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AMATEUR RADIO By GARRY CRATT, VK2YBX Random wire ant.e nna tuner for 6-metres Looking for a random wire antenna tuner unit for the 6-metre (50MHz) band? This unit can be knocked up in a few hours and will solve those antenna hassles. Most amateurs have been in the situation, during band openings, of having the necessary equipment on hand but no suitable antenna available for immediate use. After all, most amateurs do not have the space or resources to install permanent antennas for every band. This antenna tuner solves that problem by allowing a random length of wire to be pressed into service. Thus, when the 6-metre band opens (which may well be the case by the time you read this), you will be able to get to air quickly without too much bother. Antenna problems So how does an antenna tuner im- TRANSMITTER (LOW·Z) 4- prove matters? Let's take a closer look at the basic problem. The first thing that needs to be understood is that modern HF/VHF transmitters are designed to work into a coaxial cable feeder of 50-800 impedance, with a low standing wave ratio. If these conditions are not met, the efficiency will be low and the transmitter output stage may even be damaged. One type of antenna that does fulfill the necessary requirements is a ),./2 centre-fed dipole which has been carefully trimmed to the operating frequency. If it's not convenient to use this type of antenna, the problem can be overcome by installing an antenna tuning unit Construction ANTENNA (HIGH-Z) 1 (b) ) TRANSMITTER LOW-Z °lL l fr ANTENNA (Z UNKNOWN) TRANSMITTER (LOW•Z) Cl (c) .,. -t" .,. IT ANTENNA (Z UNKNOWN) 2 (d) ~ ~ Fig.I: four networks for matching a low impedance transmitter output to a high impedance load. We settled on the ,r-matching network shown at Fig.Id because it provides good second harmonic attenuation. 74 SILICON CHIP (ATU) between the antenna and the transmitter output. Basically, an ATU can be regarded as a variable RF transformer. The ATU is simply adjusted so that the transmitter "sees" the correct 50-700 load impedance, while the output from the ATU is adjusted to accurately match the antenna (or its feeder). In effect, the ATU is used to bring the antenna system (eg, a random length of wire) into resonance. There are several impedance matching schemes that can be used to transfer power adequately from a low impedance source (in this case 50 ohms) to a high impedance load. Fig.1 shows four suitable schemes. We settled on the most complicated configuration, shown in Fig. ld, which covers all possible load variations. It was chosen because of its ability to produce good second harmonic attenuation (typically 35dB) for an output load impedance of 2-3k0. As can be seen from the accompanying photo, the unit is housed in a metal diecast box. The two variable capacitors (both 10-300pF types) are mounted directly through the front of the enclosure, along with the 6-position rotary switch. Be sure to use a diecast box that does not have PCB mounting ribs, as these can interfere with any holes that are drilled in the enclosure and prevent the correct mounting of parts. The inductor, Ll, is positioned between the two variable The antenna tuner is built into a metal diecast case which provides shielding for the circuit. It should be used with an SWR meter (situated between the transmitter and the ATU). You simply adjust the Load, Tune & Inductor controls for minimum SWR. capacitors and has 6 taps to provide optimum matching. Fig.3 shows the dimensions and drilling details. Construction is best done in the following order: (1) Drill the diecast box for both variable capacitors, the switch, output insulator, SO-239 socket (transmitter input), and ground connection. (2) Mount the variable capacitors, switch, output insulator, SO-239 socket and ground lug (to which a quarter wavelength counterpoise will be connected). (3) Wind Lt. This consists of 6 turns of 1.0mm dia. copper wire, wound to a 35mm diameter, and air spaced across 17mm. Fig.4 shows the details. (4) Attach a 20mm length of tinn- L1 : 6T, 1mm DIA COPPER WIRE 35mm ID x 17mm LONG ANTENNA TUNER ed copper wire to each lug on the 6-position single pole switch. It's best to crimp the tinned copper wire after passing it through the lugs on the switch prior to soldering, so that there is some mechanical support for these taps. (5) Fit Lt. This connects between the SO-239 input socket and the "input" terminal on the 6-position switch. The coil is best preformed prior to installation, as the inside of the enclosure is quite cramped at this stage. Make sure that the coil is mounted centrally in the case so that it clears the lid of the enclosure when the unit is assembled. (6) Connect all six taps to Lt, ensuring that they are correctly spaced one turn apart. (7) Using either desoldering braid Fig.2: the final circuit of the Antenna Tuner. It is a 1r-matching configuration with switch S1 used to select taps on the inductor. or braid stripped from coaxial cable, wire the input and output variable capacitors as shown in Fig.4. Wire another braid link between one mounting lug of the SO-239 input socket and the earth lug on the rear panel of the diecast box. (8) Connect