E9D: Directional Antennas

Directional antennas — Yagis, parabolic dishes, and loaded verticals — require an understanding of how element length, spacing, and loading affect gain, impedance, and bandwidth.

This lesson covers Yagi element design and parasitic element function, parabolic reflector gain scaling with frequency, loading coils for electrically short antennas (position, Q, and effect on bandwidth), top loading, circular polarization from crossed Yagis, and how antenna Q relates to SWR bandwidth.

Parabolic Reflector Antennas

A parabolic reflector antenna focuses energy in a narrow beam by reflecting all incident energy from its focal point toward a parallel output wavefront. The gain of an ideal parabolic antenna scales with the square of frequency. When the operating frequency is doubled, gain increases by 6 dB (a factor of 4 in power ratio). This frequency-dependence is why large parabolic dishes used at microwave frequencies can achieve very high gain values.

Circular Polarization from Crossed Yagis

Two linearly polarized Yagi antennas can produce circular polarization when arranged so that their elements are perpendicular to each other on the same boom axis, with their driven elements at the same point on the boom and fed 90 degrees out of phase. The two orthogonal fields of equal amplitude, 90° apart in phase, combine to produce a rotating polarization vector — circular polarization. This is useful for satellite communications where the polarization of the received signal rotates continuously.

Yagi Element Design

Key facts about Yagi antennas:

• Driven element length: Approximately 1/2 wavelength.
• Parasitic elements: Making parasitic elements longer or shorter than resonance controls the phase shift of the currents induced in them. A longer element becomes a reflector (re-radiates toward the driven element); a shorter element becomes a director (re-radiates away from the driven element).
• Two-element Yagi reflector vs. director: A two-element Yagi with a reflector produces higher gain than one using a director at normal element spacings. This is why most two-element Yagis use a reflector.

Loading Coils for Short Antennas

An electrically short antenna has a capacitive reactance at its feed point — its electrical length is less than resonant, so it looks like a series capacitance. A loading coil is added to resonate the antenna by cancelling this capacitive reactance with equal inductive reactance.

• Most efficient position: Near the center of the vertical radiator. A center-loaded coil interacts with more of the antenna current distribution than a base-loaded coil, resulting in better radiation efficiency.
• High reactance-to-resistance ratio: Loading coils should have a high Q (high ratio of reactance to resistance) to maximize efficiency. A low-Q coil dissipates more power in resistance losses rather than radiating it.
• Effect on SWR bandwidth: Adding loading coils decreases the SWR bandwidth. The loaded antenna has a higher Q, which means it is more frequency-selective — it responds over a narrower range of frequencies.

Top Loading

Top loading adds capacitance at the top of an electrically short vertical antenna — typically by extending radial wires or using a capacitance hat. The advantage of top loading is improved radiation efficiency. By adding capacitance at the current-null point (the top), top loading modifies the current distribution to make it more uniform along the radiator, increasing the effective radiation resistance and improving efficiency compared to other loading methods.

Antenna Q and SWR Bandwidth

Antenna Q (quality factor) is analogous to circuit Q — it describes how sharply resonant the antenna is. As antenna Q increases, the SWR bandwidth decreases. A high-Q antenna stays near resonance over a very narrow frequency range; moving away from the resonant frequency causes SWR to rise quickly. Short, loaded antennas have high Q and narrow bandwidth; larger antennas near their natural resonant frequency have lower Q and broader bandwidth.

Radiation Resistance Below Resonance

For a base-fed whip antenna operated below its resonant frequency, the radiation resistance decreases. As the antenna becomes shorter relative to a wavelength, the current distribution changes and the radiation resistance falls rapidly — making the antenna increasingly difficult to feed efficiently and requiring more careful impedance matching.

E9D Practice Questions