Successful UHF DX reception is mainly dependant by factors such as local terrain and geographical location. In some areas of the world, with hot humid weather, long-haul UHF television reception is relatively common during the Summer and Autumn months. This underscores that location and height above sea level are important factors for long-haul tropospheric UHF DX.
A UHF receiving installation located on a high mountain will have a considerable advantage for deep fringe reception. One example is Fernando Garcia, Monterrey, Nuevo Leon, Mexico, who has received UHF television signals from 16 US states up to 1,600 miles via tropospheric ducting. Fernando's OTH is 1,800 ft ASL. This underscores that height coupled with an unobstructed horizon, are very beneficial for long-haul UHF TV DX.
Tropospheric propagated signals travel in the part of the atmosphere adjacent to the surface and extending to some 25,000 feet. Such signals are thus directly affected by weather conditions extending over some hundreds of miles. During very settled, warm, anti-cyclonic weather (ie, high pressure), usually weak snowy TV signals from distant transmitters improve in signal strength. Another symptom during such conditions may be interference to the local transmitter, resulting in co-channel interference (CCI), which may be in the form of horizontal lines or an extra floating picture. A settled high pressure system gives the classic conditions for enhanced tropospheric propagation, in particular favouring signals which travel along the prevailing isobar pattern rather than accross it. Such weather conditions can occur at any time, but generally the Summer and Autumn months are the best periods. In certain favourable locations, enhanced tropospheric propagation may enable reception of UHF TV signals up to 1,000 miles or more.
The observable characteristics of such high pressure systems are usually clear, cloudless days with little or no wind. At sunset the upper air cools, as does the surface temperature, but at different rates. This produces a boundary or temperature gradient which allows an inversion level to form - a similar effect occurs at sunrise. The inversion is capable of allowing VHF and UHF signal propagation far beyond the normal radio horizon distance.
The inversion effectively reduces skywave radiation from a transmitter - normally VHF and UHF signals travel on into space when they reach the horizon, the refractive index of the ionosphere preventing signal return. With temperature inversion, however, the signal is to a large extent refracted over the horizon rather than continuing along a direct path into outer space.
Fog also produces good tropospheric results, again due to inversion effects. Fog occurs during high pressure weather, and if such conditions result in a large belt of fog with clear sky above, there will be heating of the upper fog level and thus an inversion. This situation often arises towards night fall, continues overnight and clears with the sunrise over a period of around 4-5 hours.
Tropospheric ducting
Tropospheric ducting of UHF television signals is relatively common during the Summer and Autumn months, and is the result of change in the refractive index of the atmosphere at the boundary between air masses of different temperatures and humidities. Using an analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rare upper air, imparting a downward curve to the wave travel.
Ducting can occur on a very large scale when a large mass of cold air is overrun by warm air. This is termed a temperature inversion, and the boundary between the two air masses may extend for 1,000 miles (1,800 km) or more along a stationary weather front.
Temperature inversions occur most frequently along coastal areas bordering large bodies of water. This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper layers.
Even though tropospheric ducting has been occasionally observed down to 40 MHz, the signal levels are usually very weak. Higher frequencies above 90 MHz are more favourably propagated. UHF TV frequencies are especially propagated by tropospheric modes.
High elevations and mountainous areas form an effective barrier to tropospheric signals. Thus, if your receiving location is at a low elevation, tropospheric reception may be relatively poor.
In certain parts of the world, notably the Mediterranean and the Arabian Gulf, tropospheric ducting conditions can become established for many months of the year to the extent that viewers enjoy regular quality reception of television signals over distances up to around 1,000 miles. Such conditions are normally optimum during the very hot settled Summer weather.
One DX enthusiast, listening from Pakistan, has received high band VHF TV via tropospheric ducting from Qatar (1,000 miles), and southern / western India (900-1,200 miles).
Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, and Bahrain and Pakistan, has produced UHF reception ranging from 1,000 to 3,000 miles (4,500 km).
Meteorologist Bill Hepburn's tropospheric forecast maps provide a very good indication of potential tropospheric openings. See link below.
Daily UHF tropospheric scatter propagation
TV reception beyond the horizon is common every day on the VHF/UHF bands without the aid of obvious propagation enhancement. "Ground wave", a term that is more appropriate to propagation on frequencies below 3 MHz, plays no important role above 50 MHz. Beyond the horizon propagation at VHF/UHF is possible because a tiny portion of the transmitted signal is scattered by small changes in the index of refraction of air, and by dust, clouds, and other naturally occuring particles in the troposphere.
