DUMMY LOADS
To properly tune and set up an RF amplifier, a dummy load is needed since it is not good practice to use an antenna for such purposes. A dummy load simulates an ideal RF load of 50 ohms. The first design can be built for power range of 1-10 watts. A second design can be scaled for 25-100 watts depending on how many resistors are used.
 POWER SUPPLIES
Most of the transmitters and amplifiers used in micro broadcasting require an input voltage of 12 to 14 volts DC. Higher power amplifiers (above 40 watts) require 24-28 volts DC. In a fixed location, the voltage is provided by a power supply which transforms the house voltage of 110 volts AC to the proper DC voltage.
Power supplies are not only measured in terms of their voltage but current as well. A higher power amplifier is going to require a greater amount of input power as compared to a lower power amplifier. Output current is measured and specified as amps. A power supply is selected on the basis of its continuous current output which should be higher than the actual requirements of the amplifier. Power supplies operated at their fully rated output will have a tendency to overheat under continuous operation. An amplifier which requires 8 amps will need a power supply with a 10 to 12 amp continuos capacity. In most cases the following ratings are suggested for transmitters requiring 13.8 volts.
1-5 Watt Transmitter 2-3 Amps
10-15 Watt Transmitter 5-6 Amps
40 Watt Transmitter 12 Amps
Any power supply you use must have a regulated voltage output along with protection circuitry. Some reasonably priced brands include Pyramid, Triplite and Astron. There are two types of power supplies, linear and switching. Linear power supplies use a large and heavy power supply transformer. Switching supplies use a much smaller and lighter transformer. A 12 amp linear supply can weigh 15 lbs., whereas, an equivalent switcher weighs two to three 3 pounds. Switchers are rated by both voltage and wattage. Due to their higher efficiency, they do not need to be over rated as much as a linear type. One very reliable manufacturer of switching power supplies is Meanwell, which is distributed by Jameco, RSI Power, and Mouser. Do not use any of the wall transformer type of power supplies. Such units are not adequate for this application. Higher power transmitters require power supplies with an output voltage of 28 volts. A 75 watt transmitter will require a power supply with a current rating of 6-8 amps and 28 volts.
For mobile applications, voltage can be fed from the cigarette lighter socket of a car with the correct plug and heavy gauge wiring. This may not work well in some newer vehicles which are reported to have some sort of current limit protection on the lighter socket. Check with an auto mechanic about this if you are in doubt. Electrical systems on newer vehicles are rather sensitive and can be damaged if not properly understood.
Another problem with mobile operation is battery drain. A 20-40 watt transmitter running for 4-5 hours can deplete the battery to the point where the vehicle may not start. It is better to have a separate battery running parallel to the charging system with an isolator. Isolators are available from recreational vehicle accessory suppliers. Use a high capacity, deep discharge type of battery.
Lead acid batteries are not very benign. Acid can leak and spill on people, clothing and equipment. It is best to keep the battery in a plastic battery box. Vapors from the battery are explosive in confined areas. Keep this in mind for mobile vehicle operations. You might consider using a gel cell type of battery. It is sealed and can not leak. These are a bit pricey but have far fewer problems. A good quality gel charger must be used to ensure battery longevity.
Smaller gel cell batteries work really well for setting up a low power (6 watts or less) transmitter on a street corner as a public demonstration of micropower radio. Transmitters can be set up at demonstrations and rallies so motorists can tune their radios to the frequency which is displayed on large banners near the streets and listen in on what is happening. This has worked very well. Use your imagination to show how micropower broadcasting can be brought into the community.