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T9A

ANTENNAS AND FEED LINES

Antennas: vertical and horizontal polarization, concept of antenna gain, definition and types of beam antennas, antenna loading, common portable and mobile antennas, relationships between resonant length and frequency, dipole pattern

T9A011 of 11

What is a beam antenna?

Why A beam antenna is a directional antenna: it uses multiple elements (driven element plus reflectors and directors, as in a Yagi) to concentrate radiated power in one favored direction instead of spreading it evenly all around. That concentration is what produces gain, since the power that no longer goes off the back and sides shows up as extra signal off the front. Receiving works the same way, so a beam also reduces noise and interference coming from other directions.

Watch out The omnidirectional choice is the opposite of a beam by definition, and the name has nothing to do with a person or with aluminum structural beams; the word refers to the beam of radiation, like a flashlight beam.

Think flashlight: a beam antenna points the energy one way, trading coverage behind you for gain ahead.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A022 of 11

Which of the following describes a type of antenna loading?

Why Antenna loading means adding reactance, usually a coil, to make a physically short antenna behave electrically like a longer, resonant one. A quarter-wave vertical on 40 meters would be about 33 feet tall, so a mobile whip a few feet long is made resonant by inserting an inductor (a loading coil) at the base, middle or top. The coil cancels the capacitive reactance of the too-short element so the feedpoint looks resistive at the operating frequency.

Watch out A resistor would not tune anything out; it would just waste transmitter power as heat, and a base spring or beefed-up elements are mechanical fixes, not electrical loading.

Loading = coil, not mechanical. Short whip plus inductor equals electrically full size.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A033 of 11

How is the polarization of an antenna described?

Why An antenna radiates an electromagnetic wave with an electric field and a magnetic field at right angles to each other, and by convention polarization is named for the direction of the electric field. For a simple wire or whip antenna the electric field lines up with the conductor, so a vertical whip produces vertical polarization and a horizontal dipole produces horizontal polarization. Matching polarization between stations matters because a cross-polarized pair can lose 20 dB or more of signal.

Watch out The magnetic field is a real part of the wave, but it sits perpendicular to the electric field, so naming it would reverse every polarization label; the element's physical orientation is only a handy clue, not the definition.

Polarization = E for Electric field. Vertical whip, vertical E, vertical polarization.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A044 of 11

What is a disadvantage of a handheld radio transceiver's short flexible antenna compared to a full-sized quarter-wave antenna?

Why A "rubber duck" is a physically shortened antenna, usually a helically wound coil, that is electrically loaded to look resonant to the radio. Shortening an antenna lowers its radiation resistance and raises losses, so much of the transmitter power is dissipated as heat in the loading coil and matching rather than radiated. A full-size quarter-wave whip radiates a much larger fraction of the power fed to it, so it both transmits and receives better.

Watch out Polarization is set by how the antenna is oriented, not by its length, and a vertical rubber duck is still vertically polarized. Desensitization comes from strong nearby transmitters overloading the receiver front end, and a less efficient antenna would if anything pick up less of that.

Short antenna = short on efficiency. Shrinking the antenna shrinks the radiated power, not the polarization.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A055 of 11

Which of the following increases the resonant frequency of a dipole antenna?

Why A dipole's resonant length and frequency are inversely related: the half-wave length in feet is roughly 468 divided by the frequency in MHz. Cut the wire shorter and the same half wavelength now fits a shorter wave, meaning a higher frequency. So trimming a dipole is how you move its resonance up in frequency.

Watch out Lengthening it, adding series loading coils, or adding capacitive hats at the ends all make the antenna electrically longer, which lowers the resonant frequency. Loading is the standard trick for making a physically short antenna work on a lower band.

Length up, frequency down. 468/f(MHz) = dipole feet. Short wire, high note, like a short guitar string.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A066 of 11

Which of the following types of antennas offers the greatest gain?

Why A Yagi is a beam antenna: a driven element plus parasitic reflector and director elements that concentrate radiated energy into one direction. Focusing power into a narrower beam is exactly what antenna gain means, and a modest Yagi easily delivers 6 to 10 dBi or more, far beyond any omnidirectional whip. The other antennas listed radiate roughly equally in all horizontal directions, so they have little or no directional gain.

