Raytown Amateur Radio Club
Technology Net - Propagation on VHF and above
by Jacque Dupuy (JD) - NØIRS
Technology Net - Friday - August 14, 2009 8:00 PM - KØGQ repeater 145.170(-) Echolink - KØGQ-R - Node# 403841
Introduction
"JD" NØIRS has spent most of his Ham Radio career (since 1996) operating and achieving many goals in "The World Above 50 MHz", 50/144/222/432/1296 MHz. If you go to his house you'll find three towers dedicated to VHF, UHF and beyond. He hasn't forgotten the lower bands, having a Tri-Band beam and a couple dipoles. But, on the average day you'll find him on 2-Meter SSB, listening at 144.200 MHz.
JD is the founder and webmaster of the KC VHF Grid Bandits club. Currently there are over 175 members, from all over the U.S. http://kcvhfgridbandits.com
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Key elements to consider and their result:
Antenna Polarization:
Polarization is an important factor for antennas. Both antennas and electromagnetic waves are said to have a polarization. For most antennas it is very easy to determine the polarization. It is simply in the same plane as the elements of the antenna. So a vertical antenna (i.e. one with vertical elements) will receive vertically polarized signals best and similarly a horizontal antenna will receive horizontally polarized signals.
Verical Polarization is used primarily with FM signals due to the large number of mobile antennas and HT antennas being mounted vertically. Antennas with omni-directional patterns are simpler with vertical polarization as in the FM repeater world and the large number of mobile installations.
Horizontal Polarization is used primarily with single sideband due to the characteristics, horizontal polarization shows a marginal improvement for long distance communications using the ionosphere. Also, man made radio noise is predominantly vertically polarized and the use of horizontal polarization would provide some discrimination against interference from noise. SSB is the prefered choice by Hams for DX operation, so horizontal polarization is used.
Circular polarization is most often used on satellite communications and by Hams on the 1296 MHz band. This is particularly desired since the polarization of a linear polarized radio wave may be rotated as the signal passes through any anomalies (such as Faraday rotation) in the ionosphere. Circular polarization will keep the signal constant regardless of these anomalies.
Propogation Modes: The way in which radio signals at VHF and above propogate over greater distances than strictly line of sight in the troposphere.
On frequencies above 30 MHz, it is found that the troposhere has an increasing effect on radio signals. The radio signals are able to travel over greater distances than would be suggested by line of sight calculations. At times conditions change and radio signals may be detected over distances of 500 or even 1000 km and more. This is normally by a form of tropospheric enhancement, often called "tropo" for short. At times signals may even be trapped in an elevated duct in a form of radio signal propogation known as "tropospheric ducting". Tropospheric propogation effects occur comparatively close to the surface of the Earth. The radio signals are affected by the region that is below an altitude of about 2 kilometres. As these regions are those that are greatly affected by the weather. There is a strong link between the weather conditions and radio propogation conditions and coverage.
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Tropospheric bending is evident over a wide range of frequencies, although is most useful in the VHF/UHF region. Radio signals can be trapped in the troposphere, traveling a longer distance than normal before coming back to Earth's surface. During the spring, summer and fall months, it is possible to make VHF and UHF contacts over long distances up to 1,000 miles or more. When such "tropo" openings occur, the VHF and UHF bands are filled with excited Hams eager to work DX. When there is a stable high-pressure system, a mass of warm air may overrun cold air, causing a temperature inversion. Radio waves trapped below the warm air mass tavel great distances with low loss. The area between the Earth and the warm air mass is known as a duct.
Sporadic-E propagation:
Sporadic-E propagation is probably familiar to many low-band operators as the summertime "short skip" on 10 meters. It is also responsible for most of the long-distance (600km and greater) contacts on the 6-meter band. Sporadic-E is a type of ionospheric E-layer reflection caused by small patches of unusually dense ionization. These sporadic E-layer "clouds" appear unpredictably, but they are most common over the US and southern Canada during the daylight hours of late spring and summer. Signals are often remarkably strong, allowing 50 and 144MHz stations running 10 watts, and often much less than that, to make contacts 1500 km and longer with relative ease. Thousands of spectacular sporadic-E contacts made on the 50, 144, and 220-MHz bands during the past half dozen years have created considerable excitement in VHF circles.
Meteor Scatter propagation: Meteor Scatter is being used by an increasing number of Hams to work long distances.
Meteors are small particles of various compositions, They are classified into 3 main types:
1. Stony meteorites, composed mainly of silica and magnesium oxides.
2. Siderites, which contain mainly iron with a small percentage of nickel.
3. Siderolites, containing mineral and metallic elements in varying proportions.
Gases mainly carbon monoxide, nitrogen and hydrogen are also abundant and are liberated when the Meteor vaporises during It’s passage into the Earths upper atmosphere.
The mass of these objects vary considerably and range between fractions of a milli-gramme up to 1 kilo-gramme. The physical dimensions range from the size of a grain of sand to a tennis ball. As the Meteor is attracted by the Earths gravitational pull it begins to collide with molecules of air, which become entrained in the surface. The heat produced evaporates atoms and it is the collision between the air molecules and the atoms moving off the Meteorite which produce the familiar sight of a 'shooting star'. This action produces heat, light and ionisation and in general takes place around the level of the E layer at a height of approximately l00Km above the surface of the Earth. For the Meteor Scatter operator it is these 'tubes' of highly ionised particles that can be used to reflect radio signals very effectively at VHF frequencies. It is the ionised trails produced by Meteor’s that are used to Scatter and reflect radio signals. The nature of their make up suggests that the condition for optimum reflection will not last for very long.
Received signals reflected from Meteor trails are often subject to considerable fluctuations in strength. There are two main reasons for this, both of which are due to more than one reflection being received at the antenna, sometimes in phase, and adding to the signal and at other times in anti-phase and cancelling.
It must be said that some dedicated 70cms operators have had successful QSO's on this band, but compared with 2m, the combination of reduced received energy and signal duration, make the completion of QSO's very difficult for all but the very best equipped stations.
Best Times to Operate - for DX:
a. Morning 6:00am to 10:00am Night 7:00pm to Midnight.
b. Watch Weather Forecast High Pressure or Inversion layer is key.
c. Earth cools and warms drawing moisture into atmosphere.
Key events that have happened this year. Eskip openings and strong Tropo:
a. 4 openings into Florida.
b. 1 to New York New Jersey.
c. The Big One into the North East on 29 July 2009 lasted 96 minutes. Eight states worked and 42 QSO's.
d. Working Colorado from the Mobile east of Ames Iowa.
Internet Tools to help you determine good propagation:
a. Listening for propagation beacons.
b. Propagation Loggers.
c. APRS Map and how that helps us determine contact paths.
Tropospheric Ducting Forecasts: http://www.dxinfocentre.com/tropo_eur.html
Upcoming events 12 - 13 Sept ARRL VHF QSO Party. We will be on Rich Mountain in EM24 in Mena AR.
Closing - Working Weak Signal takes alot of seat time and watching weather patterns and conditions. Reward is worth it. Collecting Grid Squares. Each season brings surprises. New operators this year in area have had some very good results.