Radio Club Meeting – Tape Measure Yagi build
Radio Club Meeting – Tape Measure Yagi build
For part II of the ARRL examination study, we will study propagation of radio waves.
Radio waves spread out when transmitted from an antenna in straight lines unless they are reflected or refracted by some object. Due to this spreading and scattering, the waves become weaker as they propagate farther into the air. This limits the “range” a radio transmission can communicate over. The curvature of the Earth creates a “radio horizon” that limits the range of radio propagation. “Line of sight” propagation is when radio waves are transmitted within direct sight of the receiver. This is commonly done in VHF frequencies and higher. Lower frequencies travel as “ground waves”.
Radio waves are partially reflected when the medium through which the wave propagates changes due to a change in intrinsic impedance (a property defined by permittivity and permeability). Radio waves can even be reflected by change in weather patterns. The figure below shows the concept of diffraction (bending past an obstruction) of radio waves. Diffraction can also refer to spreading when a wave travels through a narrow medium into an open area.
Light waves also bend by “refraction” which is exactly how radio waves travel around the earth. The earth is curved and therefore the waves need to bend to propagate past “line of sight” distances. The shorter the wavelength (and hence higher frequency), the easier the wave can travel in and out of buildings by penetration of openings in solid objects.
It interesting to note that different waves received by an antenna can interfere if they are out of phase (destructive interference). This is called “multipath” which is when antennas receive waves from different paths. Moving an antenna a few feet can counterract this. Multipath propagation results in irregular fading. VHF and UHF signals propagating with multipath propagation experience fluttering or “picket-fencing” which comes from rapid variation of the signal strength. Tropospheric propagation or “tropo” is propagation of VHF or higher frequencies assisted by atmospheric phenomena such as weather fronts or temperature inversions. It is not uncommon for Tropo signals to propagate over 300 miles. Reflections can also be caused by conductors such as airplanes. Satellites reflect waves with conductive plating.
Thirty to 260 miles above the earth, the ionospheric layer resides. Atoms of nitrogen and oxygen are ionized by UV rays from the sun and become positively charged. The separation of the electrons and the creation of positive ions creates a weakly conductive region. The ionosphere is composed of many different regions. The E, F1 and F2 layers tend to reflect radio waves and the D and E regions tend to absorb waves.
“Skip” or sky wave propagation is when HF waves are completely bent back towards the earth. The conductive surface of the earth reflects the wave back and the process repeats. These “hops” or reflections allow the waves to be received at farther distances. Lower frequencies are bent more than higher frequencies. For this reason, UHF signals are rarely heard beyond the radio horizon. The MUF (maximum usable frequency) and LUF (lowest usable frequency) are the highest and lowest frequencies that can be reflected by the ionosphere without absorption. When sunspot activity increases, the makes the ionosphere more conductive and increases the MUF.
Sporadic or “E-Skip” propagation is when patches of the ionosphere become ionized enough to reflect frequencies as high as VHF and UHF. This is most common during early summer and mid winter months.
To understand the details behind antennas, the vital interface between free space and a transmit/receive system, it is important to fully understand the basic properties of antennas in order to understand their performance.
One of the main properties of an antenna is its radiation or antenna pattern. This is defined as a mathematical function of the radiation properties of the antenna as a function of space coordinates. It is important to note that this pattern is determined in the far field region (there are three main regions when studying antenna radiation: reactive near field, radiating near field, and far field). This can be a trace of the Electric or magnetic field (field pattern) or the spatial variation of the power density (power pattern). These are generally normalized with respect to the maximum value and typically are plotted in decibel scale to accentuate minor lobes. Minor lobes are any lobes that are not the major lobe. In split beam antennas, there can be multiple major lobes. The following image shows a directive antenna’s radiation pattern. Side lobes are generally undesirable and should be minimized if possible.
