By James E. Kluge, Technical Editor, Winegard Company
Crucial to the best performance of any MATV system is the choice of antenna. Other parts of the system (preamps or amps, down-lead, and splitters) either preserve the signal-to-noise ratio and the picture quality obtained from the antenna or they degrade it. Nothing but the antenna can improve the reception.
Selection of the best antenna for local conditions is more than half of the solution. Proper location, installation and orientation of the antenna are almost as important.
TV antennas are available in a variety of sizes, shapes and colors.
Therefore, selecting the one that provides the best possible performance level always involves trade offs between size, cost, performance on various channels, and durability.
The following discussion should make the selection easier.
Bigger is better
Size of the antenna probably is the single most important factor.
When all other parameters are equal, a bigger antenna will outperform smaller ones. Specifically, larger antennas having more elements can capture stronger signals, are more directional, and have a superior front-to-back ratio.
Of course, larger antennas also cost more than similar smaller models. Price must be balanced against other compromises.
Signal level
Primarily, an antenna should intercept all possible electromagnetic energy of the desired frequency and convert it to a voltage that is sufficient to mask (or make insignificant by comparison) all noise in the system that follows, including each TV receiver.
Antenna selectivity determines the relative signal level of the various desired and undesired frequencies intercepted by the antenna. Some factors that shape this frequency response are the number and spacing of the various elements (rods) plus the sizes of these elements and their lengths. One example is the design of UHF antennas having certain elements that can be broken off at pre-scored lengths. Shortening these elements increases the response or gain to the higher UHF channels. Of course, the change decreases gain at lower UHF channels.
----------- Before making polar-plot and gain measurements at the Winegard
Research Laboratory in Evergreen, CO, an engineer-technician team installed
an antenna on a rotating mast that is the top of a tilting tower. The tower
was pulled upright before the tests.
-------- These are a few of the cut-to-channel reference dipole antennas
used for gain measurements in the Winegard Laboratory.
-------- Inside the Laboratory, a technician made gain measurements using a signal-level meter. At the right is the machine for making antenna polar plots. The paper is rotated under a pen in synchronism with rotation of the antenna.
Signal-to-noise ratio In the absence of any received signal, the noise (snow) observed on the picture tube is the sum of atmospheric noise, radiated noise from the vicinity, preamp or line amp noise, and TV tuner noise.
The antenna must develop a signal that is stronger than the total noise, to produce a satisfactory snow-free picture.
After conducting subjective viewing tests, the FCC TASO committee has determined that a 45dB signal to-noise ratio provides good pictures without noticeable snow. The theoretical noise floor for a 75 ohm system is about 1.1pV (-59dBmV). There fore, for a 45dB S/N ratio, the antenna must generate a signal voltage level of -14dBmV (200pV) or more.
Most locations have atmospheric static and man-made noise that are picked up by the antenna. In addition, the amplifier chain and the TV tuner add their own noise.
More than 200pV, therefore, is required to maintain a snow-free picture. The industry has agreed to a minimum level of 1000 uV (0 dBmV) for MATV systems. And most TV receivers are designed for best performance with a minimum of 1000 uV at the antenna terminals.
Higher-gain antennas needed If an antenna now in use cannot supply 0 dBmV plus the downlead loss for all available channels, then something must be changed. One possibility is to add a preamplifier or line amplifier, but they add noise of their own.
A preamplifier mounted on the antenna will provide a stronger signal with a minimum of added noise, especially if it is a low-noise type. However, no preamplifier can improve the S/N ratio. Its primary function is to provide enough gain to cancel the downlead loss, and thus preserve the antenna S/N ratio.
These higher-gain antennas provide other benefits, in addition to strengthening the signal without adding noise.
Directivity
Antennas have more gain when they are larger and have more elements. An added bonus is that the additional elements also provide a narrower beamwidth angle which (with proper orientation) discriminates against ghosts and undesired signals while supplying higher signal levels for the desired signals.
Front-to-back ratio Higher front-to-back and front-to-side ratios are an advantage of larger antennas. The response to undesired signals and noise arriving from side or rear should be 20dB to 40dB lower than the desired on-axis signals, and such performance is easier to obtain over a broad range of frequencies when more elements are included.
Polar-pattern graphs provide the amplitude-response-versus-direction parameters. And most manufacturers can supply these graphs for all TV channels.
Occasionally, it is necessary to select a large antenna for the better directivity and front-to-back ratio.
But the large antenna might cause amplifier overload on strong channels. In such cases, installing a matched-impedance loss pad just before the overloading amplifier will reduce the level sufficiently without degrading the desired directivity and front-to-back pattern.
Antenna shapes Most antennas are manufactured in one of two basic shapes: flat and wedge. Flat antennas are built in a horizontal plane, while the wedges extend vertically also.
Wedge antennas looks like two V's or W's lying on their sides.
However, both wedge and flat types are variations of the basic Yagi Uda design.
Wedge antennas have a larger capture area for the signals, without requiring an unduly long size. Also, they have higher gain at the low end of each VHF band, while flat antennas tend to have less gain at the band low ends.
