TECHNICAL INFORMATION Below are some general technical communications information that may come in handy from time to time. Most information has come from WASES field equipment course notes. INDEX Frequency ranges & their use Wave propagation Ground conductivity The ionosphere Tilting effect Ducting Wavelength equations and lengths Clark mast anchor alternatives IPS also has an excellent page of facts and educational material which can found at http://www.ips.gov.au/Main.php?CatID=8 Frequency ranges & their use Band Range Use Very low freq. (VLF) 10 - 30 kHz submarines; not used very much as antennas are expensive and inefficient Low freq. (LF) 30 - 300 kHz mainly navigation purposes Medium freq. (MF) 300 - 3000 kHz broadcast stations (good ground waves) High freq. (HF) 3 - 30 MHz long range (eg overseas) communications Very high freq. (VHF) 30 - 300 MHz wide bandwidths are possible with resultant multi-channel operation Ultra high freq. (UHF) 300 - 3000 MHz as for VHF with more channels being available Super high freq. (SHF) 3 - 30 GHz as for UHF Extra high freq. (EHF) 30 - 300 GHz as for UHF Back to top Wave propagation Wave Mode Mechanism Frequency & Range Surface Wave energy travels close to the ground and is caused, by diffraction, to follow the earth's contours and curvature Below 500kHz approx. 1600km or more 0.5 - 1.5MHz up to 240km Direct Wave energy is transmitted along a modified line of sight path joining transmitter and receiver Above 25MHz approx. line of sight Ground reflected Wave energy reaches the receiver after one or more reflections from the ground   Space wave Consists of Direct plus Ground reflected waves Above 25MHZ approx line of sight Ground wave Surface plus space waves Below 1.5MHz as above 1.5 - 25MHz shorter ranges Ionospheric Wave energy is returned to the earth after bending and refraction in the ionosphere 3 - 30MHz Back to top Ground conductivity Since the surface wave front is in contact with the ground, it will lose some energy into the ground. The amount of energy lost depends on the conductivity of the earth at any given point. If the ground is a good conductor, part of the energy will be returned to the other wave fronts. If it is a bad conductor, much of the energy will be lost into the ground. Below is a list comparing the different type of terrain. Terrain Relative Conductivity Sea water Good Large bodies of fresh water Fair Wet soil Fair Flat, loamy soil Fair Dry, rocky terrain Poor City areas* Poor Desert Poor Jungle* Poor * city areas and jungle are particularly bad for surface propagation due to the absorption of the radio waves by buildings and foliage Back to top The ionosphere The ionosphere is the region of upper atmosphere (approx. 50 - 500km above the earth's surface) where the air is ionised by solar radiation (mainly ultraviolet). The composition of the ionosphere changes with height, and each of the atmospheric gases is ionised by a different part of the solar spectrum. This creates 'layers' of ionisation at different heights. There are usually three identifiable layers, those being E layer in the E region, and the F1 and F2 layers in the F region. When the sun sets, the ions and electrons in the lower layers recombine, but the F region collisions are less frequent and some ionisation remains all night. As solar radiation affects the ionosphere (and therefore HF communications), so do other solar activities such as sunspots, solar flares, and magnetic storms. IPS Radio and Space Services provides a service which enables HF communicators to use the ionosphere effectively using frequency predictions, and solar activity warnings. See the Links page for a link to their site. (image courtesy IPS Radio & Space Services) Back to top Tilting effect Since some of the surface wave energy is lost into the ground, the wave is retarded at points to the ground, causing the whole wave front to tilt forward. This retarding effect becomes less noticeable with increasing height above ground. Poor conducting surfaces cause high loss, greater tilt, and total absorption of energy. The angle of tilt also varies with frequency - as frequency increases, the angle of the tilt increases. The table below shows angles of tilt over good (sea water) and poor (dry ground) conducting surfaces for a range of frequencies Frequency (MHz) Angle of tilt over sea water Angle of tilt over dry ground 0.02 0° 2.5' 4° 18' 0.20 0° 8' 13° 30' 2.00 0° 25' 32° 12' 20.0 1° 23' 35° Back to top Ducting Ducting occurs when cool winds from the ocean blow across the land. This creates a band (or duct) of cool air trapped between two layers of warm air. Once radio signals (typically in the VHF and UHF bands) enter this duct, they effectively 'bounce' inside the duct, with some of the signal penetrating the top and bottom of the duct on each bounce. This anomaly can greatly extend the range of communications. For UHF, ducting can extend the range from 40km to over 400km. However, severe atmospheric disturbances, such as tropical cyclones, destroy ducting by disrupting the temperature inversion. Back to top Wavelength equations and lengths Wavelength (l) = velocity (V) / frequency (F) The length of a half wavelength dipole antenna is ½ l = 142.5 / F(MHz) Thus one element of the dipole is a quarter wavelength ¼ l = 71.25 / F(MHz) Below are some approximate quarter wavelengths for WASES HF frequencies. Note the quarter wavelength (and thus the wavelength) gets smaller as the frequency increases. Frequency ¼ l Frequency ¼ l 2MHz 27.7m 9MHz 7.6m 3MHz 19.1m 10MHz 6.9m 4MHz 15.6m 11MHz 6.2m 5MHz 12.2m 14MHz 4.8m 7MHz 9.7m 17MHz 4.1m Back to top Clark mast anchor alternatives Occasionally you will come across a situation where it is impossible to hammer the Clark mast anchor pickets into the ground, such as when the only high point around is a solid rock outcrop. This happened to us at a search we attended in September 2011, and below are some photos of the alternative anchor system employed by the responding team. Luckily there were some small shrubs that were growing in the cracks in the outcrop, and their root system was strong enough to support the mast. The spacing between the 3 sets of guys had to be adjusted from the standard 120° however the final setup proved to be suitable for the task. Back to top