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:
All radio waves travel line-of-site, but…
- VHF frequencies and higher
- ground wave (LF, MF, HF). Excellent discussion of Ground Wave Propagation from Radio-Electronics.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
- 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)
- 11-year 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
“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 (twin-lead, 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 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)
- open-wire 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:
3-element, triband (20m, 15m, 10m) beam, horizontally polarized
11-element 2m beam, vertically polarized
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
- radio signals vary over many powers of 10
- input to receiver may be one ten-billionth of a watt:
- 10 billion = 10,000,000,000.0 = 1010 [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 = 103
- transmitter power is 10,000,000,000,000 times larger!
- easier to say: 1013 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 S-unit 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!
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
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
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 S-Meter:
Radiation patterns, feedlines
Feed lines and SWR
Antenna radiation patterns
The “simplest” antenna: the 1/2-wavelength dipole
- 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 1/4-wave vertical: a vertical dipole, with a “mirror!”
Directional antennas – beams: yagis, quads, dishes, etc.