Radio Waves, Propagation and Antennas T9

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Radio Waves, Propagation and Antennas (T9)

• Doing things right.

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HF (3 MHz - 30 MHz) Propagation

• It is the unpredictable nature of HF propagation
  that makes the HF bands so much fun.
• Long distance communication (DX) is accomplished
  by the reflection of radio waves by the
  ionosphere, the upper layers of the atmosphere
  ionized by ultraviolet radiation from the sun.
• This is why the General Class License is so
  cool!!!

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Ionospheric Protection
The ionosphere and magnetosphere protect us from
harmful radiation from the sun.
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How the Ionosphere is Formed
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The Ionosphere
F2 Layer (Reflecting) F1 Layer (Reflecting) E
Layer (Reflecting) D Layer (Absorbing)
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Layers of the Ionosphere

• D Layer, Absorbing, Disappears at night
• E Layer, Reflecting, Disappears at night
• F1 and F2 Layers, Reflecting, combine into a
  single F layer at night.
• The reflective layers are responsible for sky
  wave propagation.

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Sporadic-E

• There are two natural phenomena that can
  propagate VHF signals over long distances.
• Sporadic E occurs when the E-layer reflects VHF
  signals!

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Tropospheric Ducting
Under certain conditions, VHF signals are
caught in a duct of moist warm air, giving
propagation over hundreds of miles.
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Line of Sight Propagation
Worldwide communications by line of sight is not
possible due to the curvature of the Earth
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Sky wave Propagation (Skip)
Over the horizon communication is possible by
sky- wave propagation, bouncing signals off the
ionosphere.
Occurs mostly at HF frequencies (less than 30
MHz).These are available to General Class Hams!
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Sunspots

• Sunspots peak during 11-year cycles.
• The higher the sunspot count, the more the
  atmosphere is ionized.
• Thus, higher sunspot counts support a higher
  Maximum Usable Frequency (MUF).

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VHF/UHF Propagation

• Generally line of sight
• Can be blocked by and/or reflected off mountains
  and large buildings even the Moon!
• Temperature inversions in the troposphere can
  cause ducting, and a path will open briefly for
  500 - 600 miles.
• VHF/UHF will penetrate the Ionosphere, making
  these frequencies ideal for satellite, and
  Earth-Moon-Earth (EME) operations.

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VHF/UHF Propagation
VHF/UHF signals travel only in straight lines. We
call this line of sight propagation
Direct communications are not possible because of
the mountain
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How far can I talk with these radios?

• There is never an exact answer for any radio.
  Many factors come into play!
• Geography
• Frequency
• Buildings
• Distance

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VHF UHF

• FM signals are subject to fading.
• Stronger signals trump weaker ones.
• Line-of-Sight Propagation
• Radio Horizon
• Signals are subject to multi-path distortion
• When signals hit objects, they can be refracted.
  Move the radio a bit and things might get better.
• Mobile stations are subject to picket fencing.
• Rapid fluttering sound during a transmission.

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T9B01 Why are VHF/UHF signals not normally heard
over long distances? A. They are too weak to go
very far B. FCC regulations prohibit them from
going more than 50 miles C. VHF and UHF signals
are usually not reflected by the ionosphere D.
They collide with trees and shrubbery and fade
out
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T9B03 What is the most likely cause of sudden
bursts of tones or fragments of different
conversations that interfere with VHF or UHF
signals? A. The batteries in your transceiver
are failing B. Strong signals are overloading
the receiver and causing undesired signals to be
heard C. The receiver is picking up low orbit
satellites D. A nearby broadcast station is
having transmitter problems
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T9B05 What should you do if a station reports
that your signals were strong just a moment ago,
but now they are weak or distorted? A. Change
the batteries in your radio to a different
type B. Speak more slowly so he can understand
your better C. Ask the other operator to adjust
his squelch control D. Try moving a few feet,
random reflections may be causing multi-path
distortion.
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T9B07 What is a good thing to remember when using
your hand-held VHF or UHF radio to reach a
distant repeater? A. Speak as loudly as possible
to help your signal go farther B. Keep your
transmissions short to conserve battery power C.
Keep the antenna as close to vertical as you can
D. Turn off the CTCSS tone
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Antennas

• The long and short of those magical wires

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All Forms and Shapes
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Rubber Duck
A rubber duck is the antenna you find on most
HTs. They are not the ideal antenna, but they
do serve a purpose when you need to access a
repeater.
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Antennas and Feed lines

• Antenna systems are resonant - that is, they
  respond best to a certain frequency.
• For best operation, the transmitter, feed line
  and antenna must all be tuned to resonance - or
  something that looks like resonance.
• The transmitting antenna induces a radio wave
  into the air. The radio wave travels to the
  receiving antenna, and induces a current in that
  antenna.