The theoretical maximum distance that can be worked by tropospheric scatter is limited ultimately by the line of sight distance between the TV transmitter and receiving station have of the same scattering region of the troposphere. The highest altitude for which scattering is efficient at typical UHF TV transmitter power levels is about 6 miles. An application of the distance to horizon formula shows that 550 miles (900 Km) is the maximum distance for daily tropospheric scatter.
In practice, tropospheric scatter paths will likely be shorter than 550 miles because most receiving installations are less than optimal, and scattering path losses increase with frequency. Almost all extended-distance tropo scatter TV reception involves weak signals and considerable fading. Scatter may be received any time or season. However, there are diurnal variations for scatter efficiency. Scatter levels are generally higher around 6-9 AM and 9-12 PM. Also, scatter levels are higher during the Summer and Autumn months.
Tropospheric scatter reception over distances up to around 300 miles, averages 10 - 20 microvolts, with the use of a high gain 16dB UHF deep fringe TV antenna and 2dB (170K°) noise figure masthead preamplifier.
Extremely weak UHF signals (less than 5 microvolts) can be detected using high quality scanning receivers (Icom R-7100/R-7100/R-8500, etc). When using 15 KHz narrow FM bandwidth, UHF TV audio signals are often detectable out to 300-400 miles on a daily basis.
Extremely weak video carriers are best monitored using 2.4 Khz USB mode. Signal strengths are often as low as 0.052uV (-132.6dBm). Video carriers can be often detected at scanner level up to 500 miles on a daily basis.
PC FFT spectrum analyzers enable detection of extremely weak video carriers, which otherwise would be inaudible on a sensitive scanning receiver. Using a 1Hz bandwidth, signals can be seen down to 2dB above the noise level. This means that providing there are clear channels, scatter signals can be detected up to 700-900 KM on a daily basis. With this level of sensitivity, you will also have EME capability.
UHF advantages
Fringe or DX UHF TV reception has unique technical challenges, which leave little room for error. Let's consider some of the advantages for UHF verses VHF TV reception:
1. Man-made and external ambient noise levels at UHF are at least 20dB less than VHF band I (45-70 MHz).
2. Low noise masthead preamplifiers are very beneficial at UHF.
3. UHF antennas are smaller and compact, hence relatively easy to produce high gain aerials.
4. Due to the small wavelength size, it is more practical to stack high gain UHF antennas, but with manageable physical space.
5. Height gain is significant at UHF. Usually one obtains about 0.5dB gain for every foot heigher, for heights between 25 and 35ft. In other words, the same aerial will provide about 5dB more signal at 35ft than at 25ft.
6. UHF transmitter powers are very high, typically 500kW to 1000kW.
UHF disadvantages
1. UHF TV tuner noise figures are around 10dB (typical). VHF tuner noise figures are typically 6dB. This means UHF tuners are 4-6dB less sensitive than VHF tuners.
2. Signal path loss is much greater at UHF.
3. Trees, foliage, buildings, etc, greatly absorb UHF signals.
4. Transmission coax cable line losses are far greater at UHF compared to VHF band I.
5. UHF TV aerials need to be positioned higher than VHF aerials.
6. The voltage developed by a dipole element decreases inversely with frequency. For example, a band I dipole will develope from a given field strength about 15 times as much signal voltage as a high band V UHF dipole.
To help compensate for UHF disadvantages, we need to observe the following points:
1. Use high gain (13-18dB) 'deep fringe' type UHF only TV aerial(s) (see list below).
2. Use a quality low noise (2dB or less) masthead UHF preamplifier.
3. Use low loss 75 ohm coax cable (see table below). For example, RG-11 coax. Never use RG-75U coax!
4. Position the UHF antenna at least 30 ft above ground level. Within constructional limits, the higher the better.
Path losses at UHF
Radio propagation loss through space rises with increased frequency. If TV stations operating on channel 1 (57.25 MHz) and channel 68 (795 MHz) radiate equal power, have antennas at the same height, and are equal distances from the receiving antenna, the channel 2 signal will be 10 times stronger than the ch 68 signal.
Free space path loss is proportional to the square of the frequency; so UHF will be 20 dB weaker than band 1, all other factors being equal.
