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Choosing a TV Antenna
March 1972 Popular Electronics

March 1972 Popular Electronics

March 1972 Popular Electronics Cover - RF Cafe  Table of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

Unless you happened to live close to a television broadcasting tower, receiving an acceptable signal has always been a matter of luck. Obstructions such as buildings and terrain can greatly attenuate signal strength and multipath can generate telltale ghost images and confuse the synchronization portions of signals. It was bad enough with black and white (B&W) broadcasts, but the advent of color made the situation notably worse because more information needed to be received properly in order to display a good picture. Color TV adoption really began to take off in the late 1960s, and that is about the same time when electronics and technology magazines started publishing articles like this one about how to select a roof-mounted TV antenna. A follow-up article appeared in the April 1973 issue of Popular Electronics. The December 1958 issue had an antenna selection article as well, obviously for B&W TVs.

Complete listing of recommended antennas for your viewing area

By Forest H. Belt

Choosing a TV Antenna, March 1972 Popular Electronics - RF CafeNot too long ago, the only people who tried to convince color-TV owners to buy rooftop antennas were the manufacturers of rooftop antennas. Today, any TV salesman who assures you of a prime picture with only the set's rabbit ears - well, he may disappoint you. Service technicians know; they get many requests to fix a color-TV when the only problem is a weak or ghosty signal.

So don't disdain the antenna ads. Still, for the sake of snow-free and color-true viewing, you should know what the ads try to say. Some play a numbers game, citing how many decibels (dB) of gain (sensitivity) one antenna has over another. Some ads tell of "front-to-back ratio," others of "side lobes" or some other equally technical term. Catchy names abound, too: "Color Brite," "Color Guard," "Color Spectrum," "Color Tuned," "Magic Color," "Sensar," "Stellar 2001," and so on.

Winegard uhf section - RF Cafe

Fig. 1 - Winegard uhf section fits in front of other companies' vhf units.

Antenna from Finco uses their frequency dependent principle - RF Cafe

Fig. 2 - This antenna from Finco uses their frequency dependent principle.

The important matter is what kind of picture the antenna puts on the screen of your color receiver. That may depend on where you live. How far away are the stations you watch? How powerful are they? Are they vhf or uhf? How high can you have your outdoor antenna?

The accompanying full-page chart can guide your choice. Obviously, if you don't watch any uhf stations, a vhf antenna is enough. Or, in a uhf-only area, you certainly have no use for a vhf antenna. That is, you don't unless there's a not-too-distant vhf station you can pick up with a high, sensitive antenna. In that case you might consider a powerful all-channel model. And, if you're in a vhf-only or uhf-only locality, ask around - a new station starting up soon might outmode your antenna.

Local, strong signals are usually received up to about 15 or 20 miles from the transmitting antenna; medium signals up to 30 or 50 miles; and fringe signals out to 70 or 100 miles. Vhf signals usually reach out somewhat farther than do uhf signals. Terrain modifies the TV signal. If you find hills between you and the station, consider a more sensitive antenna (from the next farther grouping). Likewise, if you live near the "far" end of a mileage grouping, you may prefer the stronger antenna even if the countryside is only mildly rolling. Beyond the mileages given above, even the best antenna brings in only a snowy picture - unless the terrain is very flat or you can put the antenna extremely high.

The chart lists the models suggested by major manufacturers for each signal category. Don't go only by price. Ask your dealer or distributor to show you the antenna you think best suits your requirements. Judge its sturdiness. Is it simple to put together and raise into position? Check its weatherproofing. Consider directionality and sensitivity (dB of gain). And, only then, compare prices.

Several popular antenna models are shown on these pages. Some have odd shapes; but don't think those shapes are accidental. They are carefully thought out, for very special reasons.

For All Channels. One example is a uhf design (Fig. 1) patented by Winegard. What appears to be a folded dipole wrapped around the boom forms the only active element. The lead-in is fastened to the opening at the bottom. There's another gap at the top of the dipole - unlike ordinary folded dipoles which are solid along the side opposite the feed-line. Two phasing bars (you can see only one in the illustration), connected to the top gap, are just long enough at uhf to act like zero impedance (a short circuit) across the gap.

This peculiarity permits tacking the whole uhf array, which Winegard calls a "tetrapole collector," onto the front of a vhf antenna and using a single downlead. At vhf, the phasing bars and the uhf dipole have no resonance. They act as mere conductors tying the vhf antenna to the block where the lead-in fastens. Far station signals in either uhf or vhf, the lead-in "sees" 300 ohms impedance.

Element Shapes and Spacing. Another patented design principle applies to the Finco (Finney Co.) antenna in Fig. 2. The company tags the idea its "frequency-dependent principle" (FDP). Short elements, the ones that pick up high-numbered channels, are spaced far apart toward the front of the boom. This trick imparts higher gain as the frequency goes up, which makes up for natural losses in the TV spectrum.

Log-periodic antenna by JFD - RF Cafe

Fig. 3 - Log-periodic antenna by JFD.

Exponential formula in GC Audiotex design - RF Cafe

Fig. 4 - Element lengths follow exponential formula in GC Audiotex design.

Another special feature narrows the front lobe of this antenna. Dipoles are not straight across. Instead, they are staggered along two electrically separate booms. Half of any given dipole is on one boom; its other half is further along on the other. In effect, this transposes the phasing of the feed centers from dipole to dipole. The twin lead-in connects directly to the ends of the two booms.

A design called the "delta reflector" adds a third feature to Finco's antenna. Staggered mounting of elements continues on the delta-shaped boom that connects at the back of the double boom. The delta array forms a closed resonant loop to smooth response across the entire vhf band. The delta reflector is said to block out signals from the rear more effectively than a straight reflector, improving the front-to-back ratio.

