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Antenna Performance Key to Good Color Reception
May 1969 Sylvania News Article

There was a time that selecting a television antenna was as important to the quality of life as buying the right smartphone is today. There were probably as many choices in antennas then as there are phones now. You might think, especially if you are not an amateur or military radio operator, that nobody worries about antennas anymore, but as I've written before there is a slight resurgence in people installing the old fashioned multi-element antennas for receiving local television and radio stations. The market's not huge, but seems to be keeping companies like Channel Master in business. Incidentally, in contrast to my aforementioned comment, dig the opening sentence of the article: "Virtually no one in this day and age goes about discussing the reception quality of his telephone."

Antenna Performance Key to Good Color Reception

Antenna Performance Key to Good Color Reception, May 1969 Sylvania News - RF CafeTechnical Section

May 1969 Vol. 36 - Issue 2

Technical Editor R. A. Humphreys

A National Pastime

Virtually no one in this day and age goes about discussing the reception quality of his telephone. The telephone works, so he takes it for granted. Yet every day millions of people discuss their television reception. "How's your picture?" is a nationally discussed question. Obviously, the person who inquires is seeking to compare his "picture" with that of someone else's. He is doing this because in his mind he is doubtful about the quality of his own reception. He has good reason to be doubtful, for he and millions of others are the victims of sub-standard reception. This is especially true of color.

"My color set is no good," is the usual explanation advanced by the householder to explain away a poor picture, and he believes it.

Ingredients of a Good Color Picture

Let's examine the facts. The vast majority of popular color receivers manufactured today are perfectly capable of delivering high-quality color pictures. To prove this, a visit to any of the plants where the sets are made will afford you an opportunity to see thousands of color receivers "lit up like Christmas trees" as they come through final test. How then is it possible that these sets give good color results at the factory and yet perform poorly in the living room? The answer is simple: at the factory they are being fed with the proper color signal, and in the home of the user, they are not. There is no mystery or magic involved in good color reception. besides the presence of a well functioning color receiver, you need only these three additional ingredients: a reasonable level of interference-free signal, a well designed color antenna, and a proper transmission line. Put them all together and you have excellent color every time. Leave one out and there goes the color picture!

The level of available signal at the receiving site is usually a matter over which you have limited control. However, the choice of which antenna to use is far more broad. Everyone and his brother has some sort of antenna that they are pushing. In far too many cases the decision is made to sue the cheapest and most readily available color antenna on the theory that, "what's the difference all antennas are pretty much the same." This simply is not true, and if the installer wished to avoid customer complaints about picture quality, he should not make this assumption. Many of the antennas on the market today are poorly suited to the task of providing quality color reception consistently over a long period of time.

Figure 1 - Variety of Model "Sizes"; Trend of Gain to Increase with Frequency

Figure 2 - Channel Response

Figure 3 - Directivity of Sylvania Model 32 VU Fringe Area Antenna

The 'Lens" of the Color Receiver

What then is a good color antenna? It is, simply stated, a device which has been designed to process each television channel individually in the most linear possible fashion and for the longest possible time. Does this mean that antenna gain is no longer important? No, the forward gain of the antenna is just as important for color as it is for black and white TV. However, since the black and white transmission utilizes only a segment of the bandwidth for each six-MHz television channel, abrupt variations in gain across a channel and from channel to channel could be tolerated.

On the other hand, color transmission utilizes a considerable portion of each six-MHz television channel, and its proper reception dictates that the color antenna present a relatively flat and linear response to the entire transmission of each channel to permit all of the transmission to reach the receiver. Failure of the antenna to do this causes the antenna to act effectively as a filter, which distorts much of the color information before it ever reaches the receiver. Further, to minimize color smear and other forms of distortion, the antenna should respond relatively little to unwanted signals. Thus, it may truly be said that the color antenna is the "lens" of the color receiver. Obviously, no one should use a poorly designed color antenna as the "lens" for a color set and expect optimum performance.

