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)
• Radial wires of ground-mounted vertical should be placed on the Earth or buried a few inches below the ground
• As a 1/2 wavelength dipole antenna is lowered to less than 1/4 wavelength above ground, the feed-point impedance steadily decreases
• As a 1/2 wavelength dipole has its feed point moved from the center to the end, the feed-point impedance steadily increases (to several thousand Ohms)
• Horizontally polarized HF antenna has lower ground-reflection losses than a vertically polarized HF antena (advantage of horizontal polarization)
• The paproximate length for a 1/2 wavelength dipole antenna cut for 14.250 MHz is given by 492/f = 492/14.250 = 34.5 ft or approximately 32 feet
• The approximate length for a 1/2 wavelength dipole antenna cut for 3.550 MHz is 492/f = 492/3.550 = 131 feet
• The approximate length for a 1/4 wavelength vertical antenna cut for 28.5 MHz is 246/f = 246/28.5 = 8.6 feet or 8 feet
• Antenna gains in dBi compare to antenna gains in dBd as follows:
  • dBi gain is 2.15 dB higher than dBd gain
  • dBi = gain relative to isotropic antenna
  • dBd = gain relative to dipole antenna

Section 7.3: Yagi Antennas

• Yagi antennas are cheap, effective
• Directional antennas have frontal lobes and side lobes
• Pointing antenna in direction shown by azimuthal map enables you to beam signal directly to other station
• HF signals can skew signal path by up to 15 degrees
• Minimize noise/interference by listening for best direction and using S-meter
• On crowded bands like 20 m, having directional antenna can reduce unwanted noise
• Dipole, ground plane, and rnaomd wire antenas use a single radiating element
• Yagis use more than one radiating elemetn - array antenna
• Main lobe/major lobe - direction of maximum field strength
• Two types of arrays: driven, and parasitic
  • Driven array: all elements are connected to transmitter, and all are driven elements
  • parasitic array: one or more elements are not connected to feed line, but influence radiation pattern of antenna
• Array pattern of radiation depends on constructive and destructive interference
• If 2 interfering waves are in phase, they reinforce each other
• If 2 interfering waves are out of phase, they cancel each other out
• For pair of dipoles, radiated fields add/subtract to create the radiated field/lobes/nulls
• In a parasitic array, antenna elements close together enough that energy from the driven element induces current in parasitic element
• Parasitic element will re-radiate power as if it were fed too

Yagi structure and function:

• Yagi is parasitic array with 1 driven element and 1 or more parasitic element
• Parasitic elements are arranged to create main lobe
• directors - elements placed in direction of maximum gain
• reflectors - elements placed in direction of minimum gain
• front-to-back ratio - ratio of signal strength at peak of major lobe to signal strenght in opposite direction
• Yagi driven element should be resonant dipole at approximately 1/2 wavelength long
• Reflector shoudl be 5% longer than driven element, placed 0.15 - 0.2 wavelengths behind driven element
• Signal from DE causes current to flow into RE
• Signal in RE is 180 degrees out of phase, so cancels signal in direction of reflected element
• Added length of RE is due to fact that there is incomplete cancellation
• Larger element creates additional phase shift due to inductive impedance
• Director element placed in front of DE increases forward gain
• Shorter element results in capacitive reactance, which subtracts some phase shift

Performance

• 2-Element Yagi (neglecting height above ground):
  • Compared to isotropic antenna, gain of 7 dBi
• Compared to dipole, gain of 5 dBd
  • Front to back ratio - 10-15 dB
• Three element Yagi:
  • 9.7 dBi
  • front to back ratio of 30-35 dB
• Additional reflectors make little difference, usually just 1
• Additional reflectors increases antenna gain

Yagi design tradeoffs

• Many things to optimize for: maximum gain, front to back ratio, SWR variation across bands, etc.
• Variables for Yagi design:
  • Length and diameter of each element
  • Placement of elements along boom
• Affect of elements:
  • More directors increase gain
  • Longer boom, for a given number of directors, increase gain, up to a certain maximum (then decreases again)
  • Large diameter elements reduce SWR variation with frequency
  • Placement of elements of tuning of elements affects gain and feed point impedance and SWR