The Half Power Beamwidth (HPBW or sometimes just beamwidth) can be determined by drawing two lines from the origin point to the -3dB (half power) point and seeing the resultant angle.
Antennas are generally compared to “isotropic” antennas. These are hypothetical antennas that radiate power equally in all directions. This is not to be confused with omnidirectional antennas, which radiate power equally in the azimuthal direction. The E and H planes are defined as the plane containing the electric field vector and direction of maximum radiation and the plane containing the H vector respectively.
The three main regions around an antenna are the reactive near field, radiating near field and far field. In the reactive near field, the radiation is reactive (eg. the E and H fields are out of phase by 90 degrees. Because the waves are not in phase and transverse, they do not propagate. In the radiating near field, the waves are not purely reactive and propagate, however the shape varies with distance. In the far field (where the radiation pattern originates from), the radiation pattern does not change with distance and the waves are transverse.
One of the major characterizing aspects of antennas is the directivity. This is equivalent to the ratio of the radiation intensity in a certain direction over the hypothetical isotropic radiator intensity.
The denominator represents the average power radiated in all directions. The function is the normalized radiation pattern as a function of both the elevation and azimuthal angles. It is also possible to calculate partial directivities in either the theta direction or the phi direction and total directivity is the sum of these two. For a highly directive antenna with a very narrow major lobe and negligible minor lobes, the solid angle can be approximated by the product of the half power beamwidths in two different planes.
Another important property is antenna efficiency, which is the product of reflection efficiency, conduction efficiency, and dielectric efficiency. This takes into account all possible loss: either from a VSWR greater than 1 due to an impedance mismatch between the feedline and the antenna and conductive losses due to Joule heating from both the dielectric and the conductive parts. The antenna gain can be defined as the product of the antenna efficiency and directivity.
The ARRL (American Radio Relay League) is an organization for amateur radio enthusiasts. In order to communicate using HAM radio, at least a technician license must be obtained. The following post is meant as a useful information guide for those wishing to obtain a license.
The ARRL provides a complete manual as a study reference for HAMs. The book is divided into nine chapters: Basic info about ARRL, Radio and Signals, Circuit components, propagation and antennas, Amateur radio equipment, HAM communication, License regulation, operating regulation and safety. The questions come directly from each chapter (35 total, 26 to pass).
For Radio and Signal fundamentals, it is important to know basic properties of waves including wavelength, speed of propagation, the relation between wavelength and frequency, identifying frequency bands, the frequency ranges of various bands used by HAMs and so forth. The fundamental equation for propagation of waves is c = fλ. Because radio waves are being transmitted by antennas through air, the speed of propagation is 300 million meters/sec. This is a constant value and therefore if frequency is increased, the wavelength decreases proportionally. This speed value is roughly equivalent to the speed of light in a vacuum. The property of radio waves used to identify different frequency bands is wavelength. HAMs tend to use the frequencies occupied by bands MF through UHF. It is important to know the frequency ranges of these bands.
In this section, it is important to know prefixes for the SI unit system, so conversions between various values can be made. The following table should be committed to memory.
The next section deals with modulation, which is a necessary function to transmit the correct signal to receiver. It is important not to set a transmit frequency to be at the edge of any band to allow for transmitter frequency drift, allow for calibration error, and so that modulation sidebands do not extend beyond the band edge. It is important to know about FM deviation (which is dependent on amplitude of the modulating signal) and that if the deviation is increased, the signal occupies more bandwidth. Setting a microphone gain too high could cause the FM signal to interfere with nearby stations. It is important to know the types of AM modulation (Double Sideband, Single Sideband, etc) and which modulation technique is best for various frequency bands. “Continuous wave” (Morse code-esque) modulation occupies the lowest bandwidth, followed by SSB modulation. The various advantages to certain modulation techniques should be understood. For example, SSB is preferential to FM because it occupies less bandwidth and has longer range. The bandwidth for each modulation technique is shown below.
