March 1957 Radio & Television News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Multipath signal interference is as prevalent today as it was at the beginning of radio communications - maybe more so since there are typically more reflective obstacles between the transmitter and the receiver. The evidence of multipath with digital communications, be it voice, Internet page viewing, or movies, is slowness in transmission as opposed to the analog case where garbled speech on radio and ghosting pictures on television are the evidence. High data rates with digital transmissions typically mask the packet errors and their necessary re-transmission; it all happens before the buffered information is presented to the listener / viewer. However, there is no such buffering of over-the-air radio or TV transmissions so evidence of multipath is immediately noticeable. I remember how sometimes simply having a large metal trash or delivery truck roaming through the neighborhood was enough to cause the Leave It to Beaver (and other) program's pictures to lose sync. Now, as then, one of the best ways to mitigate the issue is use of a directional antenna.
Problems in Metropolitan TV Reception
By Simon Holzman
Chief Antenna Engineer JFD Manufacturing Co., Inc.
Fig. 1 - The JFD "Super Helix" is an example of an antenna with a narrow polar angle (see Fig. 4B) that weakens ghosts.
Fringe areas have no monopoly on difficulties with picking up good signal. Overload, ghosting, and interference often plague prime-signal locations.
While considerable attention has been given to the reception difficulties encountered in areas distant from TV transmitting points, one sometimes forgets that television installations in metropolitan and other primary areas often suffer from problems peculiar to these locations. Noteworthy among these problems are those of ghosting, graininess, the effects of too much (or too little) signal, cross-modulation, and interference from a number of sources. In some cases, these conditions can be completely controlled. In cases where they cannot be entirely eliminated, they may be greatly minimized, as a rule, resulting in satisfactory reception.
In many localities, ghosting is a serious deterrent to acceptable viewing. This phenomenon is caused by the simultaneous reception of out-of-phase signals on the same channel. High-frequency TV waves are capable of being reradiated (as in radar) by metallic bodies. The body reradiating these waves acts as a secondary transmitting antenna and effectively "bounces" the signal in a different direction. The action of two such reflecting bodies is illustrated in Fig. 3. Since the bounced wave and the original signal travel different distances before arriving at the receiving antenna, they are out-of-phase. The phase differential governs the degree of separation of the two pictures on the screen.
To understand the cure, let us investigate two properties of receiving antennas. The first of these, the front-to-back ratio, is the ratio of the received voltage from the front of the array to the signal received from the rear, given equal power densities in both cases. The higher the front-to-back ratio, the less signal the antenna will receive from reflections coming in from in back of it.
Fig. 2 - Some interference can be tuned out with a receiver-installed high "Q" frequency-selective adjustable trap.
The second property is the antenna's polar response. This has to do with the directivity of the antenna with respect to a signal from one source. It is determined by the angle formed between the direction or position in which the antenna is most sensitive (forward pickup) and the direction or position in which it receives 3 db, or 30 percent, less signal from the same source. This may be measured by beaming a constant signal toward the antenna while the latter is aimed directly at the transmitting source, measuring the signal voltage generated in. the antenna, and then rotating the antenna without changing the source until the antenna is in a position where the signal generated in it is 3 db down.
In actual practice, this property is of interest because it describes the relative sensitivity of the antenna to signals coming in from different directions. One signal, for instance, may be an unwanted ghost. In Fig. 4 we see the effect of a wide polar pattern (A) and a narrow one (B) in picking up or rejecting such a ghost. In the former case, pickup of the ghost is almost as great as that of the primary signal. In the latter case, the angle at which the ghost comes in is outside the polar response angle, or 3-db point, and is sufficiently weaker so that its effect on the viewed picture is considerably reduced. This property of an antenna is roughly analogous to selectivity in the receiver's front end. An example of an antenna having a small polar angle, the JFD "Super Helix", is shown in Fig. 1.
Unfortunately, narrow beamwidth goes hand in hand with high gain. Often, the greater signal level, in a primary area, will overload the receiver causing buzz, fuzziness, and tearing. This excess of signal, however, is easily compensated for by the addition of an attenuator pad at the set terminals. This pad may have a fixed value, but it is preferable to use a variable type since the degree of attenuation required will vary with the location. Units of this type are readily available commercially. The use of a pad will also help minimize ghosts, by bringing the level of the spurious signal down to a non-visible value. Extreme caution must be exercised in an installation of this type to orient the antenna accurately and to keep horizontal runs of lead-in to a minimum. Horizontal lengths of twin-lead tend to act as signal-collecting devices, and will complicate the original problem.
Fig. 3 - The longer paths of the reflected signals are responsible for ghosts.
Fig. 4 - A narrow-beamed, more directional antenna, like the one in (B), will reduce amplitude of the ghost.
Fig. 5 - Receiver i.f. circuits are designed for best response (center trace) over a certain normal range of a.g.c. voltage. Abnormally high signal level (right) can deteriorate performance as much as low signal and a.g.c. values.
Fig. 6 - Tuned line is used to locate interference frequency or act as trap.
