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Block That Multipath Ghost!
January 1948 Radio-Craft

January 1948 Radio-Craft

January 1948 Radio Craft Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

It is probably safe to say that the vast majority of cellphone users never consider that their cherished devices are fundamentally radios, and with that capability they would be merely powerful PDAs. Even less likely to be thought about is that as wireless devices, an antenna is needed to establish communications. Up until the early 2000s, most cellphones had some form of obvious antenna protruding from the case - either an extendable type or a molded stub around an internal antenna. Operational frequencies at the time were primarily in the 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz bands, with 1/4 wavelengths of about 3.5 inches, 3.3 inches, 1.6 inches, and 1.5 inches, which was convenient given the physical size of phones. Always seeking to develop new features to outclass the competition, manufacturers decided to do away with any visible antenna by absorbing the entire structure within the case of the phone. Doing so took real genius level engineering with lots of trial and error, simulation, and empirical testing. The "Can you hear me now?" commercials were as much a testament to the phone as it was the network.

Have you seen what the antennas in a modern smartphone look like? You probably would not recognize them as antennas at first glance, but they work incredibly well. Some companies mold them into the plastic case and others snake them around the edge of the PCB substrate. A need to accommodate multiple bands for cellular in addition to GPS, Wi-Fi, Bluetooth, NFC, and even FM radio (usually the external ear bud wire), complicates antenna design - particularly when they need to work whether laying on a tabletop, being carried in a pocket, or resting against someone's head and being held in someone's hand. On top of that, they need to work in any physical orientation and at any speed of travel. It truly is an amazing achievement in antenna design that is taken for granted at this point. Antennas are not parts performing magic, however. Sophisticated algorithms both in the phones and in the cell towers are able to continuously arbitrate power level and cell selection to achieve signal quality to mitigate signal degradation caused by effects such as multipath and natural and manmade obstructions. The builders of the world's original wired telephony infrastructure would be proud of the work done on today's wireless version.

The television ghost images referenced in this article do not occur in today's digital transmission world. Instead, the picture is basically either there or is not there. You might get a second or so of pixelated display before the signal is dropped, but multipath and other forms of signal degradation and variation are masked by circuits and software that correct if possible and shut off if not.

Block that Ghost!

Fig. 1 - Double-image ghost cause by direct and a reflected signal.

Fig. 2 - A triple-image ghost shows there are two reflected signals.

Care and skill in making the antenna installation makes the difference between good and bad images

By Jordan McQuay

The chief complaint among new television-set owners is the presence of one or more overlapping images on the picture screen, resulting in a consistent double exposure (Fig. 1) or triple-exposure (Fig. 2) effect. Since the displaced images duplicate the main picture image in every respect, and usually with less intensity, they are appropriately known as ghosts. Their presence is not the fault of the picture tube or the television receiver. They are due entirely to inadequate or improper installation of the television antenna.

Elimination of these ghosts may require only proper siting and orienting of the existing antenna. Should this prove fruitless, a more directional type of antenna must be substituted and properly installed.

Regardless of the location - whether town or country, city or industrial district - it is possible to receive television pictures entirely free of ghost interference. Such reception can be achieved only by considering the specific problems of each television location.

Ghosts now present in existing television systems can be blocked or eliminated by means of the same general method used for new installations. The work requires a practical knowledge of antennas and reflected waves. Siting is performed by two men, equipped with tools and patience.

Direct, and Reflected Waves

The high-frequency waves used in television are similar in many respects to ordinary light waves. They travel in straight lines until their path is obstructed. The receiving antenna should be installed sufficiently high and in the clear so that it intercepts these direct waves. This means that the transmitting antenna of the television station should be visible, or "almost" visible, from the site of the receiving antenna. Reception may be possible when objects or surfaces partially obstruct the path of the direct wave, but usually the received signal is very weak.

Again similarly to light waves, the straight-line paths of television waves are affected by any kind of obstruction. Usually, the waves are diverted or reflected upon striking an object or surface. Much as a billiard ball is reflected angularly by a soft cushion, these waves, after reflection, continue their journey in a different angular direction, depending upon the original direction of the wave and the structural nature of the interfering object or surface. The waves lose some of their energy each time they are reflected. However, when reflected by large surfaces - such as steel buildings, storage tanks, or even mountainsides - very little energy may be lost.

