| EMC Issues: On commissioning the antenna, tests were made to see if the SGC-230 could tune-up the antenna on all traditional and WARC bands, as indeed it could, and to see what EMC problems would result. Surprisingly, given the intractable difficulties encountered on trying to use an end-fed antenna, the only immediate problem was that of one computer; which flagged a keyboard buffer over-run error and promptly crashed when a signal was emitted on 40m. This was a radiated interference problem. The computer in question had an unusually long coiled keyboard cable, which was probably resonant on 7MHz. A ferrite ring at the keyboard socket (right) eliminated the trouble. |  |
| Otherwise, the choke-balun arrangement was so effective at keeping RF out of the mains, that a Maplin mains intercom system (120KHz FM), which had previously emitted loud un-demodulated SSB noises when an end-fed antenna was used, became usable while the transmitter was running. The house burglar alarm system however, retained a tendency give a tamper alert when signals were transmitted on frequencies of 7MHz and above; but the problem could be eliminated by putting the alarm into 'engineer' mode. |
| Performance: As far as propagation characteristics are concerned, the antenna system described is suitable for NVIS (near-vertical-incidence sky-wave) communications at low frequencies, moving gradually to lower-angles as the frequency is increased; the frequency at which the low-angle radiation becomes useful being a function of the height, particularly at the feed point. In the author's original configuration, the feed point was only some 12m above ground, and only 7.5m above the flat roof of an annex building. Using a 100W PEP transmitter, G3YNH (near Exeter, in Devon) received signal reports of 59+ from all over the British Isles and Western Europe on 40m and 80m. On 160m, where the antenna is electrically short and the system less efficient, 59 reports appeared to be confined to a radius of a 350 - 500 km, depending on atmospheric conditions, but perfectly good 58-59 QSOs were had with stations in northern Scotland (about 1100km). The author's location is approximately 550 ft ASL, with a good take-off to the West, and nightly transatlantic 57-59 contacts could be made without difficulty on 20m. Long-path VK QSOs under suitable conditions elicited signal reports of 56-57. Later changes to the arrangement of support masts raised the average height of the antenna to 15m. This modification increased transatlantic signal reports on 20m by about 12dB, and elicited 59+ reports from Canada on 80m (under suitable conditions). The antenna therefore proved itself to be a very acceptable all-rounder, but was, of course, no substitute for an HF beam. Note that on higher frequencies, the antenna described is effectively a centre-fed long-wire. This means that the radiation pattern breaks up into multiple lobes, with gain in some directions and nulls in others. In the authors's installation. with the wire running on a bearing of 110°, signal reports gave evidence of a null in the direction of central USA on 17 and 20m. Such nulls are a fact of life with long-wire antennas and, if possible, should be taken into consideration when choosing the orientation of the wire. One drawback with the antenna system as described was its poor performance on receive on some bands until a carrier had been sent to the SGC-230 to make it tune-up. This was entirely the author's fault, for using 40m of feed-line contrary to the recommendations in the manual; but was also a necessary consequence of using a choke balun. The attenuation was not serious on frequencies close to odd-order resonances (i.e., on 80m and 20m), but was significant on frequencies close to even-order resonances (i.e., on 40m and 10m). The difference in received signal level between tuned and untuned states on 40m was about 30dB. One short press of the Morse key would bring the antenna to life, but there was no proper provision for listening outside of the amateur bands. In practice however, due to the high noise levels on the HF bands, the lack of receive sensitivity in the untuned condition was rarely a problem. |
| Control Box: In addition to receiver desensitisation when the antenna is untuned, there may also be encountered frequencies on which the tuner has difficulty in finding a unique tuning solution. If this occurs, the tuner will keep on trying to retune during the course of an SSB transmission, and thereby disrupt station operation. The problem occasionally cropped up on 160m in the installation described above. SGC is aware of it, and has provided a solution in the form of the 'reset / lock' control line (revision T onwards), which can be tied to the +13.8V supply to prevent retuning. Access to this functionality can be had by purchasing the optional 'Smartlock' unit, or equally well by building the simple control box shown below: |
(Legend: NO = Normally Open, NC = Normally Closed)