The final section of Chapter two deals with radio equipment basics. A repeater should be understood to be a station that retransmits a signal onto another channel. The following is an image of a transceiver, which transmits and receives RF signals using a TR switch to switch between each function. A repeater uses a duplexer in place of this switch to transmit and receive simultaneously.
The following are questions that are used for the HAM Radio Technician level license. The title of this section of questions is: “Feel Lines: types of feed lines; attenuation vs. frequency; SWR concepts; matching; weather protections; choosing RF connectors and feed lines.”
Why is it important to have a low SWR in an antenna system that uses coaxial cable feed line?
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
SWR or Standing Wave Ratio refers to the efficiency of an antenna. A low SWR means that the loss will be reduced in the system. Therefore, the correct answer is B.
What is the impedance of the most commonly used coaxial cable in typical amateur radio installations?
A. 8 ohms
B. 50 ohms
C. 600 ohms
D. 12 ohms
The correct answer is B. 50 Ohm transmission lines are very common in most RF systems. Television systems use 75 Ohm lines.
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 feed line
C. It can handle more power than any other type of feed line
D. It is less expensive than any other types of feed line
The correct answer is A, considering it’s ease of use.
What does an antenna tuner do?
A. It matches the antenna system impedance to the transceiver’s output impedance
B. It helps a receiver automatically tune in weak stations
C. It allows an antenna to be used on both transmit and receive
D. It automatically selects the proper antenna for the frequency band being use
An antenna tuners are used if the SWR is too high for a radio to operate properly. It matches the antenna’s impedance to the impedance of a transmitter. Automatic tuners also exist. Correct answer: A.
What generally happens as the frequency of a signal passing through coaxial cable is increased?
A. The apparent SWR increases
B. The reflected power increases
C. The characteristic impedance increases
D. The loss increases
Coaxial cables work well within certain frequency ranges, however most are not rated to go above certain ranges. You’ll need to pay more for coaxial cable that can handle higher frequencies. The reason for them not working well at high frequencies is due to loss. Correct answer: D.
Which of the following connectors is most suitable for frequencies above 400 MHz?
A. A UHF (PL-259/SO-239) connector
B. A Type N connector
C. An RS-213 connector
D. A DB-25 connector
Type N connectors are used above 400 MHz. Correct answer: B.
Which of the following is true of PL-259 type coax connectors?
A. They are preferred for microwave operation
B. They are water tight
C. They are commonly used at HF frequencies
D. They are a bayonet type connector
PL-259 type coax connectors are used in UHF, HF applications. Correct answer: C.
Why should coax connectors exposed to the weather be sealed against water intrusion? A. To prevent an increase in feed line loss
B. To prevent interference to telephones
C. To keep the jacket from becoming loose
D. All of these choices are correct
The correct answer is A. Water intrusion can cause an increase in loss. Correct answer: A.
What might cause erratic changes in SWR readings?
A. The transmitter is being modulated
B. A loose connection in an antenna or a feed line
C. The transmitter is being over-modulated
D. Interference from other stations is distorting your signal
Standing wave ratio is important for the measuring the efficiency of equipment. Modulation or interference therefore will not have an effect on the SWR. Loose connections may however cause an issue with SWR. Correct answer: B.
What electrical difference exists between the smaller RG-58 and larger RG-8 coaxial cables?
A. There is no significant difference between the two types
B. RG-58 cable has less loss at a given frequency
C. RG-8 cable has less loss at a given frequency
D. RG-58 cable can handle higher power levels
RG-58 and RG-8, although similar are different in that the RG-8 coax cable has less loss per length. Correct answer: C. The following table lists losses per feet:
Which of the following types of feed line has the lowest loss at VHF and UHF?
A. 50-ohm flexible coax
B. Multi-conductor unbalanced cable
C. Air-insulated hard line
D. 75-ohm flexible coax
Air-insulated hard line coax has the lowest loss with added insulation. Correct answer: C.