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AMATEUR RADIO BY GARRY CRATT, VK2YBX How to optimise an HF antenna for multihop operation HF transmissions can travel great distances by being bounced off the ionosphere but taking advantage of this fact is not easy. This article gives the background you need to set up and aim your antenna. Most HF communications use "sky wave" as the means of propagation. In this mode, the radio wave leaves the transmitting antenna and travels upwards to the ionosphere until it is reflected back to Earth. The ionosphere is located 100-400km above the Earth's surface and because it is made up of several layers (due to solar radiation), is an extremely dynamic and highly unpredictable medium. When the signal arrives back at the Earth's surface after reflection, it is again reflected skywards where the process may be repeated. During each TRANSMITTING ANTENNA D E F1 F2 reflection, part of the signal is absorbed and attenuated, and the amount of signal lost depends on the frequency, density of the atmosphere and the ground conditions; ie, whether land or sea. Fig.1 shows the principle of these "skip" transmissions. Ultimately, there is a limit to how often this process can be utilised. Multihop transmissions can occur over ranges up to 2400km. In order to obtain the maximum performance between two known points (ie, to minimise the free space attenuation between those two points), 70-90km 105-120km 145-190km 320km Fig.1: skip transmissions involve using the ionosphere to reflect signals from a transmitting antenna to a distant receiver. This diagram shows single skips only but multihop transmissions also occur over ranges up to 2400km. 68 SILICON CHIP it is necessary to ensure that the radio wave emitted by the transmitting antenna is propagated correctly along the path. What many operators fail to realise is that it is very important to determine the distance between the two points as accurately as possible. A deviation from the required bearing by only a few degrees can cause a substantial reduction of signal strength at the receiving point. The distance between two ends of an HF link can be determined using the following equations: CosD = SinA.SinB + CosA.CosB.CosL where D is the angle of arc of the greater circle between the two ends of the link measured in degrees, A is the latitude of the transmitting side, B is the latitude of the receiving side, and L is the difference in longitude. For stations in the northern hemisphere, latitude takes a positive sign and for those in the southern hemisphere, a negative sign. The actual distance between the two ends of the link in kilometres is obtained from the arc length of one degree of the circle, multiplied by the angle D for the circuit: Distance (km) = 40,000D/360 The direction of transmission from the transmitting end to the receiving end is given by this formula: Sin CA-B = (CosB.SinL)/SinD For the reverse direction, the following formula applies: Sin CB-A= (CosA.SinL)/SinD Having determined the antenna bearing, an appropriate frequency can be selected from ionospheric prediction charts of the world for the various seasons. These charts indicate the maximum usable frequency for the particular time of the year. C c,c, c::,c:,c::::, 0 OCN 011) 0 gc:, CICI NON 0<"'> .,....,... QC:,OQ QOC>CI Q C:::, U, .,... C:ti.t'>OO NN(")°"" co ......... Cl Cl 11)00 N NM""' ~ 160 would have been for the F2 layer. Thus, the E, Fl & F2 layers of the ionosphere should all be considered, together with the time of day and temperature. All these factors are used to determine the correct angle of radiation. Antenna polarity c::, c:,c:::,o C, C, "' c:::,c:::,c, DISTANCE 1km) USING M XIMUM FDT'S DISTANCE 1km) USING MINIMUM FOT'S 90 80 ... 70 - 60 "'z< 50 :c ~ ...... . C w ,= 160 "':cw :c 320 2!I 320 E ~ 500 z "' ;;; z "' ;;; 700 800 1000 ~ ::, 40 EXAMPLE OF A 1000km POINT-TO-POINT CIRCUIT 30 ~ 700 800 1000 i:i 1200 :;; 10 4 5 6 7 8 9 10 20 30 40 500 z w ,._, 20 ... "':cw 1500 2000 2500 3000 4000 ~ ::, I,._, 1200 ,= 1500 w z "' i:i 2000 2500 3000 4000 FREQUENCY 1MHz) Fig.2: this general propagation chart is used to determine the maximum frequency range & take-off angle angles for a transmission path. Shown is an example for a 1000km point-to-point circuit (within the rectangle) Fig.2 shows the general propagation chart. The shaded area determines the maximum frequency range and take-off angles for the point-to-point link. Selecting the angle of radiation of the transmitting antenna and the angle of radiation at the receiving antenna are two very important matters relating to the correct selection of antenna type. One method used to determine these angles is the skywave transmission graph, such as the one shown in Fig.3. The scales on the chart indicate the distance between antennae and the distance between reflection points, as well as the height of the reflective RF layer and the take-off angle. A simple example shows how this chart can be used. If we wish to determine the angle of radiation for a path of 1000km great circle distance, we can assume the ionospheric