SWR Varies with Length of Dipole Elements (8f2)

In this section we look at how the SWR of a λ/2 dipole antenna varies as the length of the elements are adjusted.

Safety First

• 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).

Initial Set-Up

• 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.

Initial Test

• 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.

Making adjustments

• 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

• 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.

• Analyse the Results.

Analyse Results

What You Last Did	What Was the Result	Next Action
Increased Length	Increased SWR	Reduce Length
Increased Length	Reduced SWR	Increase Length
Reduced Length	Increased SWR	Increase Length
Reduced Length	Reduced SWR	Reduce Length

• 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.

Reduce the Length

• 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.

• Analyse the Results.