Module 8 - PROPAGATION
- Reading Assignment
- Radio Communication
- Ground Waves
- Line of Sight
- Sky Waves
- Sunspot Cycles
- Meteor Scatter
- Auroral Ionization
- Sporadic E
- Tropospheric Ducting
- Propagation Prediction Sites
- Quiz 8
- Recommended Reading
Read Chapter 3 pages 4-1 to 4-4 in your text before you continue.
If you have questions after reading the assignment, consult the recommended reading.
Radio waves propagate by many mechanisms. The science of wave propagation has many facets. We will discuss three basic ways: Line of sight Ground wave Sky wave but first:
The ionosphere is a part of the upper atmosphere, from about 30 to 260 miles altitude, it is ionized by solar radiation. It plays an important part in atmospheric electricity. It has practical importance because, among other functions, it influences radio propagation to near and distant places on the Earth.
Review the Ionosphere in your text Page 4-3 and refer to the figure to the right.
Lower frequencies follow the curvature of the earth via groundwave propagation . The wave "clings" to the surface and thus follows the curvature of the earth.
Groundwaves are how we communicate locally on HF frequencies.
Since the ground is not a perfect electrical conductor, ground waves are attenuated rapidly as they follow the earth's surface. Attenuation is proportional to the frequency making this mode mainly useful only for LF and VLF frequencies below the ham bands.
(HF) frequencies have limited ground-wave range. The most common HF wavelengths used are 160 m, 80 m, 40 m, 30 m, 20 m, 17 m, 15 m, 12 m and 10 m. The only HF band available to Technician Licenses is the 10 meter band.
Line of sight is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes at VHF and higher frequencies.
The maximum distance for line-of-sight transmissions with two people standing on the surface of the earth is about (roughly) 25 miles, this is due to the curvature of the earth. This is called the Radio Horizon (It is slightly further than actual sight since the radio waves do bend a small amount, however the higher the frequency the less bend.)
You can extend the range of your communications on VHF and higher frequencies by increasing antenna height. Ten feet of antenna height is like doubling your power output.
This is why a repeater on a hight point is so effective in extending range on VHF and higher frequencies.
Another thing to do is to pay particular attention to your antenna and feedline losses. Losses are much higher at higher frequencies for the same given length and or type of antenna feedline.
Because VHF & UHF radio signals can travel through many non-metallic objects, radio can be picked up through walls of buildings (UHF better because of shorter wavelengths) . This is still line-of-sight propagation. Examples would include propagation between a satellite and a ground antenna or reception of television signals from a local TV transmitter.
However, ground plane reflection effects are an important factor in line of sight propagation and sometimes moving a few feet can make a major difference in reception because of multi-paths.
Radio waves can be diffracted around the sharp edges of a building or other solid object this is known as knife edge.
Mobile communications can be alternately good and bad causing a fluttering know as picket fencing, kind of like seeing a light through a picket fence.
Line of sight communications is the most important to technician class licensees since most of the allowable frequency bands are VHF and higher. You will probably find yourselves using repeaters more often than not.
However, don't deny yourself the fun of contesting and talking to hams you don't know by looking for DX (distant stations) on simplex (transmitting and receiving on the same frequency) on 6 & 2 meters. 10 meters offers these kinds of opportunities also since there is no repeater operation on that band. Making regular contacts out of your areas is something to look forward to when you obtain your General Class License and have access to the HF frequencies.
Skywave propagation (skip) is one of the modes that rely on refraction of radio waves in the ionosphere, which is made up of one or more ionized layers in the upper atmosphere. F2-layer is the most important layer for long-distance, multiple-hop HF propagation, though F1, E, and D-layers also play significant roles.
Especially during the evening on HF frequencies skip, including multiple hops are possible making long distance contacts (called DX),is common. When you are able to hear and therefore make contact with many stations the band is said to be 'open'. When the band conditions are such that you cannot hear or contact very few or no stations the band is said to be 'closed'.
The D-layer, when present during sunlight periods, causes significant amount of signal loss, because it absorbs the HF signals making daylight communications very difficult by blocking higher frequency signals from reaching the F2-layer where they can skip.
During solar maxima, or sunspot highs and peaks, the whole HF range up to 30 MHz can be used usually around the clock. During solar minima, or minimum sunspot counts down to zero, propagation of frequencies above 15 MHz is generally unavailable.
