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VHF Propagation: Introduction to Various Propagation Modes

This introduction provides an overview of VHF propagation and its various modes, including inverse square rule, visual cues, troposcatter, high pressure tropo, low pressure tropo, sporadic E, multi-hop Es, trans-equatorial propagation (TEP), Earth-Moon-Earth (EME), and aurora scatter.

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VHF Propagation: Introduction to Various Propagation Modes

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  1. VHF PropagationNOTE: This is only an introduction. Further personal propagation study is highly recommended. Many of these propagation modes are also active on 10 meters. Hint: Get on the air.

  2. Introduction (1/2) • Inverse Square Rule • ρP=1/r2 • Doubling the distance from a transmitter reduces the power density to ¼ the original value • If at 2 miles the power density is 4 watts/square meter, at 4 miles it drops to 1 watt/square meter • Kind of like paint coming out of a spray can

  3. Introduction (2/2) • D=√2h • D=Distance in miles • h=Antenna height in feet • 4/3 Rule - Radio waves propagate to 4/3 visual horizon. • Explanation • Speed of radio waves in air • At Altitude it’s colder, less water molecules to attenuate signal • Higher wave portion moves faster through upper air, so go farther • Like walking in water up to your knees – your upper body in the air can move faster than your legs in the water. • This bends the radio waves (Refraction)

  4. Visual Cues • Light waves also refract like radio waves • Super refraction – Objects over visual horizon are visible (commonly called a “mirage”). • Ground mirages on hot, calm days • Also bends radio waves • Example – California-Hawaii path. • Look for enhanced propagation on hot, still, hazy days • Es usually around longest/shortest days of year

  5. Troposcatter (1/3) • Signals refract off of particulate matter or objects in the atmosphere • Moisture, dust, buildings, etc. • Two stations may be beyond line of sight, but still able to communicate • We use this all the time • Simplex HT use in open areas outside line of sight (i.e. On opposite sides of a small rolling hill) • Hitting the repeater with no direct line of sight

  6. Think seeing car headlight beams in the air before you see the car come around the curve or over the hill Microwavers bounce off thunderstorms, precipitation, airplanes, etc. N4VBV – W4CRZ 1/2w 70cm path Military/Commercial Troposcatter use Troposcatter (2/3)

  7. Troposcatter (3/3) Airborne Particulate Mass Hill Station B Station A Actually works better if the highest particulate concentration is near one end of the signal path.

  8. High Pressure (Inversion) Tropo (1/2) • Calm high pressure system • At approx 1000’ up the air gets warmer/dryer with increased altitude vs cooling. • Ever drive up a mountain and the air gets warmer as you go up, causing your windows to fog on the outside? • This sharp change in temperature, moisture and pressure changes the air’s refractive index • This ducting condition may refract VHF/UHF signals for hundreds or thousands of miles • Has nothing to do with the ionosphere

  9. High Pressure (Inversion) Tropo (2/2) • This is called “instability” by weather folks • Steve’s SKYWARN brief • Commonly caused by frontal layers and morning radiational cooling of lower air layers • Like when the temperature drops just after sunrise • Duct thickness is important • Some ducts will conduct 70cm but not 2m • Don’t necessarily have to be “in” the duct • Ducts may appear quickly and fade quickly, but may last quite a while

  10. Low Pressure Tropo • Storm fronts approaching • Usually shorter lived than high pressure tropo (May need SSB/CW) • Watch the barometer – as the main front meets your location the duct will probably drop

  11. Tropo Ducting Cold Air Warm Air Cold Air Earth’s Surface Station B Station A Station A Warm Air Cold Air Station B

  12. Sporadic E (Es) - Charged E-Layer clouds reflect VHF (mainly 50 and 144 MHz) signals. - Multiple hops possible.

  13. Other Modes (1/3) Multi-Hop Es: Es Cloud Es Cloud Station B Es – F2 - Es Station A F2 Station B Station A Es – High Altitude Duct - Es Station B Station A Can also have combinations of above.

  14. Other Modes (2/3) Trans-Equatorial Propagation (TEP) - Between stations equidistant from the “radio” equator (Also called “chordal hop”). - Signals enter F2 layer, propagate across equator, exit same distance on other side of equator. Earth-Moon-Earth (EME) - Bounce signals off the moon. - High power, long antennas, new digital modes.

