Antenna Systems: Audio Visual, Video, Projection, Presentation Rental AV Equipment

Antenna Systems

Next to the receiver, one of the most important considerations for governing the performance of the wireless system is the type and placement of the antenna system. Unfortunately, antenna systems are one of the most misunderstood aspects of a good wireless installation. There are a number of antenna types available depending on the application, and proper selection of this critical link in the system can often eliminate larger problems. The antenna is essentially the “ear” of the receiver, and the better the “hearing” is, the better the overall system range and performance. An antenna is more than a simple length of wire. Antennas must be carefully designed to match the input impedance of the receiver, plus offer the user a degree of flexibility. The most common type of antenna used by many wireless microphones is called a quarter-wave whip, due its overall length being Ľ of the actual RF signal wavelength. There are many antenna designs on the market that can actually improve the quality of the overall system. These include:

˝ wave dipole
5/8 wave
Log periodic
Yagi

Choosing the Antenna Type

Most of the wireless microphones on the market today are supplied with Ľ wave whips that are to be mounted directly to the front or rear of the receiver. When it becomes necessary to remote mount the antenna away from the receiver, it is important to remember that Ľ wave whip antennas are never suitable for use as a remote antenna because they are a low impedance device that does not have a proper ground plane. The ground plane for a Ľ wave antenna is the chassis or circuit board of the receiver. A ground plane is effectively the other half of the antenna, with the actual aerial whip being the complement. Using a Ľ wave whip without a ground plane is actually worse than no antenna because without the impedance match that the ground plane provides, the antenna feed-coax pick-ups extraneous RF in the outer shield, and this is in turn fed directly to the receiver. Any sources of RFI such as computers, motors, dimmers, etc. can have a very adverse affect on an improperly installed antenna system.

Remote Antennas

Remote antennas provide the advantage of allowing the antenna to be located closer to the transmitter, and many antenna designs actually increase the signal gain to the receiver. The higher the signal gain to the receiver, the more reliable the operational range of the system. Many manufacturer’s offer a variety of free standing antennas including Ľ wave dipoles, 5/8 wave transformer loads, log periodics, and yagis. Each type has different characteristics that are suitable for different situations. For most installations a omni-directional antenna is best. For directional applications, Logs and Yagi’s, are a good choice. Directional antennas operate similarly to directional microphones where an RF source can be targeted while rejecting others. In directional antennas, the pattern generally provides high forward gain of the RF signal and rejection at the back of the antenna. Common TV Yagi’s can work very well provided you use one that is properly matched for the frequency range and input impedance of the receiver.

Yagi’s and Logs appear to be very similar in that both antennas have the characteristic fishbone appearance, however Yagi’s have one active element and are narrow in frequency coverage. Log's have a number of active elements, and tend to be much more broad-banded. Yagi's are forward directional to about a 60° pattern with up to 5 dB of gain, and Log's are forward directional to 60° with up to 4 dB of gain. Each 3 dB of gain will net a two-fold increase in range over a standard receiver mounted Ľ wave whip.

Antenna Placement

For any type of diversity receiver to operate properly, the optimum antenna spacing must be followed. The two antennas must be placed at least one wavelength apart or the diversity circuitry will not function as it was designed. For determining the optimum spacing it is important to know how long an actual wavelength is. The length of the wave can be calculated by the following formula:

984 f = the wave in feet, where f = the frequency in MHz.

Using this formula, a full wave at 174 MHz is 5.66 feet or 67.86 inches, which is the minimum recommended distance the two antennas would need to be spaced apart in order to have the diversity circuitry be effective. Optimum spacing at VHF is generally 10' between the antennas, and 3' at UHF to insure that they are receiving a different wave. Proper antenna placement is also critical in that the antennas should always be in line of sight from the operating transmitters and free from obstructions, especially metal.

Where is the best place to locate the antenna?

Proper antenna placement can mean the difference between a good installation, and a problem installation. There are a few issues to look at prior to choosing location for the antenna. Look at these factors:

Construction
What type of building materials were used -- wood, masonry, steel, reinforced concrete. Are there metal surfaces that could act as reflectors and cause multipath?
User Requirements
Is the user a high profile broadcaster, corporate boardroom, school, or a church. If there is a rare problem, will it be disastrous?
Cost Considerations
What is the budget of the user? Is the user planning to purchase and install their own system or use a quality contractor? Will the receivers be rack mounted?
RF Environment
Is the installation in a large metro area where there is considerable RF activity? This can have an effect on placement of the antennas.

The Best Location

In general, the best location for antenna placement is always the closest to the transmitter. The type of antenna selected also has an effect on placement as discussed earlier. Since the transmitted signal is typically vertically polarized, it is important to orient the receiver antennas in a vertical position. The best place is usually on an adjacent wall, near the vicinity of the transmitter, using the frequency length formula to determine the proper distance between the antennas. When a number of multiple systems are used in a given location it is a common practice to use antenna combiners to eliminate the “antenna Farm.” Antenna amplifier/combiners are a convenience item that allows a single pair of antennas to feed multiple receivers.