An efficient 5 band random wire antenna (40-15m)

Tackling the challenge of a multi-band random wire antenna for 40m through 15m, I pondered which length of radiating element would find that sweet spot where efficiency meets directivity across 40/30/20/17 and 15m bands.

First, I considered the fundamental resonant lengths for each of the bands:

• 40m: ~66 feet
• 30m: ~47 feet
• 20m: ~33 feet
• 17m: ~27 feet
• 15m: ~22 feet

Comparison of radiator lengths for multi-band end-fed antenna

Knowing that a non-resonant antenna can still perform well, I started calculating harmonics and looking for common multiples. A length of 68 feet emerged as a promising candidate - long enough to be efficient on 40m while providing harmonic resonance on the 20m and 15m bands..

This looked like it should provide:

• A good match on 40m with a low takeoff angle
• Decent performance on 30m with acceptable SWR
• Strong lobes on 20m, nearly full-wave resonance
• Workable patterns on 17m and 15m, though with some compromise

While not fully resonant on all bands, the 68-foot length works well with a 9:1 unun, offering a reasonable SWR on all bands and a good match for an antenna tuner, allowing for efficient operation across the desired bands. This flexibility is particularly useful as I hope to use this single antenna for multiple bands without frequent adjustments.

Next, I considered antenna orientation and decided on a northwest to southeast layout to maximise my desired coverage. This orientation should then give me:

• Good low-angle radiation on 40m for DX work
• A nice figure-of-eight on 20m covering both Europe and the Pacific
• Acceptable compromise patterns on 30m, 17m, and 15m

Further research suggested that a slight inverted-V configuration would provide the added benefit of bringing the feedpoint impedance closer to the unun's sweet spot on several bands meaning that the predicted efficiency ranged from excellent on 40m and 20m to acceptable on 15m, with 30m and 17m falling somewhere in between.

I'm confident that my chosen length of 68 feet will allow me to get most out of this simple yet effective antenna design and achieve my personal objectives. Of course, the real test will come with on-air performance so now it's time to start unspooling wire and see if the ionosphere agrees with me. Wish me luck!