a counterpoise made from from a 142cm length of heavy gauge insulated copper wire to the earth lug on the enclosure. This ensures correct loading. Don't leave the conterpoise out - it's vital for correct operation of the unit. Testing At this stage, the construction is complete and all that remains is to test the unit. This is best done in the PARTS LIST 1 diecast case, 120 x 53 x 75mm (Jaycar HB-5020) 1 SO-239 input socket 1 single pole 6-position switch 2 5-64pF variable capacitors (Dauner Electronics) 3 knobs to suit 1 output insulator (Dauner Electronics) 1 earth connector/binding post 1 50cm length of desoldering braid 1 800mm length of 1mm dia. copper wire 1 solder lug 2 3 x 10mm screws plus nuts . & washers (to mount SO-239 · socket) FEBRUARY 1990 75 35 ' ,l 35 32.5 32.5 - ~ "' .J ·I ~. N . J¥ ~__J---=5f ,._% I I "' N I I CASE BACK mMENSIONS IN MILLIMETRES CASE FRONT Fig.3: drilling details for the front & rear panels. receive mode and can easily be accomplished by connecting a 6-metre receiver to the input of the tuning unit and a random length of wire to the output. Check the table shown in Fig.5 for the nearest 6-metre repeater or beacon and adjust the tuner for maximum received signal. This in- volves first switching the inductor for maximum receiver noise and then peaking each of the variable capacitors for maximum signal. The use of a signal generator will assist at this point, although it is not essential. The transmit operation should be monitored using an SWR meter in the input line (ie, between the ATU and the transmitter). In this position, it can be used to ensure a good match between the transmitter output and the tuner input. All you have to do is adjust the Load, Tune and Inductor controls for minimum SWR. Be sure to select low power for all initial transmitter tests and note that the input and output variable capacitors are only rated at about 142cm COUNTERPOISE A short braid link must be run between one mounting lug of the S0-239 socket and the earth lug which runs to the counterpoise. This ensures a good RF earth. CONNECTIONS MADE WITH COPPER BRAIO STRIPPED FROM COAX Fig.4: the coil is supported inside the case by its own input lead and by the tapped connections to S1. The connections are made using desoldering braid or braid stripped from coaxial cable. 76 SILICON CHIP The rear panel holds the S0-239 input socket (at left), the insulated output socket, and the earth terminal for the counterpoise. POLYSTYRENE CAPACITORS Made here in Australia? Made Special to Type? Make sure that the coil is mounted centrally in the case so that it clears the lid when the unit is fully assembled. A 142cm-long counterpoise is connected to the earth lug on the rear panel. 80 watts. Application of higher power levels could result in RF arcing across the plates of these capacitors. This in turn could damage the transmitter. If a higher power unit is required, it is best to select input and output capacitors having larger spacing between the plates. Finally, the author would like to thank Peter, VK2XKB for his assistance in the construction of prototypes for this article. For information on the construction of antenna tuners suitable for VHF use, readers are ref erred to the the ARRL Handbook and the RSGB Handbook. ~ FIG.5: 6-METRE BAND REPEATERS FREQUENCY 50.010MHz 50.075 50.090 51 .020 51 .030 52.013 52.020 52.1 00 52.200 52.225 52.250 52.310 52.320 52.325 CALL SIGN JA21GY VS6SIX KH6EOI ZL1UHF ZL2MHB P29BPL FK8AB ZK2SIX VK8VF ZL2VH2 ZL2VHM ZL3MHF VK6RTT VK2RHV LOCATION Nagoya Hong Kong Honolulu Auckland Hawkes Bay Loloata Island Noumea Niue Darwin Tarana ki Manawatu Christchurch Wickham Newcastle 52.345 52.350 52.370 52.418 52.420 52.425 52.435 52.440 52.450 52.460 52.465 52.470 52.485 52.490 52.510 VK4ABP VK6RTU VK7RST VK0MA VK2RSY VK2RGB VK3RMV VK4RTL VK5VF VK6RPH VK6RTW VK7RNT VK8RAS ZL2SIX ZL2MHF Long reach Kalgoorlie Hobart Mawson Base Sydney Gunnedah Hamilton Townsville Mt. Lofty Perth Albany Launceston Alice Springs Blenheim Mt. Climie Polystryrene Capacitors - 1 0pF to 1 mF Voltage Range 63VDCW to 10,000 VDCW Tolerance - 0 .25% to 10% Allied Capacitors Australia manufactures capacitors to the specification of the customers using high quality, imported polystyrene and aluminium foil with a tolerance of 1 micron. Our capacitors are manufactured to the system of Total Quality Control. We can provide Just In Time delivery if required, together with a Certificate of Conformance if requested. Specific values between 1 0pF and 1 uF are no more expensive than standard value components. You can now design circuits to use a single capacitor rather than a number in parallel or series to achieve a desired value. Personalised labelling is available at no extra cost. Your component code or name can be built into the capacitor. Minimum runs of only 25 allow you to specify a particular value for a prototype run. Delivery lead time for short runs is 3 to 4 working days, and for longer runs is less than 2 weeks. We are also able to produce capacitor styles for applipations such as dual mount, end filled and mini style capacitors. INTERESTED? Call us now on: (02) 938 4690 ALLIED CAPACITORS AUSTRALIA PO Box 740 Brookvale, NSW 2100 FEBRUARY1990 77