VHF Radio Propagation. Joe Fleagle W0FY

Transcription

1 VHF Radio Propagation Joe Fleagle W0FY

2 Frequency Ranges Definitions are from 1940 s 30 khz 300 khz LF 10km 1km 300 khz 3 MHz MF 1 km -100 meters 3 MHz 30 MHz HF 100 M 10 M 30 MHz MHz VHF 10 M -1M 300 MHz 3 GHz UHF 1M - 10 CM 3 GHz 30 GHz EHF 10 CM -1 CM 30 GHz 300 GHz SHF 1 CM - 1 MM Visible light (Red) 500 THz 0.6 microns

3 What s a radio wave? A little physics. Alternating electric field, accompanied by a magnetic field Transverse electromagnetic (TEM) plane wave in free space Polarization is defined as plane in which electric field exists Vertical, horizontal, or circular Electric and magnetic field amplitudes are related E / H = ohms often called impedance of free space

4 What s the big deal about polarization? Doesn t matter much at HF Ionospheric prop Dec 2010 QST - KL7AJ Article Does affect local ground wave communications Matters at VHF and above Blind combinations H pol antennas can t receive V pol signals and vice versa RH CP can t receive LH CP CP antenna can receive any linear polarization with 3 db loss Scattering from objects in antenna field depolarizes signals Perfect cancellation never happens but db loss common Polarization of common ham antennas Electric field usually parallel to radiating element Not true for mag loop antennas or cubical quads

5 Wave Propagation Signal power decreases with square of distance Doubling distance reduces signal by 6 db Equation for free space path loss path loss (db) = 20 log F (MHz) + 20 log D (miles) +37 At 146 MHz the loss in first mile is 80.3 db 5 watt radio into -3 dbi gain antenna is +34 dbm. Receiver sensitivity is around -120 dbm so have 154 db of gain How far would that go? > 5000 miles

6 More Physics Radio waves want to go in straight lines. Why don t they? Reflection and refraction Speed of light in vacuum C=186,280 mph or x 10 8 meters/sec Speed of light is slower in different media Frequency doesn t change but wavelength does λ= C/F

7 Reflection and refraction both occur when wave passes from one medium to another Surface does not have to be conductive to cause reflection Change in dielectric constant and / or permeability of medium Refraction and reflection scatter waves, signals may arrive from multiple directions Both bad and good

8 Sources of Reflection and Refraction Terrain Buildings, trees, hills, the ground under the antenna Atmospheric Troposphere Region 0 to 1 mi above ground Affects 2 meters and higher, Higher freqs affected first Waves refracted by moisture, density changes in air

9 Ionosphere Region 60 to 200 mi above ground E skip, meteors, aurora affect lower VHF frequencies at times F layer propagation occurs on 6 meters at sunspot peaks Ionosphere effects rare above 220 MHz Never have true LOS only path signals arrive at antenna from multiple sources Called multi-path propagation Signals may add or cancel depending on relative path lengths

10 Local VHF Communications Ranges out to 30 miles or so - Vertical polarization most common Easy to make vertically polarized omni directional antennas Vertical polarization works better for low antenna heights Rule of thumb for distance to radio horizon D= (2H) 1/2 where D is in miles and H is antenna height in feet Radio horizon more distant than visual horizon in temperate zones Simplex range limited by antenna height more than anything else Earth curvature and ground reflections from antenna affect ground wave range

11 2M Antenna at 1 WL We want RF to go at the horizon for terrestrial communications Ground reflections play a key role Polarization matters 2M horizontal 6 ft 6.9 dbi at 16 degrees Vertical dipole 6 ft max radiation 2.66 dbi at 10 deg elevation

12 2M Antenna at 3 WL Horizontal polarization at 20 feet AGL Lowest lobe 7.85 dbi at 5 degrees Lots of lobes from multipath Vertical polarization Lowest lobe 5.79 dbi at 4 deg Vertical polarization puts energy at a lower angle of radiation which is good Higher antenna is better but 20 ft is typical of a portable operating scenario

13 Long Distance VHF Communications Higher transmitter powers (150 watts and up) and directional antennas extend range significantly Antenna gain at both ends plus higher power can add 30 db or more to path loss capability Horizontal polarization easier to implement with directional antennas

14 Everyday ranges of mi are possible at 2M and up Signals forward scattered from atmosphere SSB, CW, or digital methods offer better SNR capability than FM High power ( kw) on 6 meters can produce consistent mi range using ionospheric scatter.

