Range Considerations For Small ATV Transmitters


The range you obtain with small ATV Transmitters depend on several things. These are as follows, in order of importance:

Antennas and height thereof
Expected Picture quality
Path of propagation of signals
Geographical Location and Terrain
Presence of interference
Receiver Sensitivity
Transmitter Power

We are frequently asked by customers about performance with regard to how much range specific antenna, transmitter and receiver setups will have. While it is theoretically possible to predict this in free space, environ mental factors present on this planet will affect this prediction in an un fortunately negative manner. Like the weather for next year, this is very difficult to predict and impossible to guarantee. It has nothing to do with manufacturer or kinds of circuits. To demonstrate what is involved, we will compare an ATV system to a radio system using a 2 watt handie talkie, such as used by ams, police, or security personnel,etc. These have ranges varying from 1/4 mile in bad locations to 10 or more miles from a good location such as a hilltop. No repeaters will be assumed, just simplex operation to another HT or base setup. Typically, these units have receivers with 0.2 microvolt sensitivity at threshold and small whip antennas with zero gain or less. Bandwidth is typically 13 KHz for two way narrowband FM. Remember that rdinary TV broadcast (and most ATV) NTSC format signals need 3 to 5 MHz bandwidth depending on resolution and whether B/W or color the receiver must have this bandwidth. Since thermal noise limits sensitivity and is directly proportional to bandwidth, a TV receiver will have a much larger ackground noise level. Assuming the TV receiver and the HT have similar noise figures (3 to 6 DB is typical) then the ratio of background noise power is:

     Noise of TV receiver         5 MHz      5000000
    -----------------------  =  -------- = ----------- = 384.61 or 385 times
     Noise of HT receiver        13 KHz       13000

The TV receiver must contend with 385 times as much noise power as the radio receiver. Also, consider the fact that an 8 DB signal to noise power ratio gives adequate copy on the radio, 8 DB is useless for any picture. Some receivers need at least 10 to 15 DB just to hold sync. You would not watch a TV picture with a 20 DB S/N as it would be very snowy. And 30 DB would still be somewhat snowy. Therefore another 22 DB more signal is needed at the TV set at the minimum. 22 DB is 158 times more received signal power. For a good cable quality picture 45 to 50 DB is the minimum. For ham use the 30 DB figure might be adequate. This means then, 385 X 158 or 60,830 times the signal power is needed at the TV receiver. This is a ratio of 48 DB. However, this is not as bleak as it may sound. A good amateur installation would use directional Yagi antennas at both receiver and transmitter sites, having 15 DB to even 20 DB gain, low(< 3DB NF) noise downconverters, and possibly 100 to 200 watt linear amplifiers. This could increase system gain by 15+15+3 or 33 DB, and if a 100 watt linear amplifier is used another 17 DB (assuming a 2 watt ATV transmitter as an exciter). This is 50 DB and even more if 20 DB gain antennas are used, and also higher power up to 1 KW for a total of 70 DB possible. A station like this would have an effective radiated power of 1 to 100 kilowatts. The latter figure is typical of a UHF broadcast station.
A relatively low power transmitter with an effective antenna will far outperform a higher power transmitter with a mediocre antenna. An average receiver with a good antenna will likewise far outperform a more sensitive receiver with a simple whip or loop antenna. Remember that at UHF propagation is generally restricted to line of sight. If both receiver and transmitter sites have good antennas (10-15 element yagi or better) and there is a rela tively unobstructed path between the sites, ranges of 30 to as much as 100 miles can be obtained. On the other extreme, if simple antennas such as six inch whip antennas are used in rough or hilly terrain, 1/2 mile would be doing well. In cities with concrete and steel structures this might be reduced to as little as 100 yards. Therefore the antenna system is the single most important factor in regard to range obtained. At 915 and 1260 MHz simple whip antennas are nearly useless for significant range. And there are no cure-all circuits to improve this. For many years a lot of research has gone into small antennas and there seems no way to get around this. You have to radiate the transmitter power efficiently and at the receiving site capture as much as possible. It has nothing to do with the kind of transmitter or receiver being used. There simply are no magic tricks. Poor antennas give poor results. Miniature antennas yield miniature results. Theoretically, a small antenna can be efficient but in practice the very low radiation resist ances encountered are difficult to match without high losses in the required matching networks. Also, bandwidth can be a problem and NTSC television needs 4 MHz for good definition. Some antennas may be a problem in this regard, as for example, long yagis with gamma match feed. On the other hand, big antennas will generally give big results if properly tuned and matched. ATV operation, unlike 2M FM or HF SSB operation, is not a "plug together and play" business if you want best results. To get good results, all peripheral equipment must be compatible, properly interfaced, video levels must be correct, and you must make some effort and do your homework on your antenna system. Proper setup is needed to get best results.

All antennas have a gain factor expressed in decibels. Usually this is relative to an isotropic radiator. An isotropic radiator radiates uniformly in all directions, as does a point source of light. All the power that the transmitter produces ideally is radiated by the antenna. However this is not generally true in practice since there are losses in both the antenna and its associated feedline. The transmitted power is effectively multiplied by the antenna system gain, which is the sum of the line losses and the antenna gain (or loss for many small simple antennas). The gains in Decibels (DB) directly add and may be expressed as a numerical factor. The transmitter power and the antenna gain when multiplied equal the effective radiated power (ERP) Directional gain antennas such as yagi arrays and log periodics increase range and reduce interference and ghosting. Do all possible at the receiver before attempting improvements at the transmitter end of the path. This is the best practice, least expensive, enviromentally sound, and good engineering practice, and will reduce RF pollution for other users of the spectrum. Next, install a good directional transmitting antenna and a low loss feedline. Use only low loss coax and if more than 40-50 feet use hardline, as line losses are very high at 440 MHz and still higher at 915 and 1280 MHz. Also remember that obstructions such as buildings, hills, foliage, and large metal structures can cause shadows in the propagation path, limiting coverage in these areas. This effect becomes more pronounced at higher frequencies. Also, while some "fill" occurs in these shadow areas due to reflections, diffraction, and some scattering, multiple paths and reflections can cause ghosting problems. A good directional antenna can help to reduce these effects. The most ineffective way from a cost standpoint is to use more transmitter power. You will be quite disappointed if this is the way you try to increase range. Good antennas are far more effective than brute force and increased power is ALWAYS the LAST resort. Some typical figures follow. By the way, antenna gain also counts at the receiver in the same way. Always use the better antenna at the receiving site if at all possible.


    For more antenna information consult a good text on antennas, such as the ARRL Antenna Handbook, and also antenna manufacturers literature. Another excellent reference text is the Radio Society of Great Britain (RSGB) handbook. This latter text has much detailed information for home construction. These books may be obtained from most amateur radio dealers or directly from the American Radio Relay League, 225 Main Street, Newington CT 06111 USA.

     

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