Antennas and feed lines: separate size, pattern, and match

These questions mix three different objects: the antenna's electrical size, its radiation pattern, and the line that carries power to it. Decide which object is under test before reaching for a formula or an antenna name.

Size, current, and the power path

A resonant wire length is set by wavelength, so it becomes shorter as frequency rises. For a practical half-wave dipole in feet, divide 468 by the frequency in MHz; a quarter-wave monopole uses half that numerator. Keep the units attached until the last step, and check whether the question asks for the whole dipole or only a quarter-wave element. On a resonant half-wave wire, current is greatest near the center and approaches zero at the ends, while voltage does the opposite. Use impedance as voltage divided by current to classify any requested feed point.

Characteristic impedance belongs to the line itself. For parallel conductors it follows their spacing, radius, and dielectric, not the line's length or operating frequency. Reflection begins where the antenna feed-point impedance differs from that characteristic impedance. For a purely resistive load, SWR is the larger impedance divided by the smaller, always written as a ratio to one. A matching network at the transmitter can make the transmitter see its preferred load, but it does not remove the mismatch or standing wave already present farther down the line.

Real cable turns some RF into heat. Coaxial attenuation rises with frequency as conductor and dielectric losses grow. A reflection makes part of the energy traverse a lossy line again, so mismatch adds loss even though SWR itself is not a source of dissipation. That same attenuation weakens the reflected wave before it reaches an input-end meter, which can make a poor antenna look deceptively better on a long or lossy line.

Patterns are geometry in space, not voltage or current plots along the metal. A directive antenna's main lobe contains its strongest field direction; front-to-back ratio compares that lobe with the direction opposite it. In an array, stacking narrows the pattern in the plane of the stacking axis. If identical antennas are correctly spaced and fed in phase, add their effective apertures, form the power ratio against one antenna, and convert that ratio with 10 log₁₀(ratio).

Where the answers are lost

• Letting line length or frequency set characteristic impedance. Those distractors ignore the cross-sectional geometry that defines the line; frequency changes attenuation instead.
• Blaming reflected power on operating at resonance. Reflection instead begins when the antenna feed-point impedance differs from the line impedance; resonance alone neither causes nor rules out that mismatch.
• Believing a tuner repairs the antenna end. A transmitter-side network changes the load seen by the radio; distractors that lower the line's existing SWR move the effect to the wrong side of the network.
• Expecting line loss to make the input SWR reading larger or more accurate, or to have no effect. Those distractor directions ignore that attenuation weakens the reflected wave before it returns to the meter.
• Mixing pattern terms with conditions on an element. Current maxima, voltage maxima, forward gain, and the field direction of a main lobe are different quantities; front-to-back ratio compares opposite spatial directions, not element counts or reference antennas.

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