Digital Television frequencies

March 7, 2024
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Over-the-air (OTA) HDTV becomes more and more popular. For someone who is used to noisy analog TV pictures, it is hard to believe how amazing a quality of HDTV broadcasts can be. In fact, HDTV channels received over the air free of charge often have better quality than the same channels received through a paid satellite HDTV subscription. All you need to enjoy OTA HDTV is a HD television with a built-in HDTV tuner and an HDTV antenna.

Huh? Which kind of antenna?! If you have Ph.D in Electrical Engineering and have never heard about the antenna type called "HDTV antenna", it's not because you were a bad student. HDTV antenna has nothing to do with physics and engineering. It was invented in marketing departments. Marketing found an effective trick to boost TV antenna sales. HDTV is a hot thing these days. Call essentially the same device HDTV antenna, and it sells better. It makes people to believe they must buy an HDTV model or HDTV optimized antenna to watch HDTV broadcasts. This is very far from truth.

HDTV antenna hype created a huge misconception with regard to TV antennas used for HDTV reception. This article is an attempt to clarify this issue.

Do you know what a regular antenna is? Antenna is a piece of metal designed to resonate at a specific frequency and to be responsive over a certain range of frequencies. TV antennas are designed to work either in the range of Ultra High Frequencies (UHF), Very High Frequencies (VHF) or both. Any station transmitting in the VHF/UHF frequency bands, can be picked up by a VHF/UHF antenna and transferred to the TV set.

All television broadcasts, digital and analog, are in the VHF and UHF bands. Over 90% of the HDTV broadcasts are in the UHF, and less than 10% in the VHF band. What is important from the antenna perspective is that HDTV falls in the bandwidth of a regular VHF/UHF antenna. Not HDTV antenna, not HDTV optimized antenna, just a normal regular TV antenna. What makes a signal to be HD is its content, the way a signal is modulated, and not the carrier frequency it is transmitted on. On the contrary, the antenna knows nothing about the signal modulation and content. Hence, you don't need an HDTV antenna to pick up the HD signal. An antenna has absolutely no idea what the signal resolution is. It can be HDTV, SDTV, NTSC, whatever. It is the job of a HDTV tuner and HD television set to demodulate the signal and to present the actual content on the screen.

Well, the antenna bandwidth and frequency response are not the only parameters that are important for clear TV reception. An antenna has other important electrical and spatial properties, such as antenna gain (directivity) and high front-to-back (F/B) ratio. One might assume that an HDTV antenna should be more powerful in terms of F/B and gain parameters. Does HDTV reception impose more stringent requirements on antenna gain and F/B ratio?

There is a wrong, yet widespread belief that you need more antenna gain to receive digital television. I don't know where the hell this belief comes from, cause the situation is exactly the opposite. HDTV has much better noise and interference immunity than the analog television and can produce high quality video at significantly lower signal-to-noise ratios.

Another important specification, F/B ratio, has to do with the antenna ability to cope with a multi-path signal propagation from the towers to the receiving antenna. The higher F/B ratio is, the better is multi-path rejection (also known as ghost suppression). Without going into technical details, we must say that HDTV signal is a bit more sensitive to multi-path cause it has slightly larger bandwidth. Multi-path causes dips in the signal spectrum, whereas we want to keep the spectrum as flat as possible. When signal content is spread over a larger portion of spectrum it is more likely to be distorted by multi-path. Basically, what TV equipment manufacturers are trying to do in the so called HDTV optimization is to keep the spectrum flat in the whole frequency band. It is important for HDTV antenna to have a high F/B ratio in some areas where ghosts may be a problem. The point is, however, that most directional, old fashioned and cheap TV antennas have F/B ratio good enough to handle multi-path propagation of HDTV signal and keep spectrum distortion at minimum. If an antenna can handle an analog signal, it can handle a digital signal as well.

There is nothing specific about a TV antenna that is used to receive HDTV. When choosing an HDTV antenna, check the really important parameters such as directivity, gain, F/B ratio. These specifications are important for reception of both, digital and analog broadcasts. The HDTV optimization is probably the least important factor you should take into account.

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