  15. Other Modes (3/3) Aurora Scatter - All stations point beams north. - Bounce signals off auroral curtain. - Usually use CW/digital modes due to the “flutter” of the moving aurora. - Higher power, “mostly” northern stations, BUT remember when aurora was active in SC in October/November 2003.

  16. The Practical Part (1/3) • Listen • Propagation beacons (Usually CW) • 6m – 50.060-50.080 (Also 10m beacons 28.170-28.300) • 2m – 144.275-144.300 • Only one 2m beacon listed for SC – JT on 144.277 (CW) • 70cm – 432.3 - 432.4 • NOAA WX Radio stations • 146.52 Simplex • Distant Repeaters • Raleigh is ~ 130Miles straight-line distance, but not over flat ground; not normally heard. • Distant FM radio stations on an empty dial freq.

  17. The Practical Part (2/3) Calling Frequencies • 6m: 50.125 (50.110 for DX) (SSB/CW); • 52.525 FM Simplex (10m: 28.400 SSB; 29.600 FM for General and up) • 2m : 144.2 SSB/CW; 146.52 FM Simplex • 70cm: 432.1 SSB/CW; 446.0 FM Simplex • Hint: Program 29.60, 52.525, 146.52, and 446.0 into an old scanner and leave it on • Can also program active 10m/6m/2m/70cm repeaters at various distances/directions around your station that are not normally heard • Some enhanced propagation may start at the lower bands and move up, or the other way around

  18. The Practical Part (3/3) • For other VHF/UHF bands, check: http://www.arrl.org/FandES/field/regulations/bandplan.html • Listen/CALL on 29.6/52.525/146.52/446 FM • Use the repeater directory to try distant repeaters along the estimated ducting path • May be able to get hundreds or thousands of miles when ducting is active • Most long distance operators use SSB/CW with horizontal polarization, but if you only have vertical antennas and FM give it a try anyway • Even though you may not physically be “in the duct”, your signal may be able to get in

  19. Raleigh-Sumter -147.015 repeater output (same as ours) • 130 mile path • Just after sunrise - 800’ Antenna height - √2*800=√1600 - 40 miles under normal conditions over flat terrain - Over 3 times longer reception when ducting active - Happens regularly

  20. Sumter - Laurens • 90 mile path (146.52 simplex) • Just after sunrise • 5’ Antenna height/25w mobile • 20’ Base antenna - √2*5 (√10) +√2*20 (√40) = 9.5 - 9.5 miles under normal conditions over flat terrain - 10 times longer reception when ducting active

  21. Sumter-Athens, GA • 144 mile path • Near Sunset • 800’ Repeater antenna height - 50’ Base antenna - 47 miles under normal conditions over flat terrain - 3 times longer reception when ducting active

  22. Online Sources to Check (1/4) • http://dxworld.com/144prop.html

  23. Online Sources to Check (2/4) • http://mountainlake.k12.mn.us/ham/aprs/path.cgi?map=na

  24. Online Sources to Check (3/4) • http://www.dxinfocentre.com/tropo.html#hour48

  25. Online Sources to Check (4/4) www.dxmaps.com

  26. Weather Effects Example • See next 2 slides

  27. The Bad Part of Ducting • Repeater Interference • Repeaters coordinated based on normal conditions • May require PL to be turned on when not normally needed • May cause problems in emergency situations • Microwave link problems • Hap’s ETV microwave link discussion • Satellite Signals • Cell phone issues • Ever notice no/spotty reception near sunrise/sunset where it’s normally good?

  28. Ducting “Head Scratchers” • Ducts usually form at 800’-1000’, but may be at different levels • Near the surface • 30,000 Feet • Sometimes you might head for high ground looking for a duct and pass right through it • A 2m duct may not work for 6m • Same for 70cm and 2m • Think duct thickness as related to wavelength • Sometimes the repeater may sound “poor”, but actually be working fine • Repeater antenna is in the duct but you’re not

  29. Ducting Prediction

  30. Actual WX

  31. Actual 2 Meter Propagation

  32. References • 2006 ARRL Handbook, Chapter 20, “Propagation” • CQ Communications “VHF Propagation, A Practical Guide for Radio Amateurs” • Any internet search

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