Since many signals are radiated simultaneously and in all directions by the transmitting antenna, there is always a possibility that some of these reflected waves may reach the site of the receiving antenna (Fig. 3).

If the receiving antenna is not sufficiently directional, it will accept both the direct signal and the reflected signals. The frequency of both is the same, and therefore the television receivers - no matter how efficient or expensive - cannot differentiate between them.

 

The unwanted signals must be eliminated by the antenna system.

Fig. 3 - Where the television ghosts are born.

Fig. 4 - The dipole-basic television antenna.

Since any reflected wave travels a greater distance than the direct wave, the additional time consumed causes the reflected signal to arrive later than the direct signal. The delayed signal appears on the picture tube as an additional image, which is always displaced horizontally and to the right of the direct image. The amount of this displacement is a direct function of the additional time required for the reflected signal to travel the additional distance, with resulting displacement on the picture tube may be so small that it produces merely a blurry out-of-focus effect. More often, reflected signals travel considerably greater distances than the direct signal, resulting in more-or-less distinct multiple images on the television screen, as shown in Figs. 1 and 2.

These unwanted signals may be reflected by any number of types and kinds of large surfaces and objects. For this reason, ghost images are usually troublesome in the industrial or metropolitan districts of cities.

However, the general method of elimination of ghosts is the same for all types of television installations.

Proper Siting

Since ghost-images are the result of reflected signals arriving from directions which differ from that of the direct signal from a transmitter, their appearance on the picture tube is due entirely to insufficient directivity of the existing antenna.

This does not necessarily mean that another type of receiving antenna must be substituted immediately, because in many cases the existing antenna is improperly sited, improperly oriented, or both, due to careless or indifferent work' at the time of the original installation.

Therefore, the first logical step in blocking ghost reception is to make certain that the existing antenna is sited and oriented to obtain the best possible reception at the particular location.

At least two technicians or servicemen are needed to make a satisfactory television installation. The following more-or-less standard procedure is used to site and orient properly any type of television antenna.

One man, holding a pole upon which is mounted the antenna, is located on the roof of the house or building. The portable antenna is connected to the television receiver by a lead-in, which is loose and long enough to reach any location on the roof. A second man is located at the picture tube of the receiver to observe comparative signal strengths of the direct image and any ghost images. Some means of direct communication-such as a portable telephone or intercom-is used between the two men. Tests are conducted while the desired television station is on the air.

With the antenna held horizontally and broadside toward the direction of the station, the man on the roof explores various possible antenna sites, while the observer at the set notes comparative signal strength data for each of the various roof locations.

If 2 television stations are to be received with the same antenna, the entire procedure is duplicated for each station, and a suitable average or compromise location is selected as the best site for 2-channel reception. A similar process is used for 3- and 4-channel reception. However, antennas designed for multi-channel operation - such as folded dipoles - are susceptible to ghosts, since they lack sufficient directivity.

When the best site has been determined, the antenna is temporarily mounted so that it can be rotated in azimuth. Again using the 2-man coordination system, the antenna is revolved while changes in the received image are observed at the image tube. Some ghost effects will disappear and reappear as the antenna is rotated. The object of this search is to locate a bearing position of the antenna which provides maximum strength for the direct wave, and the least interference due to wave reflections. At such a bearing position, the antenna is fixed in place, and is then considered to., be properly sited and oriented. Usually, but not always, objectionable ghost effects are greatly minimized or completely eliminated by this process.

Fig. 5 - This simple antenna is often good.
            Service Division, Philco Corp.

Fig. 6 - Double doublet is highly directive.

Fig. 7 - Folded dipoles can be ghost free on only one channel.
           Photo by Ward Products

Fig. 8 - The Duoband operates on all channels.

Fig. 9 - A wide-band, highly directive antenna.

