Wednesday, December 25, 2013

FM Radio Station Power Part 12


Online mW to W Converter

The FM radio station at 100 mW is only .1 watt which is a mere whisper of more powerful radio stations. (1 Watt = 1000 mW)

Relative Power
Measuring the voltage at the antenna can determine the relative power measurement which is useful for tuning, i.e. to a minimal or maximum value. 

Measuring Power 

From Code of Federal Regulations Telecommunications 47
Part 80 to End, FCC 80.259
"(c) A reserve transmitter must be equipped to measure antenna current. (d) The antenna power must be determined at the operating carrier frequency by the product of the antenna resistance and the square of the average antenna current both measured at the same point in the antenna circuit at approximately ground potential."

How to Measure Amps or Watts With a Multimeter
Measuring Amps

Step 1: Select the "Current" setting on the main dial of the multimeter. Choose a current range high enough for the circuit being tested. For example, if the circuit has a current that you estimate to be around five amps, select the "10 amp" setting instead of the "1 amp" option. Choosing a setting that is too low can overload the multimeter.

Step 2: Insert the leads into the multimeter connections labeled "current." Typically, the black lead should be connected to the "common" port, while the red lead is connected to a port that matches the selected current range. This arrangement varies, depending on the unit. Consult the multimeter labels carefully to confirm that the configuration is correct for measuring current.

Step 3: Place the multimeter leads into the circuit using a series configuration. The electricity must be redirected to flow completely through the multimeter to obtain an accurate reading. Current should usually move into the red lead and exit the black lead.

Step 4: Read the amount of amperage displayed on the multimeter. Remember to consider this number in the context of the selected current range. For example, the number "10.00" may indicate 10 amps on one setting but only 0.01 amps if the multimeter is set to the smaller "milliamps" range.

Measuring Watts
Step 1: Choose the "Voltage" setting on the multimeter dial. As you did when measuring the current, ensure that the appropriate voltage range is selected. It is usually a good idea to select the highest possible voltage range to prevent an overload.

Step 2: Configure the multimeter leads into the ports marked for "voltage." The black lead can usually remain in the "common" position. Move the red lead to the port labeled for the voltage range being tested.

Step 3: Connect the leads to the circuit in a parallel arrangement. The black lead should link to a negative or grounded point, while the red lead should be touched to a point of the circuit you wish to measure. Unlike a current measurement, a voltage test does not need to be completely redirected through the multimeter.

Step 4: Read the number on the multimeter display. As with current, remember to consider the context of the multimeter range setting.

Step 5: Multiply the amount of current and the amount of voltage in a circuit to determine the watts. For example, a motor circuit that uses 5 amps and 12 volts has 60 watts of power.

Use several incrementing known wattage light bulbs as a test to verify basic power operation of the transmitter. The lamp will present a load to the transmitter that's similar to an antenna. This test may provide a visual indication of the power output from the transmitter, and verify operation of the antenna coupler. Use grain of wheat bulbs for milliwatt ratings.

P=EI, P=(I^2)R where E is voltage in volts and I is current in amps

The antenna is a small circular loop with a 1.75-inch diameter. It's measurement of resistance for the tiny loop antenna is 0 ohms on every scale from 2K to 20M. The multi-strand antenna wire is a total of 6-inches long. This indicates that power output readings and measurements can be made at the antenna coupler with no antenna needed. The coupler is where the antenna attaches to the board.

The next step is to unravel a length of wire equal to a fundamental size of the wavelength and measure its resistance. If the value is still 0, increase the fundamental, from 1/32th wavelength to 1/16th, then repeat again if the wire still has zero resistance. At 1/8th, the antenna may be too long.

FM Radio Station Part 5 Index