technician license course chapter 4 lesson plan module 9 – antenna fundamentals, feed lines &...

Technician License CourseChapter 4

Lesson Plan Module 9 – Antenna Fundamentals, Feed Lines & SWR

The Antenna System

• Antenna: Facilitates the sending of your signal to some distant station.– Back to the falling magnet

• Feed line: Connects your station to the antenna.

• Test and matching equipment: Allows you to monitor antenna performance.

The Antenna (Some Vocabulary)• Element: The conducting part or parts of an

antenna designed to radiate or receive radio waves.

• Driven element: The element supplied directly with power from the transmitter

• Parasitic element: A non-powered element that can add directivity to the antenna.

• Feed point: Where the transmitted energy enters the antenna.

The Antenna (Some Vocabulary)

• Polarization: The direction of the electric field relative to the surface of the earth.

• Same as the physical direction• Vertical – Perpendicular to the earth

• Horizontal – Parallel to the earth

• Circular - Right hand or left hand

Vertically Polarized Electromagnetic (Radio) Wave

Signal Polarization

• Determined by the transmitting antenna polarization.

• For line-of-sight communications, differences in polarization can result in significantly weaker signals. – Vertical / Horizontal = up to 30 dB loss.

• Signals that refract in the ionosphere will have their polarization randomized. (Sky-wave or Skip)

The Antenna (Some Vocabulary)

• Omni-directional – radiates in all directions.

• Directional beam – focuses radiation in specific directions.

• Gain – apparent increase in power in a particular direction because energy is focused in that direction.– Expressed in decibels (dB)

Decibels (dB)

Power Gain or Loss

• Twice ( X 2)

• X 4

• X 10

• X 100

• X1000

• Half (½)

• ¼• 1/10

•

Decibels

• 3dB

• 6dB

• 10dB

• 20dB

• 30db

• -3dB

• -6dB

• -10dB

Antenna Radiation Patterns

• Radiation patterns are a way of visualizing antenna performance.

• The further the line is away from the center of the graph, the stronger the signal at that point.

What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not

using the same polarization? (T3A04)

• A. The modulation sidebands might become inverted

• B. Signals could be significantly weaker• C. Signals have an echo effect on voices• D. Nothing significant will happen

What type of wave carries radio signals between transmitting and receiving stations?

(T3A07)

• A. Electromagnetic

• B. Electrostatic

• C. Surface acoustic

• D. Magnetostrictive

Which of the following is a common effect of “skip” reflections between the Earth and the ionosphere?

(T3A09) • A. The sidebands become reversed at each

reflection• B. The polarization of the original signal is

randomized• C. The apparent frequency of the received

signal is shifted by a random amount• D. Signals at frequencies above 30 MHz

become stronger with each reflection

What are the two components of a radio wave? (T3B03)

• A. AC and DC

• B. Voltage and current

• C. Electric and magnetic fields

• D. Ionizing and non-ionizing radiation

What is the approximate amount of change, measured in decibels (dB), of a power

increase from 5 watts to 10 watts? (T5B09)

• 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 increase from 20 watts to 200 watts?

(T5B11)• A. 10 dB

• B. 12 dB

• C. 18 dB

• D. 28 dB

What is a usual name for electromagnetic waves that travel through space? (T5C07)

• A. Gravity waves

• B. Sound waves

• C. Radio waves

• D. Pressure waves

Which of the following is true regarding vertical antennas? (T9A02)

• A. The magnetic field is perpendicular to the Earth

• B. The electric field is perpendicular to the Earth

• C. The phase is inverted

• D. The phase is reversed

What is meant by the gain of an antenna? (T9A11)

• A. The additional power that is added to the transmitter power

• B. The additional power that is lost in the antenna when transmitting on a higher frequency

• C. The increase in signal strength in a specified direction when compared to a reference antenna

• D. The increase in impedance on receive or transmit compared to a reference antenna

Antenna versus Feed Line

• For efficient transfer of energy from the transmitter to the feed line and from the feed line to the antenna, the various impedances need to match.

• When there is mismatch of impedances, things may still work, but not as effectively as they could.

Feed Line types

• The purpose of the feed line is to get RF energy from your station to the antenna.

• Basic feed line types.– Coaxial cable (coax).– Open-wire or ladder line.

• Each has a characteristic impedance, each has its unique application.

Feed Lines

• Twin-lead lines have low loss, but require impedance matching circuits, and react with surrounding conductors.

• Co-axial cables are easier to work with. The center insulation must be protected from moisture. 50 Ohm impedance is common.

• Hard line co-ax has a semi-rigid or rigid shield, and has lower losses at VHF and UHF frequencies.

Coax

• Most common feed line.

• Easy to use.• Matches impedance of

modern radio equipment (50 ohms).

• Some loss of signal depending on coax quality (cost).

Open-Wire/Ladder Line

• Not common today except in special applications.

• Difficult to use.

