USING CIRCUIT METERS

Voltmeters, ammeters, and ohmmeters are used to measure voltage, current, and resistance.

• When measuring voltage with a voltmeter, be sure to connect the leads in parallel with the circuit.  The effective range of a voltmeter can be increased by placing a resistor of known value in series with the meter, between the meter and the circuit under test.  This is what takes place internally in a selecting voltmeter when you switch from a lower to higher voltage range.
• When measuring current with an ammeter, be sure to connect the leads in series with the circuit.  The range of an ammeter can be increased by adding resistance in parallel with the meter.

Multimeters are integrated instruments capable of measuring voltage, current, and resistance.  Be careful not to switch a multimeter to measure resistance when it is selected to measure voltage on a circuit - you could very well burn up the circuitry of the meter.  Also, when measuring resistance, be careful in using the ohmmeter function - if you select a resistance range that is lower than that which are currently measuring, you will probably burn up the internal resistors.

Wattmeters are used to measure power and can either be an integrated part of your radio equipment or a separate component.  The wattmeter should be connected at the transmitter output connector for the most accurate transmitter output power measurement.  RF wattmeters usually operate at 50 ohms.

An S meter is used to measure the relative signal strength in a receiver - it is an approximation.  Two measurements side-by-side on radios with the same antennas will give different readings.

TUNING ANTENNAS

Antennas must be fine-tuned to operate properly.  Antenna tuners are devices that allow the user to use an antenna on frequencies it was not designed to propagate.  It matches the transceiver output impedance to the antenna system impedance.  If the antenna is too short or too long, or the connections are corroded or poorly made, then its resonance can be off, meaning that you will have an impedance mismatch.   Directional wattmeters are capable of measuring both forward power and reflected power so that you can calculate the total power that you are transmitting at.  A peak-reading wattmeter measures the maximum power output of your antenna.  Performing such a measurement is important to ensure that the operator is not exceeding the power output allowed by his or her license class.

Related to measuring power is the concept of the Standing Wave Ratio (SWR).  The SWR of an antenna is a ratio of how much energy is being fed back to your radio equipment due to the improper installation of an antenna.  The ratio is how much forward power is being fed to the antenna in relation to how much reflected power is being dumped back into your equipment.  Too much reflected power can be harmful, even fatal, to your radio because the only way for the circuits to release the excess energy is by heat.   Feed line will also warm up due to signal loss and high SWR's. 

To determine the total power output of an antenna using a directional wattmeter, use this equation:

Total Power = Forward Power - Reflected Power

To use your SWR meter (a type of reflectometer), you must connect it between the feed line and the antenna.  A SWR measurement of 1:1 (one-to-one) is perfect, but in reality, quite hard to achieve.  If you get in the area of 1.5:1 then you have a good ratio and little energy is being dumped back to your radio.  If you get sloppy, you can get as bad as 4:1 - that's definitely not a healthy condition for your radio.   Probably the length of your antenna is off, or you have a bad connection somewhere in your feedline.

Both directional wattmeters and SWR meters are great tools to use in ensuring that you are not feeding excessive power back through your antenna.  However, know this: over time, transistors slowly degrade due to circuit wear-and-tear and lose the ability to create strong signals.  If you were to chart periodic wattmeter measurements over time, you would notice that both the forward and reflected power readings will slowly decrease, even if you hold a properly matched impedance.  That's just the nature of the beast.  Fortunately, an SWR reading will not deviate with matched impedance because it continues to reflect a power ratio.   SWR meters are also yield reliable measurements across many bands, while wattmeters must be chosen specifically per band application for reliable results.

Dummy loads is a non-inductive and resistive load that should be used by hams for testing or tuning their radio equipment.  Instead of transmitting a test signal which may interfere with other ham communications, the signal is pumped into the dummy load to be dissipated as heat.  Dummy loads can be connected to your antenna switch as an output to make it easy for you to switch to while testing.  They are rated by power output, so you should only selective one that is rated at or above your transmitter's maximum output.

TROUBLESHOOTING TIPS

OTHER HELPFUL INFORMATION

• When switching to a new microphone, always perform an on-the-air check to make sure the signal quality is acceptable.
• When calibrating a tuning dial, frequency standards from WWV in Colorado or WWVH out of Hawaii can be used.