05.15.2009 03:00 PM
Expanding DTV coverage Part IV

When a receiver’s antenna has limited capabilities, the best way to get the signal to it is to enhance the propagation from the transmit antenna; that is precisely what circular polarization is all about. That little handheld receiver in a viewer’s hand may be constantly moving or stationary, moving down a sidewalk at 2mph or doing 60 down the highway. Whichever way it is moving or changing its orientation, you need to be sure your transmitted signal has the best possible chance to reach it.

Brief history

Circular polarization first came into use in the 1970s as a way to reduce ghosting in analog reception. It worked by causing the reflected signal to cancel itself due to the opposite circular rotation of the reflected signal compared with the direct signal. This worked well for VHF stations, but the added power requirements for UHF stations made the cost too high in most cases. Circular polarization requires a doubling of transmitter power, half going to horizontal and half going to vertical.

When DTV came around, the first receivers could not handle multipath, and circular polarization often worsened the situation because the vertical component tended to reflect off of vertical objects, such as tall buildings. This caused multipath interference, and so it was abandoned. (See Figure 1.)

Today

Straight horizontal polarization has worked well for DTV up to this point. But with the added responsibility of reaching mobile TV viewers using small handheld devices and the ability of today’s DTV receivers to handle a wide range of multipath interference, the situation has changed. (See Expanding DTV coverage III.) For ATSC M/H to work, it must be reliable; to do that, you must ensure that in most situations the receiver will have enough signal strength to decode the information sent. In other words, you don’t want your mobile TV viewers to fall of the digital cliff just because they move the receiver. As you might have guessed, any improvements to mobile viewers will also help stationary DTV receivers, allowing more viewers to use indoor antennas.

Handheld devices

The main issue with handheld devices is that they act like a dipole antenna — any type of antenna attached to the device will only serve to excite the circuit board within, and that will act as a dipole or single-polarity antenna. As the device moves, it will constantly match and mismatch to a transmit antenna that only uses one polarity. With the addition of a second polarity, even at a lesser power level, the probability of one or the other polarities matching the handheld device’s orientation increases. This is called orientation immunity. (See Figure 2.)

When a mobile TV receiver is used within a typical room, many different objects can cause reflections of the DTV signal. All of these objects are within close proximity to the receiver and will cause “small signal fading” due to the addition of out-of-phase reflected signals to the main. As the receiver is moved around the room, the level presented to it is constantly changing because of these reflected signals. With the main signal available on both horizontal and vertical planes, the probability of the stronger main signal making it to the receiver is increased.

Margins

The difference between the signal level at the receiver and the level required to be able to decode a picture, the majority of the time, is the margin. If the margin is too low, there is a high probability that the picture will be lost as soon as the receiver is moved and its orientation changes. The objective is to increase that margin to the point where there will be a large enough signal there for it to decode the picture when the receiver is moved. (See Figure 3.)

Elliptical polarization

Circular polarized antennas require twice the input power from the transmitter than a horizontally polarized antenna to achieve the same ERP in the horizontal plane. For a typical midband UHF channel, that translates to a transmitter output of about 70kW average DTV power. This would require a total of four inductive output tubes (IOTs) producing 18kW average each.

An elliptically polarized antenna provides only a portion of the input power to the vertical plane, not 50 percent. Antenna manufacturers can produce antennas with any ratio of horizontal to vertical power distribution. So, how much is enough? Dielectric has been carrying out tests to answer just this question. The first tests showed that with a moving receiver approximating a handheld device contained within a room that had reflective objects to create small signal fading or close in multipath, they determined that a range of 20-50 percent of vertical polarization improved the signal level by 4-5.5dB, with a best case ratio of 33 percent of the power going to the vertical plane. This 33 percent is a compromise between optimum reception and transmitter power levels.

Right now, TV stations at Sutro Tower in the San Francisco Bay Area are installing a new community DTV antenna that will have elliptical polarization, providing 20 percent of the power to the vertical plane. This means that a station licensed for 500kW ERP that was going to have a transmitter output of 18kW will now need to make 22kW — higher, but not twice as high, and well within the capabilities of the IOT.

On the road

Dielectric has carried out field tests comparing vertical, horizontal and circular polarized signals and has found that using circular polarization improves reception by an average of 5dB. The tests were carried out in open and wooded areas, offices and homes. The smallest improvement was seen inside a small car with many reflective surfaces. (See Figure 4.) Again, the issue in free space (i.e. outdoors) will be total signal strength, filling in where your signal is blocked, which can be accomplished with a single-frequency network. (See Expanding DTV coverage III.)

Acknowledgments

John Schadler of Dielectric contributed to thistutorial.



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