The Vertical I-Pole Antenna

Vertical Dipoles are very effective on the lower frequencies (160 through 40 meters), but mechanically somewhat unwieldy due to their extreme height. Various techniques are used to reduce the height (such as loading coils), but many of these techniques reduce efficiency and bandwidth, or are mechanically complex. The Vertical I-Pole provides a solution to the height problem without significant loss of bandwidth or efficiency and retains mechanical simplicity. This result is realized by using capacity hats on both ends. Using single wire hats maintains the mechanical simplicity required for supporting between trees for straightforward field deployment without significant impact on omnidirectionality. A pair of tall trees and a Pneumatic Antenna Launcher can be used to set up a very effective Vertical I-Pole rapidly in the field. For 20 meters and up a smaller version can be constructed from Buddipole parts, see the VerticalBuddIPoleAntenna page.



For resonant operation the width plus height should be approximately a half wave. A modelling program can be used to fine tune the dimensions in advance if resonance is desired. Resonance is not required if fed with balanced line and a tuner the antenna can be tuned on frequencies above and slightly below the halfwave with good efficiency. The lower arm should be high enough to minimize coupling to earth and avoid proximity to personnel. Ten feet is the recommended minimum lower element height. The following examples are all possible with tree supports ten feet or so above the apex.

Example Dimensions (approximate)

• Band Ht Wid (ft)
• 40m 27 27 (modelled, apex at 40-50+ ft)
• 80m 53 53 (modelled, apex at 65-75+ ft)
• 160m 71 142 (modelled, apex at 80-90+ ft)

Off Center Feed
Feeding the vertical section in the middle is inconvenient. Feeding lower down results in a better mechanical system. The impedance rises slowly with these large hats so it is still below 50 ohms. If coax is used to feed it must have a very effective balun to avoid upsetting the low angle vertical radiation pattern with a deep null straight up.

Mechanical Issues

• 12 or 14 gauge hard-drawn copper or copper-clad steel are recommended for strength
• Allow sag in the antenna to reduce stress
• Keep feedline and vertical section lightweight to reduce top wire loading (uninsulated wire is lighter)
• Avoid coax feed as it is heavy and lossy in multiband operation (and balun weight is at a bad spot)

• Sketch of Vertical I-Pole Antenna

References

• Pneumatic Antenna Launchers∞
• W4RNL Antenna Analysis Papers∞
• EZNec
• 4nec2

AlanB, WB6ZQZ

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CategoryHamRadio