E9D

Yagis, dishes and short antennas

Explain dish gain, how Yagi elements shape a beam, and why loading a short antenna lowers bandwidth and efficiency.

12 pool questions1 on the exam

Syllabus: Yagi antennas; parabolic reflectors; feed point impedance and loading of electrically short antennas; antenna Q; RF grounding

Double the frequency, and a dish gains 6 dB

A parabolic dish focuses signal in proportion to its size measured in wavelengths. Ideal gain = (π × diameter ÷ wavelength)². Doubling the frequency halves the wavelength, so the dish is twice as big in wavelengths: power gain ×4, which is +6 dB.

Gain = (π × diameter ÷ wavelength)². Bigger across, or shorter wavelength: narrower beam, more gain.

Frequency5 GHzDish diameter1 m

Remember×2 frequency (or ×2 diameter) = ×4 power = +6 dB.

Yagi parasitic elements work through their length

Only the driven element (about ½ wavelength) connects to the feed line. The other elements are parasitic: excited by its field and re-radiating. Making one longer or shorter than resonance controls the phase of its current, so the radiation adds in front and cancels behind. A two-element Yagi normally uses a reflector, which gives higher gain than a director.

Only the driven element connects to the feed line. Reflector and director are excited by its field and re-radiate with a phase set by their length.

RememberDriven ½ λ. Parasitic length = phase control. Two elements: reflector for gain.

Crossed Yagis fed 90° apart make circular polarization

Mount two Yagis on the same axis, perpendicular to each other, with driven elements at the same point on the boom. Feed them 90° out of phase: the horizontal and vertical fields then trade places and the combined field rotates.

Looking down the boom. Horizontal field + vertical field, 90° apart in time, trace a circle.

Phase between the two Yagis

RememberCrossed on one boom, 90° apart in phase: circular.

A coil resonates a short antenna but costs efficiency

An antenna shorter than resonance, or used below its resonant frequency, has capacitive reactance and a lower radiation resistance. A loading coil adds inductive reactance that cancels the capacitance. The coil's own resistance wastes power, so use a coil with a high reactance-to-resistance ratio (high Q) to maximize efficiency.

Illustrative model. Lower frequency or shorter whip: radiation resistance drops fast, and the coil's own resistance takes a bigger share.

Operating frequency (fraction of resonant)50%Coil quality (reactance ÷ resistance)150

RememberCoil cancels capacitive reactance. Shorter: Rrad falls. Coil X ÷ R high: efficiency up.

Put the loading where the current is: center coil, top hat

Radiation comes from current, which is highest at the base of a vertical and zero at the tip. A coil near the center is more efficient than a low coil. Top loading (a capacitance hat) keeps current high along the whole radiator and so improves radiation efficiency.

Shaded = current along the whip. Radiation comes from current, so more of it high up is better.

RememberCenter coil beats base coil. Top loading improves radiation efficiency.

Loading raises Q, which narrows the SWR bandwidth

Q says how sharp a resonance is. Higher Q means a narrower range of frequencies with a good match: SWR bandwidth decreases. A loading coil raises an antenna's Q, so a loaded short antenna has less SWR bandwidth.

Illustrative: a resonant antenna matched at its center frequency. Narrow dip = high Q = narrow SWR bandwidth.

Antenna Q20

RememberHigher Q = narrower SWR bandwidth. Loading coils decrease it.

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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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