80 m Dipole Bandwidth

Some recent conversation in our morning group touched upon the bandwidth of an 80 m dipole. We know that the commonly used wire diameters do not allow low-SWR coverage of the entire 500 kHz of bandwidth. Here are model results showing the bandwidth of a theoretical dipole that is fed with a 73-ohm source.

126 ft dipole; no feedline

 This first figure shows the SWR vs frequency for a 126 ft dipole at 50 ft above ground. The source (transmitter) is connected directly to the center of the dipole. Choosing the data points close to the SWR=2 line gives a bandwidth of about 230 kHz (marked in green). At SWR=3 the bandwidth is about 380 kHz (marked in orange).

The next figure is for the same dipole with a feedline having 73-ohm characteristic impedance and fed with a 73-ohm source.

125 ft dipole; 73 ft feedline

The bandwidth for this case at SWR=3 is about 360 kHz -- slightly less that the first case. Additional data points would be needed to more precisely locate the frequencies where SWR exactly equals 3.

An additional model case for feedline of 70.3 ft yielded very close to the same results as for 73 ft. There is little effect from changing the feedline length when operating with feedline characteristic impedance and source impedance close to the impedance of the dipole near resonance.

UPDATE:  I changed the feedline and source (transmitter) impedance to 50 ohms, leaving the other parameters unchanged. The model gave:

• SWR less than 3: bandwidth 280 kHz
• SWR less than 2: bandwidth 150 kHz
• minimum SWR = 1.37

For the cases of 300-ohm or 450-ohm feedline, the SWR is greater than 3 across the entire range of 3.5 to 4.0 MHz, requiring the use of an antenna tuner or other impedance matching methods to operate with a 50-ohm transmitter.


A dipole that is longer or shorter than 1/2 electrical wavelength at the operating frequency will have considerably different SWR than the above ideal cases. Likewise, using a feedline that is much different than the dipole's resonant impedance will affect the SWR significantly. Under these conditions a different definition of bandwidth is needed and relates more to the impedance range that an antenna tuner can accommodate, since the antenna will present a much higher SWR on its own.

WA5MLF

Static Crashes on 3740

Here is an annotated screen shot of the waterfall display showing the static crashes as pale or brighter horizontal strips. The time scale is about 47 seconds from top to bottom.