Inside Transmitters: Audio Visual, Video, Projection, Presentation Rental AV Equipment

Inside Transmitters

Like the receiver, the transmitter has an important role in the quality of a wireless system. Although the transmitter has an effect on the performance of the system, there tends to be less differences between what is considered good and ideal as transmitters tend to be easier to design and build.

Characteristics of the Ideal Transmitter

FCC Maximum Output Power
To have the greatest range and freedom from interference the transmitter should have an output power of 30-50 mW. Output power less than this will have a dramatic effect on reliability and range.
Efficient
The transmitter operates on a battery and should not be power hungry to achieve the maximum output power. Efficiency can be achieved through careful circuitry design. Most 50 mW transmitters will operate on a 9 volt alkaline battery between 5-8 hours continuously. Batteries will last longer under intermittent use.
Proper Propagation
The transmitter needs an effective radiating element (antenna) to have sufficient range. A separate “pig tail” trailing wire antenna of the proper length is generally preferred. Since many microphone capsule manufacturers do not design RF shielding into their cartridges, transmitters that use the microphone cable as the antenna subject the transmitter audio circuitry to unwanted RF reentering through the audio input.

In handheld wireless microphone transmitters, a cylindrical tube around 7" in length is used making antenna design a cosmetic problem. Most manufacturers use internal antennas on VHF models where a circuit trace is used as the antenna, but UHF handheld transmitters use a trailing wire antenna because the “hand capacitance” created by gripping the tube actually absorbs a great deal of the RF energy, thus reducing range. The trailing wire allows the RF signal to radiate properly without adverse effect of hand capacitance.
Sensitivity Controls
The audio input circuitry should be adjustable to allow for variances in input requirements between the multitude of microphone capsules on the market.
Low Spurious Emissions
Meaning that the transmitter should operate “clean” with a minimum of spurs that could present harmonic problems especially in larger multiple systems. The FCC typically sets limits for these spurs, but the FCC limits should be considered the bare minimum for good results especially in large multiple system installations.
Noise Reduction Circuit (Compandor)
The audio should have a compressor circuit to increase the audio performance of the system. In the VHF range, the FCC allows for +12 kHz of deviation for a total bandwidth of 24 kHz. In the UHF range the FCC allows for + 75 kHz of deviation for a total of up to 150 kHz of bandwidth. Regardless of the VHF or UHF operation, audio companding increases the usable bandwidth allowed. Most systems on the market today have some form of companding. There are a number of different types of Compandor systems in use by wireless manufacturers today. Each compandor type has benefits as well as limitations, however the compandor type is not terribly important provided the radio is quiet.
Mute Switch
For wireless microphones, this feature is not a must, but rather more of a convenience to keep the receiver “captured” even when the microphone is not on. This simply shuts-off audio without turning off transmitter which could cause a noise tail or squelch break. Most tone coded systems use a single switch which activates power on the transmitter. The receiver then has a momentary audio mute circuit to keep the receiver muted until the transmitter has been on for several seconds.

Batteries

While most people would prefer to have a power source in the transmitter that never needed replacement, unfortunately battery replacement is the trade-off for having the freedom of a wireless unit. You should always make sure that the customer uses quality batteries such as Eveready, Duracell, Kodak etc. Alkaline types are generally preferred because of the low internal resistance, and therefore lower noise in your transmitter. Lithium batteries are also excellent alternatives for people that need very long life between battery changes.

Re-Chargeable (NiCads), The Necessary Evil

Nickel-Cadmium batteries have been on the market for a number of years and can successfully be used in wireless transmitters. By nature, NiCads are only capable of 1.2 volts per cell, so you do not have the same capacity even at full charge as a standard Alkaline battery. This problem is compounded with the 9-volt type NiCads. There are two distinct type 9 volt NiCads on the market, and only one will properly operate a transmitter.
The first type such as those manufactured by G.E. and others, is a combination of 6 individual 1.2 volt cells resulting in the combined total of 7.2 volts/65 MAh. Unfortunately this battery falls below the cutoff voltage of many transmitters on the market, so it either will not work at all, or operates the unit for a very short period of time. The obvious problem occurs when the user assumes that the transmitter is defective because it won’t operate off a new battery.
Varta, Saft and others, manufacture a 7 cell NiCad that delivers 8.4 volts at 100 MAh. These types will deliver about 1-1/2 to 2 hours of operation in a 50 mW transmitter. Recently a 9.6 volt 135 MAh battery has become available. These new batteries will also work quite well, but provide only a marginal increase in operational time over the 100 MAh types.

NiCad Memory Effect Fact or Fiction?

The most common rumor associated with NiCads is the supposed memory effect wherein a poorly cycled battery will develop a memory for the short cycle. While this may have been true years ago, most NiCad manufacturers will claim that these problems were solved long ago. Why then do so many people have problems with NiCads? Actually there may be a couple of factors that lead a person to believe that memory effect has ruined the battery.
One of the worst things to do to a NiCad is to overcharge the battery with a high power charger. When a NiCad reaches full capacity the excess current dissipates in the form of heat. Since the 9 volt designs are actually 7 separate cells within the case, the internal heat builds-up to a point where the battery can no longer dissipate the excess so it vents. When a NiCad vents the compounds in the cells chemically change or even leak from their case ruining the battery. Once this has occurred the battery should be thrown away.
The proper method of charging a NiCad is to charge it in a trickle charger and promptly remove the battery from the charger once the rated charge time has elapsed (normally 7-12 hours). A NiCad battery will only lose 1% of its charge per day, so even if the battery is left out of the charger for a full week, it will only lose a maximum of 7% of its original charge. For more effective operation, only charge the battery the night before the performance, that way the battery will have a full charge for the performance, and there will be little chance of overcharging. If a the battery charger can’t be supervised, an automatic power off timer is recommended to ensure the charger does not overheat the battery. If a smart charger is used the battery can be left in the charger almost indefinitely, but smart chargers are costly.