Fig. 10 - Extended-V, an all-channel antenna.
             Photo by Premax Products

If, with the antenna properly installed and with all other components of the system functioning normally, ghosts still appear on the picture screen, a more directional antenna is required for ghost-free reception.

Ghost Effects

Although unwanted for normal television reception, the consistent appearance of ghosts on the picture screen during the installation can be utilized to good advantage, since the images provide considerable information concerning the nature and origin of the reflected waves. This data is obtained directly from the picture tube of the set, without additional analyzing equipment or expensive paraphernalia. Once determined, the information is used in selecting the proper type of directional antenna to block or eliminate the ghost signals.

When the direct image and the reflected image are well separated on the picture screen, this indicates that the signals are converging at the antenna from 2 widely different directions. In such cases, the unwanted signal usually can be effectively blocked with an antenna having only a slight amount of increased directivity.

On the other .hand, if the direct image and the reflected image are only displaced slightly, or if they are so close together that they cause a blurry effect, this indicates that the signals are arriving at the antenna from almost the same direction. In such cases, an extremely directional antenna is required to separate (in angle) the desired from the undesired signal.

When the intensity of the ghost image is weak in comparison with the direct image, the reflected signal is more easily blocked with a simple directional antenna. When the intensity of the ghost; image is stronger than the direct image, it is sometimes possible to orient the antenna with respect to the reflected signal - rather than the direct signal - if the direct signal can be blocked satisfactorily so as to prevent interference with the desired (reflected) signal.

By turning or rotating the antenna at the roof site while observing the comparative strength or intensity of the direct and reflected images, it is often possible to identify the true bearing or direction of the source of the reflected waves - such as buildings, tanks, etc. When the source of trouble is known, it is often easier to deal with its effects.

This and other information can be determined directly from the picture tube, regardless of the type of antenna, vided the antenna has been properly sited according to the general installation procedure.

When a more directional antenna is required for ghost-free reception, this previously determined site may prove adequate for the new - antenna as well. However, this roof location is not necessarily the best site for all types of television antennas. Therefore, any of the following types of directional antennas selected for installation must be individually sited and oriented according, to the standard installation procedure.

Basic Types of Antennas

There is no "ideal" antenna suitable for all kinds of television installations, because of the specific directional requirements and the individual nature of each location. In general, the best antenna is the simplest and most economical antenna which provides ghost-free reception for a particular location.

The simplest antenna - with the least directivity - is the fundamental, half-wave, resonant dipole (Fig. 4). Although tuned, this antenna is made only broadly resonant to prevent degeneration (loss of clarity and definition) the high-frequency side-band components of the received direct signal. The dipole has an impedance of about 72 ohms at its center, and is always erected in a horizontal position.

A simple half-wave dipole is sometimes adequate for rural or suburban installations, where ghosts are rare. More complex and directional antennas are needed for good reception in the metropolitan and industrial areas of large cities, where multiple-signal reflections are prolific and troublesome.

It is conventional practice to use this lightweight dipole as the initial step in all new television installations, according to the standard procedure given previously. In some cases, it provides satisfactory reception and can be permanently installed at the location. In many cases, it is found inadequate because of its lack of directivity. Since the dipole is bidirectional, there is also the possibility that reflected waves may strike the antenna from the rear (Fig. 3). This directional inadequacy is remedied by adding either a reflector or a director. These parasitic elements convert the single dipole into a 2-element antenna with considerably improved directivity.

The reflector is a rod about 5% longer than the dipole, placed parallel and a quarter-wave behind it. The resulting 2-element antenna (Fig. 5) is sufficiently directional to block all weak reflected signals which arrive at a wide angular difference with respect to the direct wave.

When a director is used in place of a reflector, the action is almost identical. The length of the director rod is about 5% shorter than the dipole, and is placed parallel and a quarter wave in front of it. As in the case of the reflector, the complete 2-element antenna has good directivity.

