August 13, 2008 - Educational Radio Net, Session 12
Jim Hadlock K7WA
This will be a discussion of propagation effects which we may experience on the VHF bands between 50 mHz and 440 mHz. Propagation on these bands can be local, near distant or far distant; contacts as far as Japan and even Europe have occurred recently on the six meter band. This discussion will not include F layer propagation in the ionosphere which is common on the high frequency bands and sometimes six meters - this subject deserves a session of its own. Tonight's presentation is intended as an introduction, much has been written and there is much to discover about propagation and I invite you to follow-up with questions and referral to the references listed below.
Most simply put, propagation is how a radio wave gets from the transmitting antenna to the receiving antenna. Like light waves, radio waves usually travel in straight lines. Propagation effects occur when the radio wave is bent or reflected by an object or some other medium on its journey from the transmitter to the receiver. If our primary purpose is reliable local communication, either simplex or through a repeater, we may experience propagation effects as a problem - for example interference on a repeater from a distant station, or noisy or broken-up signals over a normally reliable path. On the other hand, if we are trying to contact a distant station we will want to take advantage of propagation effects to extend the range of our signals. While most long-distance VHF communication takes place on SSB and CW, FM transmissions are also affected by propagation.
The Space Wave:
The ARRL Antenna Book defines the Space Wave as the dominant factor in local communication at 50 mHz and higher - this is what we commonly call "line of sight" propagation extending approximately 50 to 100 miles to the radio horizon. Distance covered by the Space Wave is limited by the curvature of the earth, the height of antennas at both ends of the path and obstacles, such as hills, that may exist in the signal path.
We have all experienced flutter and multi-path fading on our signals. These effects occur when a radio wave is reflected by the ground or some other object, resulting in some of the signal taking a slightly longer path to the receiving antenna than the rest of the signal. The differences of path length affect the phase of the received signal, sometimes interfering with itself in a way that reduces the signal strength. Moving the receiving antenna a short distance usually remedies this condition.
VHF operators often make use of mountains and other large objects as passive reflectors to extend the range of their transmissions. Other examples are reflecting signals off airplanes, orbiting objects, meteor scatter and moonbounce which will be discussed below.
Radio waves do not simply disappear or shoot off into space once they reach the radio horizon. Everything on earth and in the regions of space up to at least 100 miles is a potential forward-scattering medium. Scattering is the process that causes some of the signal to propagate beyond the horizon. Tropospheric effects occur in the lower atmosphere where boundaries between warm and cool air affect radio wave propagation. These effects are variable, but extensions of the minimum operating range occur almost daily. Locally these effects provide propagation north into British Columbia and Alaska, and south into Oregon. Tropospheric ducting sometimes occurs between Hawaii and southern California producing strong signals on the VHF bands.
Sporadic-E Propagation is caused by clouds or patches of abnormally intense ionization in the E layer of the Ionosphere at an altitude of approximately 60 miles. These clouds produce very effective propagation of radio waves above 28 mHz, sometimes as high as 144 mHz. Because the clouds may be small, propagation is often limited to an isolated geographic area. The clouds may also move, providing coverage to different areas during the opening. Single-hop Sporadic-E propagation is typically about 1300 miles although double and multi-hop propagation sometimes occurs extending the distance. Sporadic-E propagation occurs most commonly during the late spring and summer.
Auroral propagation is the result of charged particles from the sun interacting with gas molecules in the upper atmosphere. Because of the earth's magnetic field, auroras occur around the north and south magnetic poles. Sometimes the sun will emit an unusually large amount of charged particles toward the earth creating an aurora which may or may not be visible in the northern sky. During a strong event, radio waves will reflect off the aurora. Stations aim their antennas north and can make contacts to the east, west, and sometimes even south of their locations. Aurora reflected signals have a unique "swishy" sound to them making voice modes difficult to copy; CW is usually the most effective mode for aurora propagation.
Meteor Scatter and Moonbounce Propagation:
Meteor Scatter propagation utilizes the ionized trails of meteors entering the atmosphere to reflect radio signals from one location to another. There are so many small meteors entering the atmosphere that some commercial systems use this propagation mode to relay data on a regular basis. Amateurs use Meteor Scatter propagation on the 50 and 144 mHz bands for paths up to about 1000 miles. Voice and CW work with Meteor Scatter, but the advent of Digital modes has made this type of propagation much easier and more popular. Meteor trails dissipate quickly, sometimes a "ping" only lasts a second or so, other times a trail may last longer. High-speed digital signals can communicate much more data in a shorter time than voice or CW. K1JT has developed software (WSJT, Weak Signal JT) which enables relatively modest stations to enjoy Meteor Scatter digital communications.
Moonbounce propagation uses the moon to reflect radio signals back to earth. This usually requires large antennas and high power to overcome the considerable path loss to the moon and back. However, many amateurs are using K1JT's software to work moonbounce with relatively modest stations.
Contacts using Meteor-Scatter, moonbounce, and similar modes usually require scheduling and coordination and are rarely made on a spontaneous basis. Nevertheless, for many amateurs the challenge of these modes provides a great deal of fascination and interest.
In this presentation I have tried to cover some of the common propagation effects we experience on the VHF frequencies between 50 mHz and 440 mHz. While you may encounter some of them on FM, most of the "weak signal" activity occurs on SSB and CW around the established VHF Calling Frequencies (50.125 mHz, 144.200 mHz, and 432.100 mHz). A basic multi-mode radio and simple antennas are all that's required to experiment with propagation on VHF bands. The references below provide much more information.
The ARRL Antenna Book, ARRL
The Shortwave Propagation Handbook, Cowan Publishing Corp. (CQ Magazine)
ARRL Technical Information Service
Meteor Scatter and Moonbounce: http://www.arrl.og/tis/info/moon.html
Pacific Northwest VHF Society: http://www.pnwvhfs.org
Sunday, August 10, 2008
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