Comparing W8JI dipole to OCF dipole

Here are some analysis results for two forms of an HF multi-band dipole antenna for 160 m and up. The 220 ft W8JI multi-band dipole is based on the design guidelines presented in this reference on page 7. The off center fed (OCF) dipole analyzed here is based on a commercial product with total length of 270 ft, with one leg of 180 ft and the other of 90 ft. Both antennas are modeled at a height of 60 ft above ground.

This first phase explores the radiation pattern gain in dB compared to an isotropic antenna (dBi). Below are tables that give three calculated gain figures at one selected frequency on 10 different bands. The gain figures are the maximum values that occur for specific azimuth and elevation angles. The maximum horizontal gain and vertical gain do not occur at the same azimuth and elevation angles. For these horizontal antennas the maximum horizontal gain equals or exceeds the vertical gain for all cases.

The screen shots below show the calculated 2D radiation patterns in the horizontal plane for the azimuth and elevation angles that yield maximum total gain. The color key at the bottom left of each plot identifies the graph of total gain, horizontal gain and vertical gain. Click on each image for a larger view. For all plots the antenna structure is oriented from left to right. The OCF dipole has its longer element to the left.

160m W8JI	160 m OCF
80 m W8JI	80 m OCF
40 m W8JI	40 m OCF
20 m W8JI	20 m OCF
10 m W8JI	10 m OCF

While there are some differences in the radiation patterns and gains of the two designs for some of the frequencies analyzed, it is not apparent that one design has great advantage over the other with respect to these characteristics alone.

The bandwidth of each antenna design on each amateur band can be evaluated, to compare the performance across each band. The OCF dipole is intended to achieve a relatively low SWR across a good portion of each band without the aid of an antenna tuner. The W8JI dipole requires a tuner since it is designed to provide an impedance on each band that is in a manageable range for the antenna tuner.

The resulting power efficiency of each approach can be evaluated with some additional effort. Certainly the impedance and SWR profile for each design will be different. Neither design is likely to provide stellar performance across all 10 amateur bands. Each design has design / construction choices that may help to satisfy the operator across frequency ranges of the most interest.

- WA5MLF