Thursday, January 5, 2012
SWR – Antenna Adjustment
This procedure is for adjusting the SWR of an antenna using a cross-needle SWR meter.
Connect the antenna to one end of a coaxial line. (It is best if it is the same line you intend to use during normal operations.) Connect the other end of the coax to the connector on the SWR meter marked Antenna. Now connect a short jumper (keep this coax jumper as short as practical) to the connector marked Transmitter on the SWR meter. Connect the other end of the jumper to the transmitter output connector. Set the transmitter to FM or CW. Do not use the SSB mode.
Reduce the power out on the transmitter as low as it will go. Now tune the transmitter to the low end of the band for which you are tuning the antenna. Transmit and record the reading on the meter. Next tune the transmitter to the middle of the band. Transmit and record the reading. Now tune the transmitter to the high end of the band. Transmit and record the reading.
If the reading gets progressively higher from the first reading to the last, the antenna is too long and will need to be shortened. If the reading gets progressively lower from the first reading to the last, the antenna is too short and will need to be lengthened. If the second reading is lower than the first and last, the antenna is tuned in the band.
If the SWR is still high where you intend to use the antenna the most, you may adjust the antenna to lower the SWR there, but it will change elsewhere in the band.
If the SWR remains unacceptably high (greater than 2:1) at the lowest point, it means the antenna is resonant in the band of interest but the impedance of the antenna is not near 50 Ohms. All the shortening or lengthening will not bring the SWR down any lower. Other steps will need to be taken to adjust the antenna’s impedance.
Impedance adjustment will be discussed in future posts.