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The 1/2 Wave Dipole
Length of resonant dipole in feet
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T9A12 What is the approximate length, in inches,
of a 6-meter 1/2 wavelength wire dipole
antenna? A. 6 inches B. 50 inches C. 112
inches D. 236 inches
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The 1/2 Wave Dipole
Radiation pattern for a dipole antenna looking
down from above the antenna
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The 1/4 Wave Vertical
Length of vertical in feet
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The 1/4 Wave Vertical
A side view of the radiation pattern of a 1/4
wave vertical. From above the pattern is round
like a doughnut. A perfect ground would be a car
roof.
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T9A11 What is the approximate length, in inches,
of a quarter-wavelength vertical antenna for 146
MHz? A. 112 inches B. 50 inches C. 19
inches D. 12 inches
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T9A05 How does the physical size of half-wave
dipole antenna change with operating
frequency? A. It becomes longer as the frequency
increases B. It must be made larger because it
has to handle more power C. It becomes shorter as
the frequency increases D. It becomes shorter as
the frequency decreases
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The Yagi (This one is for HF)
The Favorite This antenna has 4 monoband Yagis
on the same boom. 2 elements on 40 and 20, 3
elements on 15 and 4 on 10.
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The Yagi

• The yagi is the hams favorite directional
  antenna.
• They usually consist of one driven element, and
  several parasitic (un-driven) elements.
• Reflector (longer than driver)
• Driven Element (1/2 wave dipole)
• One or more Directors (shorter than the driven
  element)

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The Yagi
Driver
The director acts like a lens
The reflector acts like a mirror
Boom
Gain
Director
Reflector
Feedline
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The Yagi
The yagi antenna focuses RF energy in one
direction, giving the appearance of getting free
power. This free power is called Antenna Gain.
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The Yagi
A 3 element HF Yagi
A VHF Yagi
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T9A08 What type of antennas are the quad, Yagi,
and dish? A. Antennas invented after 1985 B.
Loop antennas C. Directional or beam antennas D.
Antennas that are not permitted for amateur radio
stations
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Dummy Load
Use a dummy load to tune your transmitter. This
is good practice as it reduces QRM and gives the
transmitter a perfect 50 ohm load. Note Dummy
loads get hot.
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Feed lines

• Feed line connects your radio to the antenna.
• Feed lines are either balanced (neither side
  grounded) like ladder-line or unbalanced (one
  side grounded) like coaxial cable.
• Either type can be used in your station.
• Coax is more popular and easier to work with.

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Coax vs. Ladder Line

• Can be buried or run near metal objects.
• Less RFI since outer shield is usually grounded.
• Weatherproof
• Easy to handle and connect
• Unbalanced

• Very low signal loss
• Can tolerate high SWR
• Can tolerate high current
• Balanced

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Coax
All coaxial cable will feature a center
conductor surrounded by a dielectric
insulator and one or more layers of shielding and
an insulating cover.
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The Balun
The balun converts from BALanced antennas
to UNbalanced feed lines. Many antenna systems
work better with a balun between the feed point
and the coax, e.g. A Dipole!
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T9C12 Why is coaxial cable used more often than
any other feed line for amateur radio antenna
systems? A. It is easy to use and requires few
special installation considerations B. It has
less loss than any other type of feedline C. It
can handle more power than any other type of
feedline D. It is less expensive than any other
types of line
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T9C07 What is the most common reason for failure
of coaxial cables? A. Moisture contamination B.
Gamma rays C. End of service life D. Overloading
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SWR

• SWR (standing wave ratio) is a mathematical
  expression of the power going to an antenna and
  the power being reflected back.
• The idea is to get as close to 11 as possible.
• Most ham equipment has a 50-ohm impedance.
• So, we match the impedance of the equipment to
  the impedance of the coax/antenna for low SWR.
• The best way to get a good SWR is to cut the
  antenna to resonance.

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SWR Meter
Measures transmitter output power and reflected
power from the antenna system.
SWR 11 Good (Balanced) 41 Bad (Unbalanced)
SWR is a ratio a measure of how well the load
is matched to a transmitter.
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T9C03 What might be indicated by erratic changes
in SWR readings? A. The transmitter is being
modulated B. A loose connection in your antenna
or feedline C. The transmitter is being over
modulated D. Interference from other stations is
distorting your signal
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Questions?