Dipoles and Verticals

Cutting to length

\qquad L_{\text{feet}} = \frac{234}{f_{\text{MHz}}} \quad\text{(quarter-wave monopole)}$$ The monopole figure is half the dipole figure, because a monopole is one half of a dipole with the ground plane supplying the other half. **Three worked answers the pool asks:** | Band | Frequency | Antenna | Length | |---|---|---|---| | 20 m | 14.250 MHz | half-wave dipole | 468 / 14.25 = **33 feet** | | 80 m | 3.550 MHz | half-wave dipole | 468 / 3.55 = **132 feet** | | 10 m | 28.5 MHz | quarter-wave monopole | 234 / 28.5 = **8 feet** | Why 468 and not 492 — the true free-space half wavelength in feet? Because a real wire has an **end effect**: capacitance at the ends makes it behave electrically longer than it is, so the physical cut comes out about 5% short. Height, wire diameter, and nearby objects shift it further, which is why the number is a starting point. **Cut long and trim.** ## Patterns > **What is the radiation pattern of a dipole antenna in free space in a plane > containing the conductor?** > **It is a figure-eight at right angles to the antenna** Maximum broadside, nulls off the ends. Which way you hang the wire decides who hears you: an east-west dipole favours north and south. > **Which of the following best describes the radiation pattern of a quarter-wave > ground-plane vertical antenna?** > **Omnidirectional in azimuth.** Equal in all compass directions. That is a vertical's advantage and its limitation — no favoured direction to exploit, and no null to reject interference with. And the height effect, which surprises people: > **How does antenna height affect the azimuthal radiation pattern of a horizontal > dipole HF antenna at elevation angles higher than about 45 degrees?** > **If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost > omnidirectional** A dipole close to ground loses its figure-eight. The ground reflection dominates, the pattern fills in, and the radiation goes mostly upward. So a low dipole is both omnidirectional *and* a poor DX antenna — the two facts have the same cause, and the NVIS lesson in the propagation module is the case where you want exactly that. ## Feed point impedance Two questions that are mirror images and both worth understanding rather than memorising. > **How does the feed point impedance of a horizontal 1/2 wave dipole antenna change > as the antenna height is reduced to 1/10 wavelength above ground?** > **It steadily decreases** > **How does the feed point impedance of a 1/2 wave dipole change as the feed point is > moved from the center toward the ends?** > **It steadily increases** The second follows from the current distribution. A half-wave dipole has maximum current at the centre and maximum voltage at the ends. Impedance is voltage over current, so it is lowest at the centre — around 50 to 70 ohms — and rises toward the ends, reaching thousands of ohms. That is why a dipole is fed at the centre and why an end-fed wire needs a very different matching arrangement. ## Verticals and radials A vertical needs something to work against, and getting that wrong is the most common vertical-antenna disappointment. > **Where should the radial wires of a ground-mounted vertical antenna system be > placed?** > **On the surface or buried a few inches below the ground.** Not elevated, for a ground-mounted vertical — on or just under the surface, and as many as you can manage. Radial *quantity* matters more than radial length: sixteen short radials outperform four long ones. > **Which of the following is a common way to adjust the feed point impedance of an > elevated quarter-wave ground-plane vertical antenna to be approximately 50 ohms?** > **Slope the radials downward** A quarter-wave ground plane with horizontal radials feeds at roughly 30 ohms. Sloping the radials down raises it toward 50, matching coax directly. This is why commercial ground planes have drooping radials — it is impedance matching, not mechanical convenience. ## Horizontal versus vertical > **Which of the following is an advantage of using a horizontally polarized as > compared to a vertically polarized HF antenna?** > **Lower ground losses** A vertical drives current into the ground system, and soil is a mediocre conductor; those losses are unavoidable without an extensive radial field. A horizontal antenna at reasonable height couples to ground far less. The trade-off: a vertical is omnidirectional, physically compact, and radiates at low angles even at modest height, which suits a small lot. ## The random wire warning > **What is a characteristic of a random-wire HF antenna connected directly to > the transmitter?** > **Station equipment may carry significant RF current.** An end-fed wire with no counterpoise uses your station — chassis, power leads, coax shields, mains earth — as the missing half of the antenna. The consequences are real: RF in the audio, interference to everything nearby, RF burns from touching a microphone, and unpredictable tuning. The fix is a proper counterpoise or an antenna designed to be end-fed with its own matching transformer. Not more tuner. ## Check yourself 1. What length dipole for 7.150 MHz? 2. Your dipole is 20 feet up on 40 metres. Which way does it favour? 3. Your ground-mounted vertical works poorly. Radials elevated or on the ground? <details> <summary>Answers</summary> 1. 468 / 7.15 = **65.5 feet**, then trim. 2. **Nowhere in particular.** Twenty feet is well under a half wavelength on 40 metres (66 feet), so the azimuthal pattern is almost omnidirectional — and most of the radiation goes up. 3. **On the surface or a few inches below**, and as many as you can manage. Count matters more than length. </details>

Cutting to length

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