To trim an antenna to reduce standing waves, you can follow these general steps:
- Determine the resonant frequency: Use an antenna analyzer or other measuring tool to determine the frequency at which the antenna is resonant. This is the frequency at which the standing wave ratio (SWR) is lowest.
- Measure the SWR: Use the same measuring tool to measure the SWR of the antenna at various frequencies. Note the frequency at which the SWR is the highest.
- Calculate the amount of trimming required: Determine the difference in frequency between the resonant frequency and the frequency at which the SWR is highest. This is the amount of trimming required to adjust the antenna for optimal performance.
- Trim the antenna: Use a wire cutter or other appropriate tool to trim the antenna element(s) by the calculated amount. Trim the antenna in small increments and re-measure the SWR after each adjustment until the desired SWR is achieved.
- Fine-tune the antenna: Use the measuring tool to make small adjustments to the length of the antenna until the SWR is as low as possible across the desired frequency range.
It is important to note that trimming an antenna can be a complex process and may require specialized knowledge and tools. It is recommended to seek the guidance of an experienced antenna technician or amateur radio operator if you are not confident in your ability to trim the antenna yourself.
High SWR will cause permanent heat damage to your radio. Do not operate a radio with a untuned antenna for any length of time. Very short transmissions to test and tune the antenna are ok. However lengthy and back to back transmissions will quickly build up enough heat to damage the finals.
Please use this guide to tune your GMRS Mobile Radio Antenna.
1. To determine the if the antenna is too long or to short conducted the SWR test using the same power level (low) ~5 watts.
- If the SWR on repeater channel 22 is higher than that on channel 1, your antenna is too long.
- If the SWR on channel 1 is higher than that on repeater channel 22, your antenna is too short.
If your antenna is too long:
- Go to the channels you plan on using the most such as Repeater Channel 18. Set your power level to the high. Using the SWR meter to test and record your results for reference.
- Starting at the base end, using a hex key loosen the set screw. Then using a cut-off tool (not pliers) remove between 1/4” – 1/8″ increments and then get new readings to determine your progress.
- If you see your SWR reading start to climb, Stop. Your antenna is now becoming too short and trimming any more will cause increased SWRs.
- You want to tune your antenna to about a 1.20 SWR if it has a trans-matching Base Coil (Antenna stinger that are longer than 6”) or a 1.50 SWR if it is a quarter wave base or dipole antenna (Antenna stinger that are ~6”)
If your antenna is too short:
- Starting at the base end, using a hex key loosen the set screw. Raise the antenna while it still is in the base and re-tighten the set screw. Using the SWR meter to test and record your results for reference.
- If your antenna has a mid section with set screws you can also try to lengthen the antenna the same way.
- If this method fails to achieve desirable SWR readings you will have to replace the antenna with a longer one.