AN INTRODUCTION TO VHF/ UHF PROPAGATION Paul Wilton, M1CNK
OVERVIEW Introduction Propagation Basics Propagation Modes Getting Started in 2m DX
INTRODUCTION QRV on 2m SSB since Aug 1998, on 6m since Jan 2001 and 70cm since Mar 2004. 2m:12 ele ZL-Special - 100W 6m: 3 ele beam - 60W 70cm: 8 over 8 beam - 50w Yaesu FT-726
PROPAGATION BASICS Radio Waves travel in straight lines in free space. Direction can only be changed through either reflection, refraction or diffraction. For VHF/UHF, other mechanisms other than ionosphere are important.
PROPAGATION BASICS
PROPAGATION MODES Mode 6m 2m 70cm Line of Sight F2-layer TEP/FAI Sporadic E Meteor Scatter Auroral Scatter Tropospheric Refraction Tropospheric Scatter
LINE OF SIGHT As title suggests, radio line of sight required between stations. Because of atmospheric refraction, the radio horizon equals radius of curvature of earth plus a third. Multi-path propagation dominates in the real world. Height above local environment is very important. 6m 2m 70cm
F2-PROPAGATION Continuation of HF F2-layer propagation to 6m band. Occurs when the the Maximum Usable Frequency high enough to allow propagation at 6m. Can give world wide communications. 6m
TEP/FAI TEP Trans Equatorial Propagation Specialised form of F2 propagation Allows N/S paths across the Equator On 6m, allows regular contacts to Southern Africa from Southern Europe FAI Field Aligned Irregularities Rare scattering mode caused by ionospheric electrons trapped by Earth s magnetic field 6m 2m
SPORADIC-E Occurs in the summer months when small clouds of electrons form in the E layer. Produces spectacularly strong signals. Occurs on 10m, 6m and 2m. Open most afternoons on 6m between May and August. Can occur in December as well. As the cloud moves, geographic areas can rapidly change especially on 2m. 6m 2m
SPORADIC-E Cause not known but a number of theories have been put forward. Most likely cause is wind sheer. Multi-hop possible to extend range. Distances from 500km to 8,000km on 6m. Distances from 800km to nearly 4,000km on 2m. 6m 2m
METEOR SCATTER Uses reflections from the trails of ionisation left behind by meteors as they burn up in the atmosphere at a height of between 88km to 100km Reflections are short lived so either high speed CW or computer modes needed on 2m. Random Meteors occur at all times. Seasonal showers also occur, during some of these, SSB can be used. 6m 2m 70cm
METEOR SCATTER Originally high Speed CW was used, produced by a computer and recorded at the receiver by either a special tape recorder or by a computer. The received data was then played back at much slower speed to be decoded by ear. However, has been replaced by FSK441mode - part of WJST by K1JT, Joe Taylor 6m 2m 70cm
METEOR SCATTER Distances of between 1000km and 2000km can be obtained with modest stations called the poor man s DX. On 6m, reflection time is longer so SSB is can be used. Meteor Scatter is used commercially on frequencies around 6m. 6m 2m 70cm
AURORAL SCATTER Uses scattering from the cloud of ionisation produced by an aurora (Northern Lights). Favours northerly stations, although during more severe geomagnetic storms, the aurora can move south. More likely to occur near sun-spot maxima and around the equinox. 6m 2m 70cm
AURORAL SCATTER Antennas need to be pointed North for both stations. CW is generally used owing to the immense distortion introduced. Auroral CW Auroral SSB over a 560mile path between VE3SQD and WA1VRH 6m 2m 70cm
AURORAL SCATTER Distortion caused by Doppler shift of the frequency and hence get worse as frequency increases. 6m easier than 2m which is easier than 70cm. Ranges of between 500km and 2000km possible on 2m. 6m 2m 70cm
TROPOSPHERIC REFRACTION Also call lift or ducting. Most commonly seen enhancement. Caused by the changes in the speed of radio waves due to variations in air temperature and pressure refraction. Stable high pressure weather systems good indicator. 6m 2m 70cm
TROPOSPHERIC REFRACTION Enhanced refraction occurs when the rate of change of refraction is far larger than normal enhances the standard radio horizon beyond standard curvature plus a third. Ducting occurs when there is an inversion taking place either at ground level or at an elevation. 6m 2m 70cm
TROPOSPHERIC REFRACTION
TROPOSPHERIC REFRACTION Ducts can allow immense distances to be obtained especially over sea paths. Inversions are most common with high pressure anti-cyclones. 70cm can often be better than 2m owing to the smaller wavelength. 6m 2m 70cm
TROPOSPHERIC REFRACTION Text
TROPOSPHERIC SCATTER The Workhorse of DX under flat conditions available all year round. Uses scattering from turbulence in the troposphere. Allows distances of up to about 500km between suitable equipped stations. The lower the angle of take-off, the longer the range. 6m 2m 70cm
TROPOSPHERIC SCATTER For each 1 increase in take-off angle, there is a 9-12dB cut in signal strength - 1½ to 2 S-points! Take-off angle impacted by: Local obstructions Antenna height above ground - eg 10m on 144MHz is 3 take-off but 9 on 50MHz. 20m on 144MHz is 1.5 take-off Can use sloping ground to replace antenna height. 6m 2m 70cm
M1CNK 6M
M1CNK 6M
M1CNK 2M
M1CNK 70CM
GETTING STARTED IN 2M DX Basics Multimode 2m rig needs SSB or CW. Beam antenna plus rotator. Situations Clear take-off essential in direction of interest. Low-angle radiation requires beam to be elevated (ideally 10x wavelength) or at the top of a hill with sloping ground.
GETTING STARTED IN 2M DX Enhancements Low loss coax feed RG213 or better. Mast head pre-amp with minimum amount of gain to improve RX sensitivity/overcome coax loss. Power amp 100w or more.
OPERATING AIDS Beacons www.dxmaps.com maps from DX Clusters www.gooddx.net email alerts from DX Clusters www.dxinfocentre.com/tropo_nwe.html Hepburn Tropo Index http://aurorawatch.lancs.ac.uk/alerts email alerts from UK magentometers
ANY QUESTIONS?