio transmission is AM, unlike North America where FM audio is used. North American DX'ers should check during the morning hours when it is noon, or slightly after noon, at the midpoint of the path to Europe. F2 is a daytime propagation at these frequences, directly depending on solar radiation. F2 may have a very sharp MUF, hovering just below or on channel 2 for long periods. So on your radio, keep a check on known TV audio and video carrier frequencies below channel 2. Beware of false signals if radiation near these last frequencies are from your own or nearby TV sets! On a radio, video sounds like a rough buzz. On a TV, video propagated by F2 is rarely clear; though it may be strong, it will be heavily smeared.

Aurora

Aurorae can also cause DX. Southern DX'ers may never experience it, but it becomes more and more frequent at more northerly latitudes. Bureau of Standards Radio Station WWV advises of magnetic disturbances and auroral activity periodically; you can also spot auroral conditions from blackouts in reception from northerly stations on mediumwave and shortwave.

Of all ionospheric propagations, aurora is probably the one most observed on the high VHF band (but not UHF) as well as the low. But it's quite difficult to identify television video via aurora, because of very heavy distortion and interference (as you can imagine from viewing an aurora, it's hardly a perfect reflector). You may have a chance on the audio side, however. FM and VHF radio DX'ers are much more successful at auroral DX'ing, with distances typically in the 400-800 mile range, but with no specific lower or upper limit. You may find that the signal is not coming from the direction you would expect. It is bouncing off the auroral curtain, which may be as much as 45 or 90 degrees away from the direction of the station, but not necessarily at due north. Auroral activity is most likely around the equinoxes, but may happen at any time when the proper solar disturbances occur. Heavy auroral activity can also induce regular Es during the following day or two and at 27-day intervals coinciding with the rotation of the sun.

Trans-equatorial scatter (TE)

TE seldom reaches very far north of the Tropic of Cancer, but it may link up with Es or tropo as previously explained. DX'ers in the southern tier of states from Texas to Florida should be on the lookout for TE in the spring and fall; September 1 is the traditional start of the fall TE season.

Video is more disturbed and useless in TE than in any other propagation mode, since the MUF flutters with extreme rapidity. Audio channels may be readable, however, TE usually builds up after sunset, peaking around 2000 local time; it may bring in the same station for hours at a time, night after night. Distances of several thousand miles are not unusual, and even stations not strictly on the other side of the equator may come in by this method, if they're beyond 1800 miles or so. TE can reach as high as about 65 mHz, i.e., channels 2 and 3 in the American system.

Lightning scatter (LS)

The ionization created by lightning bolts during an electrical storm may produce brief bursts of DX very similar to MS, but mostly seen on UHF channels where MS is not known to reach. The storm must be at the approximate midpoint of the path; distances range up to 700 miles.

Reproduced with permission by G Hauser

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