In many metropolitan locations, extremely high signal levels are prevalent due to the proximity of transmitting and receiving antennas and lack of blocking structures. Too high a signal level can and does cause poor picture quality. The average television set is aligned for maximum bandpass with an a.g.c. voltage of approximately minus 3 volts. As the signal increases, the a.g.c, level goes up, shifting the grid characteristics of the r.f. and i.f. amplifier tubes that are controlled. When this occurs, each of these tubes operates on a different portion of its curve, and its effective input capacitance changes. This, in turn, causes deterioration of the bandpass characteristic of the set, as shown in Fig. 5, and coarse, hazy pictures with poor definition may result. There is also a tendency to poor synchronization due to pre-triggering of the sweep oscillators. This condition may be eliminated by the same attenuator pad mentioned previously. The pad should be adjusted for a barely sufficient signal on the weakest channel. If some channels are still overloaded, a switch type pad should be used, so that it may be switched into the antenna system only on those channels where it is needed.
Cross-modulation, or windshield wiper, is another symptom of too much signal. It usually manifests itself as a dark vertical bar sweeping horizontally back and forth through the picture. This particular trouble is usually caused by a single channel much stronger than the others. TV tuners are rarely selective enough to eliminate this annoying feature particularly on channels adjacent to overly strong ones.
If all channels are sufficiently powerful, an attenuator pad may be used to reduce the overall signal level. A preferable method, however, is to install an adjustable high-"Q" trap in the lead-in, and set it for sufficient attenuation of the undesired channel. This is the equivalent of a frequency-selective attenuator. These adjustments are usually quite critical, and care should be taken when they are made. The interfering channel can be identified by turning the channel selector to the unused channel positions, and noting which channel is most often seen. A commercial version of such an attenuator that may be adjusted for rejection on the high v.h.f. band is shown in Fig. 2.
In an urban area, there are many sources of interference. Interference caused by amateur radio transmitters is usually of a frequency below 50 megacycles. This, as a rule, can be eliminated by the use of a high-pass filter. This type of filter should be installed as close to the set's antenna terminals as possible, so that there will not be pickup of interference along the lead-in wire beyond the filter.
The previously mentioned selective high-"Q" traps will be of great use in removing interference caused by powerful local FM transmitters.
Pulse-type interference, one form of which is ignition noise is often the most difficult type to eliminate. In its milder forms, putting a tight twist in the downlead may be sufficient to minimize it. Care should be taken in the original installation to keep the antenna and lead-in as far as possible from sources of ignition noise. Grounded metal screening, used to shield the antenna from the street or other source of ignition noise, is often used with considerable success.
In exceptionally noisy areas, shielded 300-ohm twin-lead or coaxial cable should be used. The outer shield should be grounded at as many points as possible. When using 75-ohm coaxial cable, a balun impedance transformer must be installed at the top to match the antenna to the characteristic impedance of the line. Another transformer must be used at the set to match the line to the balanced 300-ohm input of the front end.
Both shielded 300-ohm cable and coaxial cable have comparatively high signal loss. For this reason, a moderately high-gain antenna may be needed to compensate for these line losses.
It is often difficult to determine the source and frequency of an interfering signal. A convenient gimmick that the technician can carry with him is a 20-inch length of 300-ohm twin-lead, with lugs attached to one end and the other end cut straight across and left open-circuited. A piece of aluminum foil should be wrapped around the twin-lead for a length of about 2 1/2 inches, as shown in Fig. 6. This forms a capacitively loaded half-wave line. In use, the lugs are attached to the antenna terminals of the set in parallel with the antenna lead-in, which remains connected. The interference is then tuned in, and the foil moved up and down the line until a point is found at which the interference is at a minimum.
Diathermy interference is rare today, but, when present, is extremely annoying. The best cure for this condition is to locate the source and notify the Federal Communications Commission. For several years, it has been illegal for a diathermy machine to radiate and cause interference.
Most cases of insufficient signal in metropolitan areas are caused by blocking of the direct signal by a tall structure, such as an apartment house. In cases such as this, reception may often be obtained by means of a high-gain, sharply directional antenna, an example of which has been shown, oriented to receive a bounced signal from some large structure to the side or rear. When this means is ineffective, an attempt should be made to get permission to install the antenna on the roof of the structure that is blocking the signal. A third alternative is the use of a tower or telescopic mast, enabling the antenna to be even with or above the edge of the roof of the blocking structure. A 30-foot height will usually be more than sufficient.
Reception problems caused by the use of indoor antennas are difficult, if not impossible, to solve. Generally, the best cure is an outdoor roof installation.
A final difficulty occasionally observed is phase shift due to high voltage standing-wave ratio. The phase shift manifests itself as a ghost which will change position with variations in the fine tuning adjustment. This is almost always due to faulty installation techniques. Be sure the lead-in has minimal horizontal runs and does not run parallel to metallic objects closer than 6 inches. Often, the condition can be eliminated by removing several inches of lead-in from the slack left behind the set. This slack should also be kept as short as possible.
Posted February 12, 2020 (original 2/17/2014)