UHF receiver system sensitivity
Reducing the total receive system temperature is very important for weak signal TV reception. By placing a low noise (2dB or less) UHF preamplifier at the masthead (near the antenna), ideally all coax cable losses and the TV tuner noise figure are negated. This means that providing there is not excessive coax cable loss, the total receive noise figure will be 2dB or less.
A 0.5dB (35K°) noise figure UHF preamplifier is ideal for receiving weak signals. However, 0.5dB noise figure preamplifiers are either very expensive or hard to find. Also, 0.5dB noise figure preamplifiers may only realise their full potential in quiet rural areas. This is because man-made terrestrial noise peaks toward the optical horizon. Also, the 6 MHz total demodulated bandwidth of a UHF TV signal is relatively wide. Wider bandwidth equals greater noise, narrower bandwidth equals less noise. Hence, many DXers, living in a city or suburban area, may see no discernable difference between a 1dB and 2dB noise figure UHF TV masthead preamplifier.
High loss balun transformers can also degrade the total system noise figure. For example, assuming a low quality 300/75 ohm balun with 2dB signal loss is placed before the input of a 2dB noise figure masthead preamplifier, the masthead preamp would now have a 4dB noise figure (2+2=4)! This underscores the importance of carefully selecting a quality low loss balun, and minimizing any signal loss before the input of a masthead preamplifier.
Most high gain UHF TV aerials now have inbuilt printed circuit board baluns. Hopefully the signal loss of these baluns are no more than 0.5 to 1dB.
Let's assume that we are not using any UHF masthead preamplifier and there is 4dB coax cable loss between the antenna and TV:
1. Balun signal loss 1dB.
2. Coax cable loss 4dB.
3. TV tuner noise figure 10dB.
4. System noise figure 16dB (1+4+10 = 15).
5. A 15dB system noise figure is a poor receiving system for deep fringe UHF TV reception.
Now let's assume that we are using a 2dB noise figure UHF masthead preamplifier, 1dB loss balun, and there is 4dB coax cable loss between the antenna and TV. The following table will help you appreciate the significant improvement to weak signals by placing a low noise amplifier at the masthead:
dB Degrees °K
Typical UHF TV tuner noise figure 10 2610
20 meters of RG6 coax cable 4.0 439
Typical balun transformer loss 1.0 75
4 x belling lee connectors 0.25 17
Total losses 15.25 dB 9000
In the above example the Receiver System Sensitivity equates to 15.25 dB or 9000 K°.
Now we will add a low noise preamplifier (LNA) right at the masthead (antenna terminals):
dB Degrees °K
Masthead LNA 2.0 150
Typical balun transformer loss 1.0 75
4 x belling lee connectors 0.25 17
Total losses 3.25 dB 330
Providing the gain of the LNA exceeds the losses between it and the receiver (TV set), the above losses are negligible.
In the above example (with a LNA at the antenna feed) the receiver system sensitivity equals 3.25 dB (330 °K). This translates to a very significant 12 dB improvement. In other words, your UHF antenna array equates to a net improvement of making the antenna FOUR times as large!
Low noise UHF TV preamplifiers
A preamplifier (LNA) is a mast or antenna mounted amplifier used to either eliminate or minimize noise "snow" on the TV screen. At UHF frequencies, snow is electrical noise that is generated by the TV receiver, other electrical devices, and to a much lesser extent, the atmosphere. The objective of any antenna installation is to deliver enough signal to the TV set to override the noise (snow). A "weak" signal is one that is not strong enough to override the level of the noise in the set. Another obstacle is the signal loss incurred while traveling through the transmission line connecting the antenna to the TV set. An antenna that has acceptable gain at the antenna, but encounters excessive loss due to long cable runs, will also deliver a "snowy" or "noisy" picture to the TV set due to not enough signal makes it to the set to override the noise level. A pre-amp would be used to override the losses in the transmission line as well as the (typical 10dB) noise level of the TV set tuner.
Low noise (2dB or less) masthead preamplifiers are designed to improve the signal to noise ratio. This means increasing signal levels, but with minimal noise contribution.
High noise (4dB+) masthead preamplifers also increase signal levels, but they also increase noise, which can degrade picture quality on weak signals. Ideally we need a high signal to noise ratio (high signal/low noise) in order to improve weak signals.
In order to maintain the masthead preamplifier noise figure, the masthead voltage gain should be no less than 3dB greater than the TV tuner's noise figure after coax cable losses.
