General/Chapter 7 Study Guide

Chapter 7: Antennas

Section 7.1: Antenna Basics

Review of terminology:

• Elements - conducting portions of an antenna that radiate or receive signals
• Polarization - orientation of electric field radiated by antenna
• Feedpoint impedance - ratio of RF voltage to current at antenna feedpoint
• Resonance - when feed point impedance is purely resistive, with no reactance
• Radiation pattern - graph of signal strength in every direction
• Azimuthal pattern - shows signal strength in horizontal directions (birds eye view)
• Elevation pattern - shows signal strength in vertical directions (side view)
• Lobes - regions in radiation pattern where signal is being radiated
• Nulls - points at which radiation pattern is at a minimum
• Isotropic antenna - radiates equally in every possible direction 9horizontal or vertical)
• Omnidirectional antenna - radiates a signal of equal strength in every horizontal direction
• Directional antenna - radiates preferentially in one or more directions
• Gain - concentrating transmitted or received signals in a specific direction
• dB - decibels, units of gain
  • dBi = gain with respect to isotropic antenna
  • dBd = gain with respect to dipole antenna

Section 7.2: Dipoles, Ground Waves, Random Wires

Dipoles

• Dipole = 2 electrical polarities
• Dipoles have a figure-8 radiation pattern, signal is strongest broadside to antenna
• Actual ground installation changes radiation pattern
• Half wave dipole - 1/2 wavelength total, feedpoint in center, each half of the dipole is 1/4 wavelength
• Voltage: lowest at feedpoint, highest at ends
• Current: highest at feedpoint, lowest at ends
• Length of a dipole in feet: 492/f
• Feedpoint impedance of center-fed, free-radiating dipole should be 72 Ohms
• Becomes several THOUSAND Ohms when feedpoint is connected at the ends
• Physical thickness of wire can electrically lengthen wire, meaning you need a shorter antenna
• Height above ground affects resonant frequency
• Insulation, nearby conductors, and method of insulation all affect resonant frequency and length of dipole
• Start near free-space length, and use SWR/antenna tuner to trim dipole
• Exam asks to approximate resonant length for dipole. Use 492/f - pick the closest value to that

Ground plane vertical antena:

• Dipoleantenna, with missing half: ground plane acts as electric mirror
• Ground plane consists of sheet of metal or ground radial wires
• Often installed vertically, making them omnidirectional (good for VHF/UHF)
• Base impedance is 35 Ohms (half of dipole)
• Sloping ground radials down will increase impedance
• Downward angle of 30-45 degrees will match 50 Ohm impedance of coax cable
• Impedance increases as feed point moved further from the "center"
• Start with half of the free-space 1/2 wavelength length: length in feet is 246/f
• Example calculation: approximate length of 1/4 wave ground vertical antenna that is resonant at 28.5 MHz is:
  • Free space length = fsl = 246/f = 246/28.5 = 8.6 ft

Mobile HF antennas;

• Mobile HF antennas are ground-plane antennas
• Vertically oriented whip antennas - thin steel rod mounted atop vehicle surface
• Full-size 1/4 wavelength whip not feasible below 10 m
• Loading techniques used to physically shorten/electrically lengthen an antenna
• Loaded antennas have reasonable impedanc, but inefficient
• Screwdriver antenna: whip with adjustable loading coil at base

Random wires:

• Feed point impedance and radiation patterns are totally unpredictable
• May have lobes at multiple angles and multiple locations
• Connected to transmitter output or to antenna tuner without feedline
• Antenna, radio, station equipment are all part of the antenna system
• May result in significant RF currents/voltages on station equipment
• Can result in RF burns
• If impedance can be matched, can give excellent results

Effect of height:

• Feed point impedance and radiation pattern affected by antenna's physical height above ground
• Presence of electrical image created in conducting ground below antenna affects performance
• Image is electrically reversed
• As antenna and image get closer, they start to short each other out
• Close to ground level, feed point impedance is near zero
• Above half wavelength, impedance varies. Goes through maxima and minima
• Maxima at 1/4 wavelength, then 1 wavelength
• Ground also affects radiation patterns: real radiation patterns composed of energy received by antenna, and energy reflected from ground
• Direct and reflected signals take different times to travel, and may combine, cancel out, or anywhere in between
• This leads to new lobes/nulls pattern
• Depending on height, you can have most radiation going UP, or most radiation going out via side lobes

Polarization:

• Polarization can affect amount of signal lost due to ground resistance
• Radio waves reflected from ground: lower losses when polarization is parallel to ground
• Radiation pattern consists of reflected waves combined with direct waves not reflected
• Lower reflection loss means higher maximum signal strength
• HF DX done with vertical antennas - lower losses at low angles (but at higher angles, still more effective than horizontally polarized)
• Ground-mounted vertical: not polarized efficiently, but still generates stronger signals at lower angles of radiation from horizontally polarized antennas at lower heights
• DX or HF bands use verticals, because horizontally polarized antennas require unreasonable hieghts

Section 7.2 Summary

• A capacitance hat on a mobile antenna is used to electrically lengthen/physically shorten an antenna
• A corona ball on an HF mobile antenna reduces high voltage discharge from the tip of the antenna
• A disadvantage of a shortened mobile antenna is a smaller bandwidth
• A disadvantage of a directly-fed random-wire antenna is that you may experience RF burns if you touch it
• To adjust the feedpoint impedance of a quarter wave ground plane vertical antenna, slope the radials downward
• If a ground plane antenna's radials are changed from horizontal to downward, the feed point impedance increases (maximum angle makes them into a dipole, which has double the feed point impedance of a ground wave vertical antenna)
• For a dipole antenna in free space, the radiation pattern is a figure-eight at right angles to the antenna
• Antenna height affects horizontal (azimuthal) radiation pattern of a dipole by making antenna omnidirectional if low (less than 1/2 wavelength high)