SWR meter and dummy load
The primary purpose of an SWR meter is to determine the amount of reflected power in an antenna system. The accuracy of inexpensive meters is relatively unimportant, and 10% is sufficient. Whether the SWR is 1.1:1 or 1.3:1 is not crucial. The accuracy of the SWR value is determined by how well the forward and reverse readings agree. Therefore, any SWR meter should have reasonably close SWR values. Where accuracy does matter is the indicated forward power. If the meter reads 100 watts, is the actual power 90 watts, 100 watts, or 110 watts? In practice, an accuracy of 10% is sufficient. Even the best SWR meters have an accuracy of only about 5%.
A dummy load is used to check the operation of a transmitter and can also be used to check the SWR meter. A good dummy load should show an SWR of approximately 1.1:1 across its entire operating range. If your SWR meter suddenly shows an SWR of 5:1 with the dummy load, it's likely that either the meter or the dummy load is faulty. If your antenna's SWR suddenly changes drastically, check it with the dummy load. If that measurement is correct, there's something wrong with the antenna system.
Tuning an antenna with an SWR meter is easy with an antenna tuner. You simply tune to the lowest SWR (lowest reflected power). Tuning an antenna to the location where you install it means adjusting the length (or coil tap, or capacitor) to achieve a low SWR across the desired frequency range.
An SWR meter is not a laboratory instrument, and for amateur use, an accuracy of 10% is sufficient. Using the meter requires extensive reading of the ARRL Handbook and Antenna Book (and other sources) and extensive practical experience. Diagnosing problems in antenna systems based on SWR measurements requires a good understanding of antenna principles and logical troubleshooting skills.
Terms used in the communications world
- 27MHz - Frequency band (Citizen Band).
- AM (Amplitude Modulation) - A type of modulation used by CB radios in which the amplitude of the carrier wave is varied.
- Antenna - Device that transmits and receives radio waves.
- Channel (channel) - Specific frequency within a band on which communication can take place
- CTCSS (Continuous Tone-Coded Squelch System) - A system that uses subtle tones to filter out unwanted communications.
- DCS (Digital-Coded Squelch) - A digital version of CTCSS, which uses digital codes to filter out unwanted communications.
- Duplex - Communication using two frequencies, one for transmitting and one for receiving, often used in repeater systems
- Frequency - A frequency, e.g., 118.230 MHz, on which communication takes place.
- Frequency band - A specific range (band) of frequencies. For example, 108 to 400 MHz is the Airband, which covers all aviation communications, so there's a frequency band for every sector.
- Gain - Amplification of a signal, often expressed in decibels (dB).
- Impedance - Opposition of a circuit to alternating current, often expressed in ohms (Ω).
- Modulation - The process of modulating a signal to convey information. Common types are AM (Amplitude Modulation) and FM (Frequency Modulation).
- PL (Private Line) - Motorola's brand name for CTCSS. Also a designation for a PL-259 (UHF-Male) connector.
- Propagation - How radio waves travel through the atmosphere.
- Repeater - A device that retransmits a received signal on a different frequency, thereby increasing the range of communication.
- RF (Radio Frequency) - The frequencies of electromagnetic waves used for radio communications.
- Scan - A function that allows you to scan for frequencies on which communications can be heard.
- Simplex - Communication in which the same frequency is used for both transmission and reception.
- Skip - Phenomenon in which radio waves are reflected by the ionosphere and thus travel over great distances.
- SSB (Single Side Band) - A variant of AM modulation that is more efficient in bandwidth and power
- Squelch - Setting that reduces background noise by turning off the audio when no signal is received.
- SWR (Standing Wave Ratio) - Ratio that indicates how well an antenna is tuned to the transmitter frequency.
- UHF (Ultra High Frequency) - Frequency band from 300 MHz to 3 GHz, used for television broadcasting, mobile phones, and two-way radios.
- VHF (Very High Frequency) - Frequency band from 30 MHz to 300 MHz, often used for FM radio, television transmitters, and maritime communications.
- Wavelength - The distance between two peaks of a wave, often measured in meters