Antenna specifications Mechanical specifications include the number of elements, the dimensions, weight, color, finish, material type and thickness.
Color of an antennas has no effect on performance. Some customers prefer antennas in colors rather than plain aluminum. Any sort of finish satisfies that request.
It is important to the durability, however, how a color is obtained. A color-dyed finish (often used with less expensive models) has the same durability as plain metal. But if the color is obtained by anodizing of the aluminum, the finish will be hard, durable and resistant to corrosion that shortens the life-span of unprotected antennas. Anodizing is worth more than it costs when an antenna is used for several years.
Electrical specifications are not as easy to measure or determine.
Most antenna manufacturers have full specs but seldom publish them or make them available to antenna installers unless asked.
Antenna installers sometimes evaluate the practical performances of several models before finding one that is satisfactory for a certain locality. Then they probably continue to reorder this model until presented with a strong performance or price incentive. Their choice of a superior antenna would be easier if these installers first narrowed the field by using an evaluation comparison from electrical specifications.
Gain
Gain of an antenna is measured by comparing its signal (under identical conditions) with that from a standard reference dipole which is resonant to the test frequency. This must be done separately for each TV channel.
In this case, identical conditions mean that both antennas are immersed in a field of the same strength and pattern during the measurements.
If a manufacturer wants to inflate the gain rating of an antenna, he can specify the gain in reference to an isotropic radiator.
Of course, an isotropic radiator is a theoretical device that cannot be duplicated in practice. An antenna rated this way would appear to have a higher gain by 2.15dB. It seems more practical to rate gain according to measured gain rather than by theoretical gain.
There is another source of confusion between gain ratings. Antenna gain is not constant between the various channels. Therefore, it can be specified in these different ways:
According to the channel giving the highest gain.
As an average of all channels.
As an average mean between the highest and lowest gain.
Or as a typically ideal gain figure that cannot be realized in practice.
Gain represents only one of several parameters which contribute to an antenna's performance.
Therefore, the Winegard Company does not specify the gain, but provides a Figure of Merit that summarizes several performance factors. If an installer is interested in more details of performance, engineering specifications can be obtained for specific models from the Winegard engineering department.
---------- These are excerpts from a Winegard Chromstar engineering-specifications
sheet. Polar plot graphs are included for all VHF and four UHF channels, plus
other electrical and mechanical specifications.
Bandwidth
Bandwidth is not a critical factor now, but it was often a problem in the early days of color TV. Any antenna with a sharp slope or a low-gain area within the station bandwidth could cause weak or smeared color.
Antennas are now classified as single channel, VHF or UHF. They are satisfactorily flat in response across their rated bandwidth and will produce good color quality.
The response of many antennas includes the FM radio band. That's good if the antenna is to be used with both television and FM radios. Otherwise, the FM-band response can cause serious TV interference problems in metropolitan areas that have several strong FM stations.
Traps might be required to minimize this interference.
Some Winegard VHF antennas are designed to attenuate FM frequencies. When FM is desired, certain antenna directors can be shortened to increase the gain in the FM band. Instructions are provided with the antennas.
Directivity
Directivity specifications indicate sharpness of the antenna's main front lobe. It expresses in degrees the angle between the half-power (-3dB) points. If multipath ghosts, co-channel or other interferences are a problem, an antenna having a very narrow beamwidth could be the answer.
Perhaps a larger antenna is needed. Or the narrower beamwidth might require horizontal stacking of two identical antennas. On the other hand, vertical stacking of identical antennas will minimize airplane flutter or ground noise.
Front-to-back ratio
Front-to-back ratio indicates the antenna response to signals arriving from the rear as compared to those arriving at the front. A 10-to-1 ratio means that the signal strength at the back of the antenna is only 10% (-20dB) of the on-axis signal strength. This ratio is slightly different from channel to channel.
If beamwidth and front-to-back ratio are primary considerations, polar-pattern charts should be com pared and evaluated.
Impedance
Output impedance of the antenna varies with frequency, but seldom more than a 2-to-1 ratio.
Better antennas hold the variation to 1.5-to-1. Nominal antenna impedance is designed to be 75 balanced or 300 ohm balanced, depending on the type.
Twinlead transmission line can be connected directly to a 300 antenna. But for 75 ohm coaxial cable, the 75 ohm balanced antenna must be transformed to 75 ohm unbalanced. Some antennas provide a cartridge board or a preamplifier inside a weatherproof housing that's attached to the boom, and it offers a choice of switching to 300 ohm balanced or 75-ohm unbalanced as needed to match the downlead.
Comments
Antenna installers who have unique reception problems are urged to consult the antenna manufacturer's technical staff, who will assist in providing better TV pictures.
----- One method of offering choices of balanced or unbalanced impedances
and 75 ohm or 300 ohm impedances is a cartridge board mounted in a waterproof
housing that attaches to the antenna. The photographs were taken before and
after the installation.
Also see:
Preventing damage from transients
Locating AC leakage in industrial plants