SWR Bandwidth of antennas

 SWR bandwidth (High Frequency antennas) 

In general most radios work perfectly well if the SWR they see is 2:1 or less. In addition to satisfying the transmitter the losses in coax are not going to be excessive with a 2:1 or better SWR.

Most of the time I will be talking about dipole antennas to keep it simple. Other antennas are very much the same but there are exceptions. 

The 2:1 SWR bandwidth is simply how many kilohertz are between the low 2:1 SWR point on an antenna to the next 2:1 point.

Starting with my 80 meter dipole, when set for CW it has a SWR of 2:1 below the bottom of the band at 3,435 kHz. The SWR then goes down to a minimum at 3,535 and back up to 2:1 at 3,635.  The bandwidth between 3,435 and 3,635 is 200 kHz. So my dipole has a 2:1 SWR bandwidth of about 200 kHz.  

When tunes for the phone band it has a 2:1 at about 3,608, minimum SWR at 3,708 and again a 2:1 SWR at about 3,825. This bandwidth being just over 200 kHz using these numbers. Remember that our instrumentation is not perfect and there might well be an error these numbers. However the SWR bandwidth will change with frequency. It gets larger as we go up in frequency. On my 20 meter dipole the 2:1 points are 13.5 to 15 MHz with minimum at 14.200

That is a measured bandwidth of 1.5 MHz. On 10 meters measured the 2:1 points are 27.630 and 29.5 MHz. With the minimum SWR at 28.5. This is a bandwidth 1.87 MHz.

These numbers were read at my transmitter. One thing to always remember when taking readings at the input or transmitter end of your coax is the SWR is going to read lower than it really is because of losses in the coax feedline. These losses will always be higher at the higher frequencies. With good coax the loss should be small and probably can be neglected up to 10 MHz or so. Losses on 10 meters may need do be considered. 

The actual 2:1 bandwidth on my antennas is a bit less on the higher bands due to coax loss but this gives you a good idea of what to expect from good antennas and reasonable lengths of good coax.  That is 200 kHz bandwidth on 80 meters, maybe 500 kHz on 40 meters ( dipole will cover the complete band), 1.5 MHz on 20 meters and 1.8 MHz on 10 meters. 

On 10 meters EZNEC modeling program gives  1.55 MHz as the 2:1 bandwidth without loss,  whereas I measured 1.87 MHz on mine with probably 1.5 dB of loss. With 1.5 dB of loss a SWR reading at the transmitter of 2:1 means that true SWR is more like 2.5 at the antenna. However, the transmitter is only concerned with what it see connected to it and on 10 meters any dipole cut within 1 MHz of your operating frequency should be well under 2:1 at the antenna and less at the transmitter. 

Always check the SWR on your antenna at some frequency where it should have a high SWR. If it does not have a high SWR when it should, then you probably have excessive loss in your coax.