reflection point will occur half way between the two stations. For F2 layer reflections, the effective height can also be assumed to be 300 kilometres. By measuring a straight line between the antenna location (left hand corner of graph) and the assumed reflection point of 300km in height, the angle of radiation for the "take-off" can be read by extending the line to the scale at the top of the graph. For the example shown, the take-off angle is 28°. It should be noted that this angle actually changes with time (according to the ionospheric conditions) and temperature. For example, the E layer, which exists about 100km above the Earth's surface, is primarily active during the day. Hence, the angle of radiation of the signal is lower than it 30 TAKE OFF ANGLE 1°) 20 For long distance communications, the polarity of the antenna is relatively unimportant. When ionospheric paths are involved, the rotation of polarisation within the ionosphere generally has a negligible effect on the performance between vertical and horizontal antennas. Antennas should thus be selected for the highest effective gain at the expected take-off angle. This can be determined without any regard to polarisation, provided that the same kind of antenna is used at both ends of the link. It should be noted, however, that noise, whether man-made or natural, normally tends to become vertically polarised and so receiving antennas using this polarity will be more susceptible to noise pick-up. Horizontally polarised antennas are also preferable because the •angle of radiation can be more easily varied to suit the path requirement. This is done by changing the height of the antenna above the ground. From antenna theory, if a ¼-wave antenna is a ¼wavelength or less above the ground, the radiation is essentially upwards. Raising the antenna further tends to lower the radiation angle towards the horizon. Horizontally polarised antennas are best used at a height of less than one wavelength above the ground or where the normal beam angle is more than 15°. In general, horizontally polarised antenna systems are more useful for 10 _____,0 1200 OO 1400 1600 1800 GREAT CIRCLE DISTANCE Fig.3: this skywave transmission graph allows the take-off & arrival angles to be calculated. By drawing a straight line between the antenna location (left hand corner) and the assumed reflection point (this example assumes a height of 300km), the take-off angle can be read by extending the line to the scale at the top of the graph. OCT0BER1991 69 AIR CONTAINING ...-_..-MOISTURE AIR CONTAINING MOISTURE~- HORIZONTAL VERTICAL Fig.4: vertical (left) & horizontal electrolytic grounding systems. Condensation is formed when moisture in the air is extracted by natural salt contained inside the electrodes. The resulting solution trickles down a bed of coarse granulated metallic salts, thus forming an electrolyte which then seeps into the ground. short and medium range links; ie, for distances of 500-Z000km which may require angles of radiation between 25° and 50°. Vertical antennas tend to have their maximum radiation at lower angles. Electrolytic grounding Because the Earth is not a perfect conductor, the ground has an enormous influence on the actual angle of radiation of the antenna. Conventional methods of grounding are often used. This normally involves burying copper mats or rods in the ground and connecting heavy duty cables between these rods and the equipment in question. An alternative method for electrical and electronic grounding is now available in the form of self-contained electrolytic systems. These systems are designed to create their own earth by producing a reliable and constant supply of electrolytic solution. The electrolytic chemical reaction between the grounding electrode and the earth is enhanced by this solution, resulting in a ground system exhibiting consistently low resistance between the electrode surface and the surrounding earth. Fig.4 shows both vertical and horizontal electrolytic grounding systems. These create a network of "roots" in the soil, thus allowing for the dispersion of RF current into the surrounding earth. Condensation is formed when moisture in the air is extracted by natural salt contained inside the electrodes. The resulting solution trickles down a bed of coarse granulated metallic salts, thus forming an electrolyte. The metallic salts in these grounding systems are claimed to have no adverse environmental impact on the soil. The chemical properties of the electrolyte are similar to that of salt water and just like salt water, the electrolyte is an efficient conductor, with a low resistance to RF ground currents. Breather holes at the top of the copper tube allow the salt within to absorb moisture from the atmosphere, thus forming an electrolytic solution. The electrolyte then seeps out through the weep holes located near the bottom of the system into the surrounding soil, establishing a network 90 Fig.5: the radiation angle for the main lobe of a long wire antenna varies with the length of the antenna. An antenna four wavelengths long, for example, will have a take off angle of 25 °. 