The Rugged Elements

Therefore, to develop the Sylvania line of color antennas, we were faced with the problem of designing a set of broadband, high-gain, directional, and effectively linear devices. But that's just the beginning. As a primary goal, our antennas would have to continue delivering their designed performance for three years, four years, and longer. While we were at it, we took particular care to include the FM spectrum as well. As a necessary measure to meet these goals, these antennas would be fabricated from materials substantially superior to those in general use today by other rooftop antenna makers. Mechanical design would also be of primary importance. We did this not only to achieve a superior mechanical product, but particularly to insure the electrical integrity of the antenna, including both relative parts alignment and continuity, for the longest possible time. Remember, an antenna is truly an electrical circuit. The longer each of the antenna parts resists the elements and corrosion, the longer the antenna will permit the receiver to deliver good color pictures. Simply stated fine electrical design means little if the mechanical design is flimsy. The instant an element is bent out of alignment or snapped off - there goes electrical performance!

As an example of this, when our field experience clearly indicated the need for an antenna which could cope with the high incidence of salt and toxic corrosion so prevalent in many residential areas of the nation, salt corrosion tests were conducted. The results not only indicated a need for a completely aluminum antenna, but also one on which all surfaces of all parts, both inside and out, were treated with the immersion alodine process before assembly. This same process is now carried out with every Sylvania production antenna.

The Cost Factor and Long Life

For some reason many service and installation men are reluctant to sell quality antenna installations, perhaps in the mistaken notion that the customer has "shot his roll" when he purchased his color receiver. To the average householder, the entire matter of obtaining a color picture in his living room is indeed a mystery. Bluntly, he doesn't know what he needs, and it's up to the service man to intelligently tell him.

Your customer wants good color pictures. This is why he is constantly comparing his result with others. He is also willing to pay to get it, as evidenced by the thousands of householders who subscribe to community antenna systems to obtain better pictures. It logically follows that informed customers are also willing to pay for quality antenna installations - they are less expensive in the long run.

An antenna's initial performance is one thing - but what happens to performance after the antenna has been on the end of a mast for a year or two? The key to all of this is durability - durability through the materials used, through mechanical design, and through construction details.

The centerfold of this article examines these and other performance criteria in detail, and the degree to which the Sylvania line of color antennas meets them.

Ten Requirements of Good Color Antennas

Figure 4 - Driven Elements / Snap - Lock Joints / Double Boom Form Continuous Transmission Path

Figure 5 - Overall Rugged Construction; Parts of High Quality Aluminum and Plastic

Figure 6 - Full Alodine Protection of All Aluminum Internal and External Surfaces

1. The antenna must have sufficient gain for the area in which it is installed. This is sizing the antenna for the particular location where it will be used. By sizing we mean selecting an antenna of such gain that the TV set will be provided with enough signal to properly drive it. Suitable signal strength would range from 200 μV to 2000 μV. The Sylvania broad line of color antennas provides a model for every location (Figure 1), from the near-suburban to the deep fringe areas.

The antennas provide more gain as frequency increases. This compensates for the propagation and feed line losses which increase with frequency. Figure 1 indicates the trend of the Sylvania line of antennas to show increasing gain with increasing frequency. The gains shown correspond to antenna response at mid-low VHF band (channel 4), FM (88 MHz), mid-high VHF band (channel 10), and mid-UHF band (channel 50).

2. Ideally, antenna response should be essentially linear across the six-MHz band of each channel. This assures that all of the transmitted video, color, and audio information is faithfully introduced to the transmission line. Wide variations in gain across a channel can destroy picture fidelity, particularly in a low-signal area. For example, a droop in antenna gain towards the high end of a channel (Figure 2A) will result in loss of color performance. Sylvania antennas are designed for a gain performance of ± one dB across each channel (Figure 2B).

3. The antenna should be directional, that is, signals appearing at the sides and rear should be largely cancelled or suppressed. These unwanted signals, if not greatly attenuated relative to the forward gain of the antenna, commonly appear in the set as ghosts, reflections, co-channel interference, and noise. Any of these can play havoc with a color picture. Figure 3 indicates the excellent front-to-back ratio of one antenna of the Sylvania line. At the two typical frequencies (channels 11 and 47) shown, the rear 180-degree response of the antenna is less than 10 per cent of the forward pickup. Notice the freedom of large, interference-producing side lobes.