• Need an antenna tuner to make impedance match – but this allows a lot of flexibility.

• Theoretically has very low loss.

Antenna Impedance

• Antennas have a characteristic impedance.• Expressed in ohms – common value 50 ohms.• Depends on:

– Antenna design

– Height above the ground

– Distance from surrounding obstacles

– Frequency of operation

– A million other factors

Impedance – AC Resistance

• A quick review of a previous concept: impedance.– Antennas include characteristics of capacitors,

inductors and resistors

• The combined response of these component parts to alternating currents (radio waves) is called Impedance.

Standing Wave Ratio (SWR)• If the antenna and feed line impedances are

not perfectly matched, some RF energy is not radiated into space and is returned (reflected) back to the source.– Something has to happen to this reflected

energy – generally converted into heat or unwanted radio energy (bad).

– Can produce high voltages or currents in the transmission line.

Nothing is Perfect

• Although the goal is to get 100% of your radio energy radiated into space, that is virtually impossible.

• What is an acceptable level of loss (reflected power or SWR?)– 1:1 is perfect. ( 1 to 1)– 2:1 should be the max you should accept (as a general

rule).• Modern radios will start lowering transmitter output power

automatically when SWR is above 2:1.– 3:1 is when you need to do something to reduce SWR.

Test and Matching Equipment

• Proper impedance matching is important enough to deserve some simple test equipment as you develop your station repertoire.

• Basic test equipment: SWR meter.

• Matching equipment: Antenna tuner.

SWR Meter

• The SWR meter is inserted in the feed line and indicates the mismatch that exists at that point.

• You make adjustments to the antenna to minimize the reflected energy (minimum SWR).

Antenna Tuner• One way to make antenna matching

adjustments is to use an antenna tuner.

• Antenna tuners are impedance transformers (they actually do not tune the antenna).– When used appropriately they are effective.– When used inappropriately all they do is make

a bad antenna look good to the transmitter…the antenna is still bad.

How to use an Antenna Tuner

• Monitor the SWR meter.

• Make adjustments on the tuner until the minimum SWR is achieved.– The impedance of the

antenna is transformed to more closely match the impedance of the transmitter.

Which of the following is a common use of coaxial cable? (T6D11)

• A. Carry dc power from a vehicle battery to a mobile radio

• B. Carry RF signals between a radio and antenna

• C. Secure masts, tubing, and other cylindrical objects on towers

• D. Connect data signals from a TNC to a computer

What, in general terms, is standing wave ratio (SWR)? (T7C03)

• A. A measure of how well a load is matched to a transmission line

• B. The ratio of high to low impedance in a feedline

• C. The transmitter efficiency ratio• D. An indication of the quality of your station’s

ground connection

What reading on an SWR meter indicates a perfect impedance match between the

antenna and the feedline? (T7C04)

• A. 2 to 1

• B. 1 to 3

• C. 1 to 1

• D. 10 to 1

What is the approximate SWR value above which the protection circuits in most solid-state

transmitters begin to reduce transmitter power? (T7C05)

• A. 2 to 1• B. 1 to 2• C. 6 to 1• D 10 to 1

What does an SWR reading of 4:1 mean? (T7C06)

• A. An antenna loss of 4 dB

• B. A good impedance match

• C. An antenna gain of 4

• D. An impedance mismatch

What happens to power lost in a feedline? (T7C07)

• A. It increases the SWR

• B. It comes back into your transmitter and could cause damage

• C. It is converted into heat

• D. It can cause distortion of your signal

Why is it important to have a low SWR in an antenna system that uses coaxial cable

feedline? (T9B01)

• A. To reduce television interference

• B. To allow the efficient transfer of power and reduce losses

• C. To prolong antenna life

• D. All of these choices are correct

What is the impedance of the most commonly used coaxial cable in typical

amateur radio installations? (T9B02)

• A. 8 ohms

• B. 50 ohms

• C. 600 ohms

• D. 12 ohms

Why is coaxial cable used more often than any other feedline for amateur radio antenna systems?

(T9B03)

• 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 type of feedline

What generally happens as the frequency of a signal passing through coaxial cable is

increased? (T9B05)

• A. The apparent SWR increases

• B. The reflected power increases

• C. The characteristic impedance increases

• D. The loss increases

What might cause erratic changes in SWR readings? (T9B09)

• A. The transmitter is being modulated

• B. A loose connection in an antenna or a feedline

• C. The transmitter is being over-modulated

• D. Interference from other stations is distorting your signal

Which of the following types of feed line has the lowest loss at VHF and UHF? (T9B11)

• A. 50-ohm flexible coax

• B. Multi-conductor unbalanced cable

• C. Air-insulated hard line

• D. 75-ohm flexible coax

technician license course chapter 4 lesson plan module 9 – antenna fundamentals, feed lines &...

Documents

decibels db slide

antenna system antenna

magnetostrictive slide

skip slide

antenna fundamentals

radio signals

db loss

signal polarization