80 70 60 I I \ 30 ~ 20 ..... -... 10 J 70 4 5 6 8 LENGTH IN WAVELENGTHS SILICON CHIP 9 10 11 12 of roots which reduces the resistance between the rod and the surrounding earth. Resistance values of 5f.l or less are achievable, even if in high resistivity or dry soils. The advantages of the electrolytic rod system are numerous: low resistance, less corrosion, fewer rods needed, smaller area required and virtually maintenance-free operation. No watering or addition of chemicals is necessary. In addition, the effective service life of the electrolytic grounding system has been extrapolated (as the system was only developed during the 1970s) to well beyond 25 years. This length of service for the electrolyte can be attributed to the low dissolution rate of the salts and the use of copper tubes. In order to determine whether RF currents are equally distributed between antenna and ground, a simple test fixture can easily be made. This "jig" is made usinga toroid (normally powdered iron) capable of operating up to 30MHz. The idea is that either the grounding wire or the single wire antenna feed will be fed through the toroid. A coil consisting of several turns of insulated wire is wound around the toroid and each end connected to a small lamp. The RF voltage induced in the coil will then be sufficient to light the lamp, depending on the RF current flowing in both grounding lead and antenna line. For optimum antenna performance (when energy is equal in both legs of the antenna), the brilliance of this lamp should be the same in each position. Constructors may wish to fabricate two such jigs, one to be inserted in the grounding leg and one to be inserted in the antenna feed leg. In this way, the amateur can be sure that equal currents are flowing in the ground wire and antenna feed line. Note: this approach is only suitable for single wire feed, not coaxial cable. The photo accompanying this article shows the simple construction of this current sensing device. Our sample u5ed a 6.3 volt "PEA" lamp and several turns of PVC coated wire. This arrangement was sufficient to light the "PEA" lamp, even when using a low power 5-10 watt transceiver on the 28MHz band. 1 Long wire antennas There are many types of HF anten- BRAND NEW, AMORPHOUS SILICON SOLAR PANELS, CONSTRUCTED ON IMPACT RESISTANT GLASS For the more serious cost effective applications. This one will almost fully charge a fully discharged 12V.6.5AH This simple "jig" can be used to check that the RF currents are equal in both legs of the antenna. It is made by winding several turns of insulated wire on an iron-powdered toroid & connecting the two ends to a small lamp. get cell in one day! Minimum charging current into a 12V battery. 700mA. Voc: 16V (max) 20V DIMENSIONS: 300mm X 900mm. ONLY Order Solar Panel : SP3 DELIVERY BY SKYROAD EXPRESS sgg 2 WATT-12V SOLAR PANEL nas, ranging from multi-element directional antennas to the simplest long wire type. Those operators who live in urban areas have to compromise regarding the type of HF antenna they use. This compromise may be due to cost and/or size considerations. Practically any antenna will enable an amateur to make good contacts under some conditions of propagation. Perhaps one of the easiest and simplest HF antennas available is the "long wire" antenna. As can be seen from Fig.5, the radiation angle for the main lobe of a long wire antenna varies with its length. Generally, a wire antenna only qualifies as a "long wire" if it is more than one wavelength long at the frequency of operation. As can be seen from the graph of Fig.5, an antenna four wavelengths long will have a takeoff angle of 25°. If the antenna is made longer, the directional characteristics will be changed. Instead of the typical doughnut radiation pattern of a ½-wave antenna, the main lobe splits into various sub-lobes. The longer the antenna, the more the maximum lobe becomes "end-on" in response. A low angle of radiation from a long wire antenna can be enhanced by tilting the antenna down towards the direction of transmission. Further reading (1) "EMC Technology", Vol.6, No 1, Jan/Feb 1987. (2) Grounding Systems Data - XIT Grounding Systems, Lyncole Industries Inc, 22412 South Normandie Avenue, Torrance, CA 90502. Phone (213) 320 8000. (3) Communications International Magazine, September 1990. (4) "Radio Electronics", July 1989. (5) "CB Action" magazine, May/June 1988. (6) ARRL Handbook. SC .1.m:...;. r.~1;:i:.1;~ RCS Radio Pty Ltd is the only company which manufactures and sells every PCB [, front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. Can charge 0-12V batteries at approx. 150mA. More than adequate for maintain• ing 12V batteries in cars and boats, or charging batteries in portable appl iances. DIMENSIONS: ONLY , .. ~,.. 150mm X 300mm ,£:-~ ~ Order Solar Panel: SP2 ·'" · DELIVERY BY SKYROAD EXPRESS s29 . 1 WATT-6V SOLAR PANEL Experimental delight! One of these panels will fully charge a 6V-500mA HR nicad battery pack in about 4 hours! 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