Skywave modes of radio communication operate by bending (refracting) radio waves (electromagnetic radiation) through the Ionosphere. During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and cosmic rays. This effects the path (propagation) of the radio wave in complex ways which can both facilitate or hinder local and long distance communications. Forecasting of skywave modes is of considerable interest to amateur radio operators. They utilize frequencies within the High Frequency or 'HF' radio spectrum which are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency, a limit on the highest frequency usable for communications
This propagation relies on reflecting radio waves off the intensely ionized columns of air generated by meteors. While this mode is of very short duration, often only from a fraction of second to couple of seconds per event, digital Meteor burst communications allows remote stations to communicate to a station that may be hundreds of miles up to over 1,000 miles (1,600 km) away, without the expense required for a satellite link. This mode is most generally useful on the 6 meter VHF band.
Auroral Ionization occurs at 100 km altitudes within the auroral oval backscatter radio waves, perhaps most notably on HF and VHF. The radio-auroras are observed mostly at high latitudes and rarely extend down to middle latitudes. The occurrence of radio-auroras depends on solar activity (flares, coronal holes, CMEs) and annually the events are more numerous during solar cycle maxima.
Sporadic E propagation can be observed on HF and VHF bands. It must not be confused with ordinary HF E-layer propagation. Sporadic-E at mid-latitudes occurs mostly during summer season, from May to August in the northern hemisphere and from November to February in the southern hemisphere. There is no single cause for this mysterious propagation mode. The reflection takes place in a thin sheet of ionization around 90 km height. The ionization patches drift westwards at speeds of few hundred km per hour. There is a weak periodicity noted during the season and typically Sporadic E is observed on 1 to 3 successive days and remains absent for a few days to reoccur again. The events usually begin at dawn, and there is a peak in the afternoon and a second peak in the evening. Sporadic E propagation is usually gone by local midnight.
Sudden changes in the atmosphere's vertical moisture content and temperature profiles can on random occasions make microwave and UHF & VHF signals propagate hundreds of kilometers up to about 2,000 kilometers (1,300 miles) beyond the normal radio-horizon, this is called Tropospheric Ducting. The inversion layer is mostly observed over high pressure regions, but there are several tropospheric weather conditions which create these randomly occurring propagation modes. A typical example could be the late summer, early morning tropospheric enhancements that bring in signals from distances up to few hundred kilometers for a couple of hours, until undone by the Sun's warming effect.
Forecasting of skywave modes is of considerable interest to amateur radio operators and commercial marine and aircraft communications, and also to shortwave broadcasters. Real-time propagation can be assessed by listening for transmissions from specific beacon transmitters or by using several propagation programs.
We will present two such forecasting programs, but, there are several more at http://www.dxzone.com/catalog/Software/Propagation/
What is PropNET?
PropNET is the name given to an innovative project that uses PSK31 to create an RF-based digital peer-to-peer network. Participants simply download PropNetPSK (a Windows program), enter a few station-specific parameters in order to take part. Most PropNET participants automatically identify themselves over-the-air, typically a few times an hour and pass traffic to other participants.
If a receiving PropNET station "catches" (decodes) the transmission, the details are logged locally and the event is plotted on a local map. If the receiving station is connected to the Internet, the "catch" is also reported to a globally accessible website. The process repeats over time, resulting in a significant amount of network-exchanged information that can be used for many purposes, including the observation of ones own transmission quality.
However, you don't actually have to participate by transmitting. You can just 'lurk' or listen by going the 'real time activity' menu tab on the propnet website and define your parameters, (it really is pretty self explanatory)and it will show you by lines on the U.S. map stations that heard a particular station and where they are. This give you the ability to see if the band is 'open', or not.
The figure will show you what the screen looks like.Click here to see the PropNet webpage.
What is PropView??
PropView is a free application that uses the included VOACAP, ICEPAC, and IONCAP engines to graphically display band openings between two geographic locations over a specified 24 hour period. It can also build schedules for the IARU/HF beacon network and automatically QSY (change frequency on) your transceiver to monitor each scheduled beacon.
It predicts what frequencies might be open. The graph shows the maximum usable frequency, the minimum usable frequency, solar position, open bands, time and frequency bands.
Put a little simpler, it tells you when the band might be open on a particular band.
This program is for HF mainly but you get the idea.
The more you practice the quiz the better you will do on the test.
You can take the quiz as often as you wish.
In this module you have been introduced to the mystery of propagation. Look at the recommended reading and do some internet searches to gain more knowledge on the subject.
In subsequent modules we will study how to operate these different types of receivers.
View the complete Technician Question Pool in Chapter 11 of your Ham Radio License Manual.
Review the Term Glossary in Chapter 10 of your Ham Radio License Manual
Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.