This is the class outline for week 4 of the SCARS 2018 Technician / General Class.
Week 4 Outline
Technician Chapter 4, Propagation, Antennas and Feedlines
General Chapter 7, Antennas
General Chapter 8, Propagation
Instructor: Gary Skaggs WB5ULK
Week 4 Question Pool
Today’s class will concentrate on the above listed topics, and the question pools that you can download here:
Propagation
Good PDF Article on RF Propagation
All radio waves travel lineofsite, but…
 lineofsight
 light
 VHF frequencies and higher
 ground wave (LF, MF, HF). Excellent discussion of Ground Wave Propagation from RadioElectronics.com
 VHF and UHF refracted, diffracted by objects in path (“knife edge”)
 pass through objects if wavelength shorter than opening(s)
grounded jail cell: passes 2m signal, blocks HF  multipath: cancellation of signal by reflections out of phase
“picket fencing,” mobile flutter  meteor scatter, EME bounce, tropospheric ducting
Ionosphere
 solar radiation, esp. UV, knocks electrons from oxygen, nitrogen atoms
 resulting ions have a charge and collectively act as a conductor
 conductors reflect (refract) RF
 different ionospheric layers present at different times (solar effect)
 11year sunspot cycle peaks enhance ionization
 MUF: maximum usable frequency; higher pass through
 LUF: lowest usable frequendy; lower are absorbed
 Between LUF and MUF, waves refracted (reflected) back to Earth
 Earth, conductor, also reflects RF
Antenna Fundamentals
“No piece of equipment has as great an effect on the performance of a radio station…”
1/4 wave whip antenna – half a dipole
beam (directional) antenna
Feed lines, impedance
 feed line (twinlead, ladder, coax)
 feed point: where feedline meets antenna
 antenna is resonant when capacitve reactance cancels inductive reactance, leaving only resistance
 Like ohm’s law (R = E/I), impedance = (rf) V/(rf) I
 amateur radios want to “see” 50 ohm impedance
 feedline and antenna must also be 50 ohms, unless transmatch (antenna tuner) or balun used
 some common feedline types:
Antenna terminology
 antenna comprised of an array of one or more elements, which may be driven (connected to feedline), or parasitic
 impedance depends on shape, (effective) length of antenna
 also depends on height of antenna above ground
 never ideal! Formulae for antenna length, configuration, height, are a starting point! Must trim and make other adjustments…
 intrinsic antenna impedance – tune antenna (easier to cut wire than add it back on!)
 antenna height, obstructions, etc. – tuning may help, or reconfigure
 wrong feedline impedance – balun
 bad feedline (e.g., water in coax)
 corroded connectors, inappropriate connectors – coax seal

Sources, “fixes” for high SWR:
 SWR is related to ratio of reflected to forward power
 because all feed lines have loss (resistance), energy bouncing back and forth is wasted as heat
 Amateur transmitters expect 50 ohms impedance
 If feedline or antenna have different impedance: mismatch
 perfect match: no reflected power, SWR = 1.0:1
 roughly 1.5:1 is not bad
SWR – Standing Wave Ratio
When EM radiation encounters an impedance change, some is reflected.
For example, glass has a difference impedance for light than air. When you look through a window
some light is reflected back at you. When there is an impedance change getting RF from transmitter
to antenna, some energy is reflected, some goes forward. coaxial cable (coax)
 openwire feel line
 balanced, unbalanced, balun
 characterization impedance
 amateur coax: 50 ohms
 TV cable: 75 ohms
 TV twin lead: 300 ohmsat 2:1 and above, transmitter may be damaged; most automatically cut back on output powertransmatch, match box, antenna tunerpolarization: RF waves may have horizontal, vertical, or circular polarization
Part of W5JA antenna farm:
3element, triband (20m, 15m, 10m) beam, horizontally polarized
11element 2m beam, vertically polarized
rotor
Antenna gain
isotropic – fictional source, akin to naked light bulb; radiates equally in all directions
omnidirectional (omni) – radiates equally in horizontal directions
has gain over isotropic, by directing RF toward horizon
directional antenna: beam
dBi, dBd: strength relative to isotropic (dBi) or dipole (dBd), in decibels
Decibels
 radio signals vary over many powers of 10
 input to receiver may be one tenbillionth of a watt:
 10 billion = 10,000,000,000.0 = 10^{10} [1.0 => move decimal 10 places right]
 one 10 billionth = 1.0/10,000,000,000 = 0.0000000001 = 10^{10} [move decimal point 10 places left]
 the LOG of a power of 10 is how far you have to move decimal from 1.0 (negative moving to left)
 out of transmitter, power may be kilowatts: 1 killowatt = 1000 watts = 10^{3}
 transmitter power is 10,000,000,000,000 times larger!
 easier to say: 10^{13} larger than received signal!
 TEN decibels is one power of 10 larger (in power)
 so, transmitted signal is 130 decibels (dB) stronger that received signal (+130 dB)
 or, received signal is 130 decibels (dB) weaker than received signal (130 dB)
 3 dB is about double the power; 3 dB is about half the power
 6 dB is 3 dB larger than 3 dB (duh!), so it is four times the power
 An Sunit is 6 dB. Has to do with the RST system, which we’ll discuss later…
Repeating, an increase of 10 dB means 10 times as much power. An increase of 20 dB (think of it as “10 dB more than 10 dB”) is 100 (10×10) times more power. A change of 30 dB means 1/1000th the power.
These are the ONLY questions dealing with decibels, so memorize them if you have to!
T5B09 (B)
What is the approximate amount of change, measured in decibels (dB), of a power increase from 5 watts to 10 watts?
A. 2 dB
B. 3 dB
C. 5 dB
D. 10 dB
T5B10 (C)
What is the approximate amount of change, measured in decibels (dB), of a power decrease from 12 watts to 3 watts?
A. 1 dB
B. 3 dB
C. 6 dB
D. 9 dB
T5B11 (A)
What is the approximate amount of change, measured in decibels (dB), of a power increase from 20 watts to 200 watts?
A. 10 dB
B. 12 dB
C. 18 dB
D. 28 dB

Sound level comparisons:

Typical radio SMeter:
Radiation patterns, feedlines
Feed lines and SWR
Antenna radiation patterns
The “simplest” antenna: the 1/2wavelength dipole


Example problem:
 Design a dipole to operate on 28,200 kHz
 – what formula do we need?
 – what do we plug into the formula?
 – calculate the length, in ??
The 1/4wave vertical: a vertical dipole, with a “mirror!”

Directional antennas – beams: yagis, quads, dishes, etc.
