G9D

Specialized HF and VHF antennas

Choose antennas for NVIS, multiband, receive-only and loop uses, and explain traps, stacking, log-periodics and end-fed feeds.

12 pool questions1 on the exam

Syllabus: Specialized antenna types and applications

NVIS wants a low horizontal dipole

NVIS (near vertical incidence skywave) sends signal almost straight up so it drops back close by, for short-skip links such as 40 m in daytime. A horizontal dipole 1/10 to 1/4 λ above ground radiates strongly overhead. At ½ λ high it has a null straight up, and a vertical radiates little overhead.

NVIS wants energy sent nearly straight up. A low horizontal dipole does that; at ½ λ it fires almost nothing overhead.

Dipole height0.2 λ

Straight-up signal95% of best

RememberLow horizontal dipole, 1/10 to 1/4 λ up, for NVIS.

End-fed half wave: very high impedance. One support: inverted V

Feed a half-wave wire at its end and you are at the voltage maximum: the feed point impedance is very high. Slide the feed point all the way out to see it.

Impedance = voltage ÷ current. Current is greatest at the center, voltage at the ends, so the impedance climbs toward the ends.

Feed point, center to endend

Feed point impedancevery high

A dipole hung from one central support, ends sloping down, is an inverted V.

Same half-wave dipole, one tall support: the ends droop, so it is an inverted V.

RememberEnd-fed half wave: very high Z. One center support: inverted V.

Traps and loading coils make one antenna fit many bands

Antenna traps are parallel LC circuits that enable multiband operation: they block the higher band's signal from the outer wire, and act as loading coils on the lower band.

A trap is a parallel coil and capacitor tuned to the higher band. It isolates the outer wire there, so one antenna resonates on two bands.

A screwdriver mobile antenna has a motor that changes the base loading inductance to retune.

The motor changes how much of the coil is in the circuit: the loading inductance changes, and so does where the antenna resonates.

Coil in use55%

Base loading inductancemedium

The catch: multiband antennas have poor harmonic rejection.

RememberTraps: multiband. Screwdriver: motor-tuned base coil. Cost: harmonics.

Stacking narrows the beam up and down; log-periodics cover a wide band

Stacking horizontally polarized Yagis narrows the main lobe in elevation, not in azimuth.

Stacking squeezes the main lobe in elevation (side view). From above, the beam is no narrower. Two antennas at ½ λ give about 3 dB more.

A log-periodic has elements whose length and spacing vary logarithmically along the boom. Its advantage is wide bandwidth, not extra gain per element.

Every element is the same ratio longer than its neighbor, and so is every gap. The working part slides along the boom with frequency.

Signal frequencymiddle

RememberStack: narrower in elevation. Log-periodic: wide bandwidth.

Loops have nulls broadside; Beverages are for receiving

An electrically small loop (under 1/10 λ around) has nulls broadside to the loop and its strongest signal in the plane of the loop. A halo is a loop lying flat: it is omnidirectional in its own plane.

Small loop: signal arrives best in the plane of the loop. The nulls are broadside to it.

A Beverage is a long low wire for directional receiving on MF and low HF.

Long, low, terminated wire: a quiet directional receiving antenna for MF and the low HF bands.

RememberSmall loop: null broadside. Halo: omni in its plane. Beverage: directional receiving.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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