AMPLIFIERS
Although one-half to one watt may be perfectly adequate for very localized neighborhood radio coverage, higher power will be required to cover larger areas such as a town or a portion of a large urban area. In order to increase the output power of a low power FM exciter or transmitter, an amplifier or series of amplifiers are connected to the output of the transmitter. Amplifiers are also referred to as amps, and should not be confused with the unit of current also called amps.
Amplifiers are much simpler in design and construction than a transmitter. Most of the amplifiers used in micropower broadcasting employ only one active device, an RF power transistor, per stage of amplification. By convention most broadcast amplifiers have an input and output impedance of 50 ohms. This is similar to audio speakers having an impedance between 4 and 8 ohms. When an RF amplifier with a 50 ohm input impedance is attached to the 50 ohm output impedance of a transmitter, this matching of impedances assures a maximum flow of electrical energy or power between the two units.
A mismatch between any elements in the chain from transmitter to amplifier to filter to antenna will reduce the efficiency of the entire system and may result in damage if the difference is rather large. Imagine the results if a high-pressure water pipe four inches in diameter is forced to feed into a 1/2” water pipe with no decrease in the action of the pump feeding the four inch pipe. In an RF amplifier the RF power transistor will heat up and self-destruct under analogous conditions.


An RF power amplifier consists of an RF power transistor and a handful of passive components, usually capacitors and inductors which are connected in a particular topology that transforms the 50 ohm input and output impedances of the amplifier to the much lower input and output impedances of the RF power transistor. Detailed circuit theory of this interaction between the components is not covered in this primer.
Amplifiers can be categorized as either narrow band or broad band. Narrow band amplifiers are tuned to one specific frequency. Broad band amplifiers are able to work over a specified range of frequencies without tuning. Most of the amplifiers that have been used in micropower broadcasting are of the first type. A tunable amplifier can be a bit of a problem for those without much experience. In a typical tuned stage amplifier there will be two tuning capacitors in the input stage and two more in the output stage. If not correctly adjusted, the transistor can produce unwanted sideband spurs at other frequencies both within and outside of the FM band.
To make set up easier for the average micropower broadcaster, a broad band amplifier is preferable or one with a minimal amount of tuning stages. Several designs are available.
Broad band designs are not as common due to the degree of design experience required to create a functional unit. It seems a number of kit providers are content not to optimize and improve their amplifier designs. Free Radio Berkeley is now offering amplifiers that are either no tune or minimal tune designs in several different ranges of power. Certain broad band designs may be too wide in their range of frequency coverage and will amplify the harmonics equally well. For FM broadcast purposes the width of frequency coverage should be for only the FM band, about 20-25 Megahertz wide.
Selecting the right amount of power is important since you should only use enough power to cover the desired area. Unfortunately there is not an easy answer to the question of how much area a certain amount of power will cover. Antenna height is very critical, five watts at 50 feet will not go as far as five watts at 500 feet. Assuming you do not have a 10 story building or a convenient 500 foot hill to site your antenna and transmitter on, experience in urban environments has yielded the following rough guidelines. With an antenna approximately 50 feet above the ground, one-half to one watt will yield an effective range of one to three miles; five to six watts will cover out to about one to five miles. Ten to 15 watts will cover up to eight miles. Twenty to 24 watts will cover up to 10-12 miles and 30-40 watts will cover up to 15 miles. Coverage will vary depending on terrain, obstructions, type of antenna, etc. If your antenna is very high above average terrain, you will be able to go much further than the figures given above. Quality of the radios receiving your signal will be a determining factor as well. Since the power levels are rather low in comparison to other stations, an external antenna on the receiver is highly suggested, especially an outdoor one.
It is very important to provide adequate cooling for RF amplifiers. This means using a properly sized heat sink and an external cooling fan. Heat sinks have heat dissipating fins which must be placed in an upward pointing direction. Overheating will cause premature failure of the transistor. A cooling fan, usually a four to five inch square box fan, will offer extra insurance. It should be placed so that the air flows over the fins of the heat sink.
Under no circumstances should an amplifier/transmitter be operated without a proper load attached to the output. Failure to do so can destroy the output transistor. When testing and tuning, a dummy load is used to present a load of 50 ohms to the transmitter/amplifier. It is very bad practice to tune a unit with an antenna attached. Use a dummy load of proper wattage rating to match the transmitter output wattage.
An output filter must be used between the transmitter/amplifier and the antenna. More on this in the filter section.
Heavy gauge (12-16 AWG) insulated stranded wire is used to connect the amplifier to the power supply. Observe correct polarity when making the connection. Reversing the polarity will result in catastrophic failure of the transmitter. Red is positive and black is negative or ground.