Watch out The isotropic radiator is the tempting one, but it is a theoretical point source that radiates equally in every direction with 0 dBi gain by definition; it is the reference all other gains are measured against, not a high gain antenna. A 5/8 wave vertical and a J pole give only a small amount of gain by squashing the pattern toward the horizon.

Gain means squeezing the pattern. Isotropic = 0 dBi reference; only the multi-element beam (Yagi) really focuses.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A077 of 11

What is a potential drawback of using a handheld VHF transceiver inside a vehicle that lacks an externally mounted antenna?

Why A vehicle body is largely metal, so it acts like a shield (a partial Faraday cage) around a handheld radio and its rubber duck antenna. RF has to leak out through the windows and gaps, so both transmitted and received signal strength drop noticeably compared with an antenna mounted outside on the roof or trunk. That is why mobile operators use an external mag-mount or permanently mounted antenna and a short adapter cable to the HT.

Watch out The SWR-based choices are red herrings: the metal body may detune the antenna slightly, but the real, dominant problem is attenuation of the signal by the surrounding metal, not a bandwidth or reflected-power effect.

A car is a metal box. Metal box equals shield. Get the antenna outside the box.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A088 of 11

Why is a 19-inch-long vertical antenna often used on 2 meters?

Why A quarter-wave vertical is the standard mobile/handheld whip because it works against a ground plane (the vehicle roof or radio chassis) to act like half of a dipole. The quarter-wave length in feet is roughly 234 divided by the frequency in MHz, so at about 146 MHz that is 234/146 = 1.6 feet, or roughly 19 inches. At resonance the antenna presents a near-resistive, low feedpoint impedance that matches 50-ohm coax well.

Watch out A resonant half-wave on 2 meters would be about twice as long, roughly 38 inches; a 19-inch whip is half of that. And a simple quarter-wave whip has essentially no gain over a dipole.

Quarter-wave feet = 234/f(MHz). 234/146 = 1.6 ft = 19 inches. Half-wave would be 38.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A099 of 11

What is an advantage of a 5/8-wavelength whip antenna for VHF or UHF mobile service compared to a 1/4-wave antenna?

Why A 5/8-wavelength vertical is longer than a quarter-wave, so its current distribution squeezes the radiation pattern down toward the horizon instead of spreading it upward. That concentration of energy at low elevation angles shows up as gain, typically a few dB over a quarter-wave whip, which is exactly what you want for mobile work over the horizon. The tradeoff is that a 5/8-wave whip is not naturally resonant, so it needs a loading coil at the base to match 50 ohms.

Watch out The claim about a higher radiation angle is the opposite of the truth: the whole point of the 5/8-wave length is a lower takeoff angle. Its feedpoint impedance is not inherently lower or better matched either, which is why the matching coil is required.

Longer whip, flatter pattern: 5/8 wave pushes RF at the horizon, which means gain.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A1010 of 11

In which direction does a half-wave dipole antenna radiate the strongest signal?

Why A half-wave dipole has a doughnut-shaped pattern: current flows along the wire, and radiation is strongest at right angles to that wire, which is called broadside. Plotted in the plane of the antenna this looks like a figure-8, with two lobes perpendicular to the wire and deep nulls off each tip. That is why you aim a dipole by turning it sideways to the station you want to work.

Watch out Radiating off the ends is exactly backwards, since those are the null directions; an equal pattern in all directions describes an isotropic radiator, which is only a theoretical reference, and the feed line direction has nothing to do with the pattern.

Dipole = figure-8: strong off the sides, deaf off the ends.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

T9A1111 of 11

What is antenna gain?

Why Antenna gain is not power creation, it is power redistribution. An antenna focuses the same transmitter output into a narrower pattern, so the signal in the favored direction is stronger than it would be from a reference antenna such as an isotropic radiator (dBi) or a half-wave dipole (dBd). Gain is always quoted relative to that reference and in a specified direction, and whatever is gained in one direction is lost in others.

Watch out The idea that gain adds power to the transmitter is the classic trap: a passive antenna has no amplifier, it only concentrates the power it is already fed. Impedance has nothing to do with gain, it is a matching issue.

Gain is focus, not free watts. A flashlight reflector, not a bigger battery.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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