Greater directivity can be provided with a 4-element antenna, known as the double doublet (Fig. 6); it is also known as a stacked array of two 2-element antennas. This consists of 2 dipoles, one above the other and connected in phase, and 2 reflector elements, one above the other and unconnected. The combination is a good one. It discriminates against undesirable ground reflections, thus providing a more distinct picture than is possible with a single dipole-and-reflector unit. The double doublet is broadly resonant, so broad that it might be classified with the wide-band or special types of antennas which follow. It has very pronounced directivity characteristics which make it extremely effective in blocking unwanted ghosts. It is frequently possible to minimize or eliminate ghost effects merely by changing the symmetrical position of one or more of the antenna elements, even by setting them at an angle with the strict horizontal. Again, it is important to realize that every television installation must be treated individually according to the specific directional problems posed by each location.

Folded Dipoles

The directional characteristics of the folded dipole are about the same as those of the simple-dipole. Somewhat similarly, the folded dipole can be used alone or with a reflector (Fig. 7), depending upon the degree of directivity desired.

For general television use, the folded dipole has several advantages. It has an impedance (center) of 300 ohms, which permits exact matching with standard 300-ohm ribbon lead-in. Because of this relatively high impedance the folded dipole has a wide band-pass characteristic, which permits operation over a wide range of frequencies. Although resonance is determined by the length of the folded dipole, the dimension is not critical. For this reason, the folded dipole - sometimes known as a wide-band antenna - has only fair selectivity.

This sacrifice of selectivity also affects, to a lesser degree, the directivity of the antenna. Therefore, it is more difficult to block unwanted ghost signals with any of the conventional types of folded-dipole antennas.

Although this antenna is usually capable of receiving several television channels, ghost-free reception of only one channel is certain. When a folded dipole is oriented for ghost-free reception from one station, such a fixed position of the antenna will rarely provide ghost-free reception of the other television channels, since the locations of the transmitting stations and all of the various sources of image reflections will be different with respect to the site of the receiving antenna. Any attempted use of folded-dipole antennas for reception of more than one television station must be based on a compromise orientation which, from the outset, precludes any possibility of ghost-free reception on all of the desired channels.

An important variation of the basic folded dipole - known as the Duoband antenna (Fig. 8) - has a bat wing addition to a normal dipole, permitting wide-band operation of the antenna in both the high- and low-frequency television bands. Admittedly, there is only slight improvement in the directivity of antenna on any of the channels in the low-frequency band. When used on any of the high-frequency channels, however, the small bat wings provide a very sharp directional pattern, permitting ghost-free reception on at least 1, and sometimes 2, of the so-called upper channels. If greater directivity is needed on any of the lower channels, a more-or-less conventional reflector element is attached a quarter wave behind the large folded dipole.

A combination of 4 folded dipoles, connected in phase (Fig. 9), provides exceptionally high directivity over a wide range of operating frequencies. As for all types of folded dipoles, however, its chief drawback is that when the antenna array is oriented for ghost-free reception on one channel, multiple-image interference may often hopelessly mar reception on other television channels.

Fig. 11 - The Di-Fan, not highly directional.

Special Types

A number of other antennas have been designed for all-channel reception, but only a few of these special types are sufficiently directional to provide ghost-free reception of a single channel.

Kings' Electronics

Fig. 12 - This antenna turns to best position.

The extended V-type dipole (Fig. 10) combines some of the better features of the simple dipole with an ability to operate equally well on any television channel. Entirely ghost-free reception is usually possible on one channel, depending on the orientation of the antenna. This is the only television antenna which compensates for the wave distortion normal to all television signals, and therefore gives a much clearer picture. With an impedance rating of 300 ohms, a standard-transmission line can be connected directly to the antenna. Another wide-band type of antenna is the Andrew Di-Fan (Fig. 11) which operates with equal efficiency on any television channel. It has characteristics similar to the previous type, but is somewhat less directional. Ghost-free reception is usually possible on one channel, depending on the antenna orientation.

Last, but by no means the least in importance, is a rotatable television antenna (Fig. 12) which can be oriented by remote control (from the receiver) to provide ghost-free reception on any desired television channel. In operation, the antenna is rotated with the receiver switched to the desired channel. By observing the received image (or images) on the picture tube, the antenna can be properly oriented for optimum or ghost-free reception. A fixed antenna can then be installed in many locations.

 

 

Posted December 6, 2019

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