15 More exotic propagation modes Meteor scatter Ionized gases in meteor trails reflect VHF signals Reflection footprint is quite small, short duration Ranges to 1200 mi possible MHz optimum frequency range, effects rapidly grow weaker with increasing frequency Usable on 6 and 2 meters Snotel Aurora Borealis Ionized gases from solar particles around north pole reflect VHF signals Usable through 222 MHz, have never heard a signal on 432 MHz Range to 1000 mi or so. Only mode that works is CW due to extreme decorrelation of signal

16 Additional propagation modes Tropospheric ducting Autumn is best time needs quiet stable weather pattern 2 meters and up phenomenon very rare at 50 Mhz Warm dry air slides in over cooler moist air producing a temperature inversion Midwestern inversions can provide ranges out to 900 mi or so Most common path SW to NE, also to E or SE as the path tends to lie along weather fronts

17 E Layer Skip Mostly a 50 MHz phenomenon but can happen at 2M and rarely 222 MHz Single hop ~ 900 mi to 1400 mi At 50 MHz, multi hop Es can reach 4000 mi or more. Can happen any time of year but May to early August is prime time. Peaks around the summer solstice (20-21 June) EME - moonbounce High power, big antennas usually 144 MHz and up but some stations doing it on 50 MHz

18 Operating modes Aurora is purely CW due to extreme waveform distortion Meteor scatter used to be CW or SSB during major showers Now all digital, FSK441 usual mode Don t need to wait for shower any more due to digital mode Mostly scheduled between two stations Ionospheric scatter SSB, CW, digital JT65 mostly scheduled Tropospheric ducting SSB, CW, even FM - random contacts E Skip SSB, CW, FM, digital like PSK31 random contacts mostly Path duration may be rather short minute or two Can be backscatter from common point on earth EME Used to be CW, now all digital JT65

19 Propagation Sites on Web Troposphere propagation predictions- Hepburn Real Time conditions APRS network of 2 meter stations &Map=NA Has VHF, UHF, and HF bands Real time chat page with contact map Meteor scatter chat page

20 Grid Squares What s a grid square? British amateur Maidenhead suggested dividing world up into longitude and latitude squares major squares are 20 deg longitude, 10 degrees latitude Longitude starts at international date line and proceeds east Latitude starts at south pole and proceeds north Minor squares are 2 deg longitude 1 degree latitude 1 degree latitude is about 69 miles, 1 degree longitude depends on latitude, STL area 2 deg longitude is about 108 miles long. There are sub minor squares defined by more letters that are 2.7 mi in latitude, 4.5 mi long here in STL area My house is in square EM48rp» First, third, and fifth characters longitude, 2, 4, and 6 latitude

21 VHF Awards In January 1983 ARRL adopted grid squares for VHF awards and contest scoring VUCC VHF- UHF century club Work 100 different squares on 6 or 2 meters 50 squares for 222 and 432 MHz 25 squares for higher bands like 1296 and up VHF contests multiply QSO points times total grids worked for score FFMA award 50 MHz only, work all 488 grid squares in continental US Extremely difficult, only 5 people have done it to date One is here in STL, all from midwest, Texas, NM

22 Reading References Meteor scatter VHF Meteor Scatter Propagation QST April 1957 VHF Propagation by Meteor Trail Ionization QST May 1974 Meteor Scatter Communications QST Jan 1986 VHF Meteor Scatter an Astronomical Perspective QST June 1986 Tropospheric Propagation The Weather that Brings VHF DX QST May 1983 Tropospheric Scatter Propagation QST November 1983 VHF Propagation and Meterorology QST March 1984 Sporadic E propagation Sporadic E Propagation at VHF: A Review of Progress and Prospects QST April 1988 Eleven Years of Sporadic E QST March MHz Sporadic E QST July 1994 Aurora Radio Aurora QST January 1985 Practical Radio Aurora QST march 1990