4. To cover all present TV channels allocated by the FCC, the antenna must be receptive to VHF (54 to 88 MHz and 174 to 216 MHz) and UHF (470 to 890 MHz). It is also highly desirable for the antenna to include FM (88 to 108 MHz). All antennas of the Sylvania broad line were specifically designed to cover both the VHF and FM bands. For areas in which UHF is present, Sylvania provides antenna models which cover all bands (VHF, FM, and UHF).

5. Output impedance of the antenna should be matched to transmission line. Mismatch between antenna and lead wire can cause color smear and cut down on efficiency of the antenna. l! is vital that a well matched transmission line be used. Basically, there are two impedances used in home receiving antennas: 300 ohm, and 75 ohm. A 75-ohm antenna is often effectively used in high noise areas and feeds directly into 75-ohm coaxial cable which provides complete shielding. The Sylvania broad line of color antennas is available in both 75 and 300-ohm impedances.

6. An antenna that has all of the previous electrical performance characteristics is of little value unless electrical integrity of all elements to feed lines is maintained over a long period of time. The antenna is an electrical circuit; therefore if an element loses contact with the feed line, a part of the circuit is lost. Obviously, when this happens the antenna no longer functions as designed. Wind, rain, ice, and other environmental conditions combine to make this requirement a formidable task for the antenna designer to achieve.

Sylvania achieves this electrical integrity in its antennas through use of the double boom design, in which the booms themselves are the transmission lines. The rectangular boom provides a large, flat surface for solid mechanical and electrical contact with the positive-lock snap joints (Figure 4), which retain the driven elements. Carefully-formed shoulders on the snap joints assure long-lived electrical contact with the elements. This design makes the elements, effectively, electrically integral with the double boom transmission line.

7. The antenna must be strong enough to withstand the strong winds and other climatic influences encountered in its roof-top environment. Mechanical design is a primary consideration in meeting this requirement. Features of the Sylvania antenna design in this respect are:

• Double boom construction - much stronger than a single boom design (Figure 5). Booms connected and reinforced by sturdy bridge cross ties.

• One-half inch, seamless elements - a Sylvania exclusive. Being physically stronger, the elements hold their alignment much longer. The greater surface area provides a high Q. The absence of seams means better radial symmetry throughout the length of the element-all factors which provide a more efficient electrical element.

• No braces required from boom to mast - permits mounting an antenna rotor directly beneath the antenna. Rotor life is increased through elimination of a long mast extension.

• End caps on ends of booms - prevent corrosion-causing water traps.

8. As an additional requirement for long antenna life, booms and elements should be built of high quality aluminum tubing. Skimping here again means short antenna life. All Sylvania antennas incorporate rectangular cross-section, seamless booms of structural grade 3005-H-29 aluminum with 1-inch by 1-inch by 0.041-inch walls (Figure 6). Elements are half-inch seamless aluminum with 0.0165-inch walls. High-strength, injection molded plastic is used for all non-metal parts. Rivets, U-bolts, and all other hardware are of heavy-duty, industrial grade.

9. The antenna should be protectively coated to resist the effects of salt and other corrosive atmosphere. This coating should be applied to inner and outer surfaces. All aluminum parts of Sylvania antennas are processed through a seven-bath, weather-resisting alodine solution prior to assembly. This assures that all surfaces are completely covered. The high-conductivity coating penetrates the metal surface to a nominal depth of two-mils (Figure 6).

10. The antenna must be easy to install. There should be a minimum of loose parts and subassemblies in the box, and installation instructions should be clear. To ease the installer's job, Sylvania has incorporated these features into its broad line of color antennas:

• Quick, set-up snap joints lock the elements into place in minutes.

• Major parts are preassembled.

• All hardware is in place.

 

 

Posted June 20, 2019

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