Two verticals, three patterns
Feed two quarter-wave verticals a half wavelength apart and the phasing between them decides the pattern. Three cases, all asked:
| Spacing | Phasing | Pattern |
|---|---|---|
| 1/2 wavelength | in phase | figure-eight broadside to the array axis |
| 1/2 wavelength | 180 degrees out of phase | figure-eight along the array axis |
| 1/4 wavelength | 90 degrees out of phase | cardioid |
The reasoning, so you can reconstruct rather than memorise:
In phase, half-wave spacing. Broadside, the path lengths are equal, so the signals add. End-on, one is half a wavelength further, so they cancel. Maximum broadside.
180 degrees out of phase. Now broadside cancels (equal paths, opposite phase) and end-on adds (half-wave path difference plus half-wave phase difference = in phase). Maximum along the axis.
Quarter-wave spacing, 90 degrees out of phase. In one direction the path difference and the phase difference cancel exactly, giving full addition; in the opposite direction they add to a half wavelength, giving cancellation. The result is unidirectional — a cardioid, heart-shaped, with a deep null off the back.
The cardioid is the one worth building: two cheap verticals and a phasing line give a directional low-band antenna with no tower.
Making a wire antenna unidirectional
What is the effect of adding a terminating resistor to a rhombic or long-wire antenna?
It changes the radiation pattern from bidirectional to unidirectional.
Without a termination, energy reaching the far end reflects and travels back, producing a bidirectional pattern. A resistor absorbs it, so only the forward wave remains. You lose the power dissipated in the resistor and gain a front-to-back ratio — which is exactly the Beverage receiving antenna’s design from the General track.
What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased?
Additional lobes form with major lobes increasingly aligned with the axis of the antenna
Longer wire, more lobes, and the main ones swing round toward the wire’s direction. A long wire is not a broadside radiator.
The named wire antennas
| Antenna | What it is |
|---|---|
| Folded dipole | a half-wave dipole with an additional parallel wire connecting its ends |
| Two-wire folded dipole feed point | approximately 300 ohms |
| Zepp | an end-fed half-wavelength dipole |
| Extended double Zepp | a centre-fed 1.25-wavelength dipole |
| G5RV | a wire antenna centre-fed through a specific length of open-wire or ladder line |
| Off-centre-fed dipole (OCFD) | fed away from centre to create a similar feed point impedance on multiple bands |
The folded dipole’s 300 ohms is worth understanding rather than memorising: the second wire halves the current at the feed point for the same power, and impedance is voltage over current, so it rises by a factor of four from about 75 ohms. That is also why a 4:1 balun feeds one from 75-ohm coax.
The OCFD trick generalises the same idea. A dipole’s impedance varies along its length — General’s “steadily increases toward the ends” — and it varies differently on each harmonic. Choose a feed point where several bands happen to present similar impedances, and one antenna feeds them all through one matching arrangement.
Height, and the takeoff angle
This is the material that decides whether an HF antenna works for DX.
How does the radiation pattern of a horizontally polarized antenna vary with increasing height above ground?
The takeoff angle of the lowest elevation lobe decreases.
Higher antenna, lower takeoff angle, better DX. It is the single most reliable improvement available on HF, and it is why “get it higher” is always good advice.
How is the far-field elevation pattern of a vertically polarized antenna affected by being mounted over seawater versus soil?
Radiation at low angles increases.
Seawater is a far better conductor than soil, so the ground reflection is stronger and the low-angle reinforcement larger. This is why coastal and shipboard verticals outperform inland ones, and it is the same soil-conductivity effect as the efficiency question in the parameters lesson.
How does the radiation pattern of a horizontally-polarized antenna mounted above a long slope compare with the same antenna mounted above flat ground?
The main lobe takeoff angle decreases in the downhill direction
A slope tilts the effective ground plane, which tilts the reflection, which lowers the takeoff angle downhill. A hillside is a free antenna improvement in one direction — and a penalty in the other.
Check yourself
- Two quarter-wave verticals, quarter-wave spacing, fed 90 degrees out of phase. Pattern?
- Your folded dipole reads about 300 ohms. What balun ratio feeds it from 75-ohm coax?
- You have a choice of two sites for a 20 metre dipole: 10 metres up on flat ground, or 10 metres up on a slope facing your target direction. Which?
Answers
- Cardioid — unidirectional, with a deep null off the back.
- 4:1 — 300 / 75. (A 4:1 balun is a 2:1 turns ratio, from the General transformer lesson.)
- The slope, facing the target. Sloping ground lowers the takeoff angle in the downhill direction, which is what DX wants.