In this section we look at how the SWR of a λ/2 dipole antenna varies as the length of the elements are adjusted.
· Keep the transmitter power low (5 Watts or less). Damage can occur to transmitters running high power into mis-matched antennas.
· Do not adjust the lengths of the dipole elements whilst transmitting.
· (There are internationally recognised safety limits regarding how close it is safe to be to a transmitting antenna. These are available on the National Radiological Protection Board’s website).
· Connect the λ/2 dipole to the SWR meter with feeder.
· Connect the transmitter to the SWR meter with feeder.
· Set the transmitter to low power.
· Transmit either pressing the Morse key or by using FM.
· Measure the SWR.
· Switch off the transmitter and measure the length of the λ/2 dipole.
· Where the SWR is higher than 2:1, when you make adjustments these can be about 5% of the length at a time.
· Once the SWR is below 2:1, then you will need to make smaller adjustments each time you adjust.
· Remember to switch off the transmitter before making adjustments.
· Increase the length of the Dipole by about 5%. Make sure you make the same increase on both halves of the λ/2 dipole.
· Transmit either pressing the Morse key or by using FM.
· Measure the SWR.
· Switch off the transmitter and measure the length of the λ/2 dipole.
What You Last Did |
What Was the Result |
Next Action |
Increased Length |
Increased SWR |
|
Increased Length |
Reduced SWR |
|
Reduced Length |
Increased SWR |
|
Reduced Length |
Reduced SWR |
· Once you get the SWR below 2:1, then make smaller adjustments.
· Don’t worry if you can’t get the SWR below 1.2:1.
· Once the SWR is below about 1.2:1 then STOP.
· Switch off the transmitter.
· Decrease the length of the Dipole by about 5%. Make sure you make the same decrease on both halves of the λ/2 dipole by the same amount.
· Transmit either pressing the Morse key or by using FM.
· Measure the SWR.
· Switch off the transmitter and measure the length of the λ/2 dipole.