Impedance matching:

• Yagis with desirable radiation patterns have impedances that don't match 50 Ohm coax
• Typical impedance is 20-25 Ohms, creating SR of 2:1
• To match impedances, can use gamma match
• Gamma match - short section of parallel conductor transmission line, uses driven element as one conductor
• Adjustable capacitor used to gamma match SWR to 1:1
• Gamma match advantage: driven element need not be insulated from boom
• Other techniques: beta match (hairpin), omega match, impedance transformers, transmission line stubs
• VHF/UHF Yagis, can make element diameters larger

Section 7.3 Summary

• An azimuthal projection map shows true bearings and distances from a particular location
• An HF antenna that minimizes interference is a directional antenna
• To increase the bandwidth of a Yagi, use larger diameter elements

* The approximate length of a Yagi driven element is 1/2 wavelength

• For a three-element, single-band Yagi, the director is the shortest element
• For a three-element, single-band Yagi, the reflector is the longest element
• On a Yagi, increasing boom length and adding directors increases gain
• On a Yagi, front-to-back ratio means the power radiated in maximum gain direction to power radiated in opposite direction
• ON a directive antenna, the main lobe is the direction of maximum radiated field strenght from antenna
• Gain of 2 three-element Yagis, horizontally polarized, spaced vertically 1/2 wwavelength apart, ocmpared with gain of 1 three-element Yagi, will be 3 dB higher
• A Yagi design variable to optimize forward gain, front to back ratio, or SWR bandwidth includes:
  • Physical boom length
  • Number of elements on boom
  • Spacing of each element along boom
• The purpose of a Yagi gamma match is to match the LOW feed point impedance to 50Ohms
• Using a gamma match for impedance matching of Yagi to 50 Ohm feed line has advantage of not requiring elements to be insulated from boom

Section 7.4: Loop Antennas

Loop antennas enclose an area 1 wavelength or more in circumference

• Loops can be any shape, but can't be too narrow
  • Quad lop: 4 sides, each 1/4 wavelength long
  • Delta loop or triangular loop: 3 sides, each 1/3 wavelength long
• Direction of maximum signal is broadside to the loop
  • Horizontally oriented loop good for vertical waves, local/regional contacts
  • Vertically oriented loop good for singnals pointing to horizon: DX, etc.
• One wavelength loop acts like 2 dipoles end-to-end
• Feed point is point of maximum current, other point of max current is 1/2 wavelength from feedpoint
• Shorter/longer loops (non-integer multiples of wavelength) have higher feed point impedance
• Can use loops in arrays: Yagi concept, but with quad loops for elements, is quad antenna
• Driven element of quad antenna is 1 wavelength is circumference, so 1/4 wavelength on each side
• Quad/delta loop has reflectors 5% longer in circumference, directors that are 5% shorter in circumference
• Two-element quad/delta has same gain as three-element Yagi
• Yagi has better front-to-back ratio than quad/delta loop
• Quads have more restrictions/complexities on constructions
• Polarization of a horizontally oriented loop is always horizontal, regardless of feed point
• Polarization of vertical loop depends on feed point location
• Feed point at midpoint of bottom or top leads to horizontal polarization
• Feed point at vertical side results in vertical polarization
• Rotation/orientation (square or diamond) doesn't matter

Section 7.4: Summary

• The loops of a two-element quad antenna must be configured so the reflector element is 5% longer than the beam element, to use it as beam antenna
• Each side of a driven element of a quad antenna is 1/4 wavelength
• Forward gain of a 2-element quad antenna is about the same as a 3-element Yagi
• Each side of a reflector element of a quad antenna is more than 1/4 wavelength
• Gain of a two-elemetn delta loop is about the same as gain of two-element quad loop
• Each leg of a symmetrical delta loop antenna is approx. 1/3 wavelength