Multi-band Dipole Impedance and SWR

Related to my earlier posting, here are some plots of impedance magnitude and SWR for some variations of the W8JI-style multi-band dipole with balanced line feeder. All cases use a height above ground of 60 ft.

The first two plots below show SWR and impedance magnitude vs frequency for:

• 220 ft dipole with 220 ft 450-ohm feed
• 220 ft dipole with 186 ft 450-ohm feed

The color legend at the top right identifies the two curves. Click on the image for a larger view. The SWR data are computed using a 450-ohm source, matching the balanced transmission line impedance. An antenna tuner is used by the operator to match the transmitter's 50-ohm output. The antenna analysis software can be set to use a 50-ohm source (or any impedance for SWR calculation) or can be told to calculate the required (L & C) matching components at each frequency, if desired.

SWR - 220 ft Dipole at 450 ohms

Impedance - 220 ft Dipole

The next two plots are for:

• 202 ft dipole with 60 ft 300-ohm feed
• 220 ft dipole with 60 ft 300-ohm feed

In these cases the balanced transmission line has 300 ohm characteristic impedance. SWR data are calculated for a 300-ohm source.

SWR - 202 & 220 ft Dipoles

Impedance - 202 & 220 ft Dipoles

Both plots show the peaks and valleys at higher frequencies for the shorter antenna.

As always, these models cannot predict the exact conditions that will be seen at a typical ham residential installation of HF antennas, due to nearby buildings, trees, fences, etc. not included in the model. A commercial antenna installation with lots of clear area around the antenna (e.g. AM broadcast stations) will have much closer agreement between model and real world. The NEC software has been used extensively for many decades in designing commercial and military antennas, and is applicable to many ham radio antenna designs.

The simple models are capable of indicating the general behavior of a given antenna's shape, dimensions, height, feeder length and impedance. The models also predict the effects of changing a given antenna's parameters. The "cut and try" method is always important to achieving the optimal performance at a given site.

- John