All preamplifierss have a power supply that plugs into an AC outlet inside the house that lowers the voltage and sends it up the coax to power the amplifier which is mounted up near the antenna. The benefit is that you get amplification before any line loss or noise and you don't have to run AC up to your roof.
Masthead preamplifiers are powered via a indoor injector unit, which is usually placed near the TV. A injector is usually connected to a AC/DC adapter. One output sends 12-24 dc volts via the coax cable up to the masthead preamp. The other output of the injector is connected to the TV tuner input.
NEVER install a regular splitter between the power supply and the pre-amp or you will short circuit the system and it won't work. You can install a special splitter that is power passive on only one port. Or put a DC Block on the outputs that don't run to the power supply. Also you can't put any type of matching transformers between the power supply and the pre-amp. Here are some guidelines to consider before purchasing a UHF preamplifier:
1. Preamp noise figure should be 2dB or less.
2. Preamp voltage gain should be around 20dB for most applications. Too much gain (25dB+) can sometimes lead to overload problems.
3. Lowish gain (12-14dB) preamps will likely also need another line amp, further down the line to compensate for the signal voltage loss.
4. UHF only preamps usually offer better performance than VHF/UHF combined units.
5. Strong signal handling ability is usually best with GaAsFET design preamplifiers.
6. Look at the signal handling specification.
Suggested list of UHF TV preamplifiers
Wineguard AC-4990 UHF TV preamp: 20dB gain, 2dB nf (good overall performer).
RDX UA-900 UHF GaAsFET preamp: 20dB gain, 1dB nf (no longer in production?).
Wineguard AP-4800: 29dB gain, 2dB nf. (Overload problems).
Channel Master CM-7775 UHF preamp (overloads more than the AC-4990).
Sitco low noise PA-24 series VHF/UHF preamps.
Televes (Spain) make a 14dB gain, 2dB nf UHF preamplifier called MRD (minimum rising device). The MRD preamp is especially designed for the Televes Dat-75 high gain UHF TV antenna. This is the combination currently used by the writer.
Research Communications model 98030 wideband 0.5dB noise figure GaAsFET preamp (very good, but expensive)!
After extensive tests, one DXer found that the Wineguard AC-4990 UHF TV preamp had general equal performance to the RDX UA-900 UHF GaAsFET preamp.
Johansson (Belgium) make a variety of quality low noise UHF TV preamplifiers.
The Alcad BR-105 low noise 1.5 dB, 14dB gain UHF balun preamp has equal noise performance to the JIM-75 GaAsFET wideband scanner preamp. The writer currently uses the BR-105 as a 14db indoor line amp.
UHF line amplifiers
Certain UHF TV preamplifiers have a relatively low gain figure. For example, the Televes MRD UHF preamp has a quoted gain specification of only 13dB. In this case, a second line UHF line amp will be needed further down the line to compensate.
This applies when less than 13dB signal is present at the TV tuner's input. Why is a second line UHF amp sometimes necessary? Consider the following:
1. Masthead preamplifier gain = 20dB.
2. Coax cable loss between masthead preamp and TV = 10dB.
3. Signal voltage at TV is insufficient to override the TV set's 10dB noise figure. A UHF line amp is needed.
The minimum input dB level of a line amp is its own noise figure + 3dB.
1. Line amplifier typical noise figure = 3dB. line amp gain = 15dB.
2. 3 + 3 = 6dB minimum input signal level to drive the line amp.
3. Assuming the UHF masthead preamplifer has 20dB gain, this means that no more than 14dB of signal loss BEFORE the indoor line amp, in order to maintain the masthead preamplifier system noise figure.
This means that a maximum of 14dB signal loss before there is insufficient gain to drive the line amp.
The noise figure of the line amp should be below the receiver noise figure. Since most UHF TV tuners have a typical noise figure of 10dB, a line amp noise figure of 3dB is adequate.
Let's assume a UHF TV masthead preamp has 20dB gain and 2dB noise figure. The average TV tuner likely has a 10dB noise figure. This means we need minimum 13dB gain at the RF input of the TV set. The following will highlight this:
1. Masthead preamplifier gain 20dB.
2. TV tuner noise figure 10dB.
3. Gain required for masthead noise figure to become new system noise figure 13dB.
4. Gain available for coax cable loss without affecting new system noise figure = 7dB. (20-10 = 10; 10-3 = 7dB.
Recommended high gain UHF TV antennas
Televes Pro-75 (replaced by the Dat-75).
Televes Dat-75.