Take a close look at the reflector elements up front, too. They are not solid. Insulators divide them, to aid electrical breakup of longer elements so high-band vhf "cells" form. The object, of course, is to improve performance on channels 7-12, which is poor in some TV antennas.

Ordinary spacing, called "yagi spacing," places elements the same distance apart along the antenna boom. Gavin sells antennas of this design. Element lengths vary across the low vhf band, to spread the gain. As usual, the long elements operate in thirds for high-band vhf. Responses off the sides, called "side lobes," necessitate a slight forward-sweeping of reflectors - which also strengthens the front lobe and raises gain. Short directors aid high-band gain.

Editor's Note: Over the past year, with only a few exceptions, outdoor TV antennas have not changed much. However, the selection of an antenna, especially for color TV, is important enough that we decided to update a chart of recommended antennas which appeared almost a year ago in Electronics World. Major differences include new model numbers and some price changes.

The Log-Periodic Idea. One design formula expresses a logarithmic relationship between the velocity of TV signals and the size and spacing of antenna elements. JFD Electronics pioneered this "log-periodic" principle. Gain goes up as frequency rises, and impedance across the low and high spectrum stays smooth.

An all-channel log-periodic model is pictured in Fig. 3. Twin booms with alternating half-dipoles accomplish feed transposition as already described, but the halves of each dipole are directly opposite each other. Several forward elements incorporate insulators. However, the JFD insulators are capacitive to "tune" the elements for high-band resonance.

Recommended TV Antennas for Various Signal Areas - RF CafeNote that uhf array up front. Each set of flat dipoles (there are two, mounted in wedge formation) is stamped from one metal plate. Spacing and lengths of the dipoles follow the log-periodic formula, in the uhf band. The tapered configuration, both vertical and horizontal, captures uhf signals efficiently. Farther up front, the half-discs are broadband directors. JFD calculates they deliver twice the gain of linear directors.

GC Electronics, under the Audiotex brand name, markets a line of antennas that follow a different logarithmic formula. In Fig. 4, note the curved pattern outlined by the element lengths. This special tapering, say designers, improves broadband response. The dipoles are broken up by insulators, but not into thirds. The short outer stubs make a few of the driven elements parasitic to others, smoothing gain across the bands. You can't see them plainly, but small insulated wires transpose the feed between each successive pair of elements.

Interestingly, the uhf array sandwiches in between the main vhf array and some high-band vhf directors up front. Insulators break those directors up into parasitic directors for the uhf band. This antenna thus has multiple use of elements to develop higher gain at high band vhf and at uhf, yet keeping overall response smooth.

Multi-Feature Type. Fig. 5 exemplifies a high-gain all-channel Jerrold Electronics Corp. model called the "VU-Finder." Elements are spaced Yagi-style. Element lengths get shorter linearly from back to front. The feed harness is transposed, but it is through the unique disc-shaped boom insulators which have imbedded conductors. Every element is driven, with shorter elements acting as directors for longer ones, and longer ones acting as reflectors for shorter ones.

Jerrold uses circular insulators - RF Cafe

Fig. 5 - Jerrold uses circular insulators.

Parasitic elements that appear to be part of the uhf array boost high-band vhf gain too. A specially shaped bow-tie in the middle of the front array is the only driven uhf element. Jerrold named the patented design of the bow-tie an "extended resonance uhf dipole." The projections at each corner are angle-aluminum. The bow-tie itself is not flat; it is molded with half-cylinder depressions toward the sides.

Two V-angled booms carry the uhf parasitic elements, forming a corner reflector. To concentrate the front lobe and boost gain even further, another boomful of parasitic directors extends out in front of the bow-tie.

Indoor/Outdoor. Two unusual antennas are the JFD Stellar 2001 and the Winegard Sensar. Both belong to a new breed of pre-amplified antennas designed for either attic or rooftop installation.

Their amplifiers are solid-state and are part of the mast-mounted antenna. Coaxial cable connects the antenna-and-amp unit to a power-supply distribution network at the set. The manufacturers claim a performance radius of 40 to 70 miles, Keep in mind, though, that very broadband devices such as these depend on fairly smooth terrain for any real distance.

Which brings up another point. Despite the need for a really good signal for acceptable color reception, you just might be situated where the signal is good enough that you can get by with an indoor antenna. Try it, but don't be disappointed if the unit that gives you a near-perfect black-and-white picture still doesn't "cut it" for color.

Gavin makes an indoor model with two uhf loops, one slightly smaller for high uhf channels. Some models have knobs to tune and orient the elements for ghost-free reception. You may have to retune for each station.

JFD makes a complex indoor antenna. You can switch the elements as well as move them around for various ghost conditions. The dipoles telescope, too, for best vhf reception, and an inductive-capacitive circuit in the base lets you tune each station.

Channel Master sells an elaborate amplified indoor antenna called the "Chroma 1." The vhf dipoles telescope, while the uhf element is a trapezoid-shaped wire loop, inside of which is a small trapezoidal metal plate. A coaxial cable from the amplifier (in the base) feeds the signal to an impedance-matching uhf/vhf splitter that connects to the TV set. Base controls rotate the uhf antenna, switch from uhf to vhf, and tune the antenna-matching circuit for best performance on each station.

You take the first step to dependable color-TV reception when you recognize the need for a really good antenna. The second step is figuring out what antenna is "really good" for your house. The third step is to buy and install the antenna of your choice.

 

 

Posted November 12, 2017

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