Multi-band Dipole SWR on 75 m

As part of my continuing effort to get acceptable performance of the W8JI-style multi-band dipole (described in this article), I'm working with the original suggested 110 ft dipole length to cover 80 - 10 m. My current feed line has:

• 84 ft of 450-ohm ladder line
• a 1:1 choke balun
• 12 ft of 50-ohm coax cable to the tuner (I may shorten this)

When operating on 3.74 MHz, I find that the antenna tuner is not able to do better than 2.6:1. The goal of the antenna is to present an impedance to the tuner on most frequencies that is manageable.

Focusing on 3.5 to 4.0 MHz, I collected SWR data with an antenna analyzer. I also used my transceiver at low power to evaluate what the antenna tuner could do. The plot below shows the analyzer SWR data plotted along with the tuner's best effort (at a few frequencies) at matching to 50 ohms for the transceiver. (Click on the image for a full-size view.)


Although the design guidelines are intended to avoid resonance within the lowest band (80 m in this case), the degree of mismatch within most of the operating range is not acceptable. I plan to lengthen the dipole elements by several feet to see if better results can be obtained below 4 MHz without sacrificing reasonable operation on higher bands.

- John