Triax Quad stacked BB GRID compact cost-effective high gain TV DX array with mesh reflector (840 x 480mm).
Fuba XC-391d ch21-69, 91 element UHF TV yagi.
Antenna Performance U-92 Passive Wave UHF TV 14dB gain Antenna.
Channel Master Parascope model 4251 7ft parabolic dish.
Blake JBX21WB high gain, UHF only antenna.
Triax Unix 100.
Hills X type SF91WB, 91 element UHF TV yagi.
Jaycar LT3182 ch21-69, 91 element UHF TV yagi.
To enable reception of a variety of deep fringe UHF TV signals, high gain antennas which can be rotated is required, mounted - within reason - high as possible and clear of surrounding objects including trees and buildings.
If other VHF aerials are used, the UHF aerial should be mounted at the very top of the mast. At UHF a wideband system is most efficient. One covering the whole band IV-V (470-860 MHz) spectrum allows for operational ease.
Published gain figures for UHF TV aerials are sometimes suspect. For example, the Televes Dat-75 has a quoted maximum gain of 19dB. No indication is given if the gain spec is dBi or dBd. A more realistic figure is 19dBi (16.8dBd). The dBi figure is often quoted because the forward dB gain figure looks more impressive.
Why is sharp forward directivity also important. A good example is comparing a 4-bay bowtie array and corner reflector X type yagi of similar forward gain. While both types of UHF antennas would provide equal strengths on DX signals, the co-channel interference would be far less on the yagi. This highlights the importance of having a narrow forward directional pattern.
Outdoor antennas
The antenna system is the most important part of a UHF TV receiving system. Trees, foliage, houses, etc. greatly absorb UHF energy. So, you can see that quality low loss coax and high gain antennas mounted as high as possible are necessary.
Television signals are strongest when the station transmitting tower and the home receiving antenna are in line-of-sight. If the line-of-sight is blocked or weakened by mountains, buildings or trees, the signal, likewise will be weakened or lost. The signal will also grow weaker as it travels farther.
"Gain" is the measure of an antenna's sensitivity-- and its ability to pick up signals. It is measured in decibels (dB). Gain figures can be in dBi (reference to theoretical isotropic radiator), or dBd (reference to dipole).
The theoretical isotropic antenna has 2.15 dB less forward gain relative to a half wave dipole.
The half wave dipole has 2.15 dB more forward gain relative to an isotropic antenna.
In other words, a 10 dBd gain directional antenna can also be expressed as having 12.15 dBi gain.
The farther away from the station tower, the more gain the antenna should have. Gain can also vary from channel to channel. For example, an antenna's advertised gain rating may be at Channel 20, but the gain may be much less at Channel 69. Make sure the dealer guarantees that the antenna purchased is for channels in your area.
Where buildings or other obstructions cause "ghosts," an antenna with good directivity is recommended. Directivity is the ability to receive only those signals at which an antenna is pointed. Highly directive antennas have narrow receiving angles (measured to degrees) and high "front to back ratios." To insure the best reception aim the antenna carefully.
If a good VHF antenna installation is already on hand, it will probably be less expensive to add a good quality UHF deep fringe type antenna on the same mounting mast. The separate UHF antenna also will permit pointing to VHF and UHF antennas independently.
Installation
The best antenna location
* Higher is usually better. Six to eight feet above the roof should be adequate for domestic reception. Deep fringe reception will benefit from greater antenna height.
* Ideally, buildings, trees, or other obstructions shouldn't block the line-of-sight to the optical horizon.
* The shorter the lead-in line, the less signal is lost.
Tips on installation
* Check the area to be sure there are no power lines nearby that could touch the antenna, lead-in lines, or metal extension ladder.
* Ground the antenna mast electrically, using No. 6 or larger wire and standard ground rod to help protect the antenna and TV set from lightning.
* If a separate VHF antenna is used, mount it 5 or 6 ft below the UHF antenna.
* Keep coax lead-in line free of splices and sharp bends.
* For rotator use, leave enough slack in the lead-in line for rotating the antenna.
* Secure twin lead to stand-offs or tape coaxial cable to the mast to avoid strain on antenna connections.
* Form the lead-in line into a half loop where it enters the house, so rain water will drip off. Seal the entry with a waterproof material.
* Be careful to weatherproof all exposed external connectors. This includes applying coax seal (Tandy) to dipole connections, and F connectors.
* Excess line coiled in the wall or behind the set c