| The SGC-230 must be supplied with power even when receiving on an untuned antenna, because it powers-down with all of the inductors in series with the antenna. At power-on, all tuning elements are removed until the first transmission, and so the bypass button allows this condition to be achieved without turning off the PSU (which, to avoid earth loops, should preferably be the same as the one used by the transceiver). Tuning, or retuning, is prevented by moving the switch to the 'Lock' position; while pressing the 'Retune' button in the auto position forces the coupler drop all tuning elements and retune on the next transmission. It might appear, from this, that the 'retune' and 'bypass' buttons perform the same function; but the 'retune' button forces the tuner to branch to the 'retune' routine regardless of the data in memory, whereas the 'bypass' button allows the tuning to be taken from memory when possible. It is useful to be able to drop all tuning elements when changing bands, and using the bypass button enables retuning to take place in the shortest possible time after having done so. Note that the 'Tuned' LED has a series resistor of 150Ω, while the other LEDs have 620Ω (for approx. 20mA @ 13.8V). This is because the 'Tuned' line already has 470Ω in series with it inside the coupler unit. In the author's installation, an additional high-brightness 'Tuned' LED with a 680Ω series resistor was placed inside the clear junction box on the mast; the idea being that the LED in the control box, with its much lower series resistance, would steal current from it and prevent it from lighting once the control box was built. In practice, the LED in the junction box continued to work. Additional Notes: The UK retail price of the SGC-230 in June 2002 was £369. While this may seem like a substantial investment; it is worth noting that it would cost about as much to buy the components required (don't forget the weatherproof housing), and it is difficult to imagine a more straightforward solution to the problem of how to radiate on all HF bands. Were I to build this installation again, I would try a W2DU ferrite bead balun (see references below), installing small ferrite rings on the composite cable supplied with the tuner. A W1JR-type toroidal cored balun is probably a better choice when using thin coax, but the cable in this case is too thick to be wound around a small toroid, and a large core, having a longer magnetic path, may not provide sufficient choking inductance on 160m. The change to a W2DU balun would reduce the length and losses of the feed line, reduce the weight and windage on the mast, and improve the aesthetics of the installation. Calculated losses for the 40m of RG213 feedline were 1.03dB at 14 MHz for 1:1 VSWR. For 2:1 VSWR at 14MHz, feedline losses increase to about 1.5dB.  Don't use long lengths of RG58 co-ax, especially the 'cheapernet' varieties which have tinned conductors. Conductors for antennas and feedlines should be made from plain copper, enamelled copper, or silver-plated copper. Aluminium is also suitable, but aluminiun to copper junctions must not be exposed to rain. Silvered conductors must be coated to prevent sulphide formation. Other materials have high resistivity and should not be used in RF power transmission applications. Tin-plated wire should never be used in antennas and feedlines. For more information on materials science , see: From Transmitter to Antenna, Chapter 2.In many installations, the tuner and dipole may work perfectly well with no balun whatsoever. In this case, there may be some radiation from the feed line, but this is largely irrelevant in the absence of EMC implications. The balun however, guarantees good behaviour and makes the success of a first-time installation more likely. The installation described above was dismantled in May 2002 and the equipment passed to Andy Cowley M1EBV in Bristol. Reasons for dismantling the system were not due to dissatisfaction with its performance, but due to a desire to increase transmitter power to the UK legal limit (400W PEP). When the SGC-230 housing was opened after 18 months on the mast, the interior was found to be completely dry and pristine; although it should be noted that during the period of use the 12V supply was always left on, so that the 9W or so of static power dissipation would help to prevent internal condensation. Andy's installation is shown below. The SGC-230 and the choke balun are hidden inside a wooden box, and the antenna is fed with ladder-line. The antenna and the ladder line are made from 19x0.2mm silver-plated ETFE coated wire. The antenna is 34m long , with an average height of about 12m. Even with about 10m of ladder line (115mm spacing, PTFE insulators) the antenna gives almost identical performance to that obtained by the author using about the same length of dipole at the same height (average S7 to S9 between Bristol and Aberdeen on 160m). The only problem experienced was that the line had to be pruned in order to enable the SGC-230 to find a match on all HF amateur bands. |
| © D.W.Knight G3YNH, Original November 2000, updated January 2007. David Knight asserts the right to be recognised as the author of this work. |
| Notes and References |
SGC Web Site. (Manuals and other docs are available for download) "The ARRL Antenna Book", 19th Edn 2000, ISBN 0-87259-804-7.1) Input impedance of centre-fed dipoles, ch 2, p2-6
2) KI6WX Directional Wattmeter: "The Tandem Match" ch 27, pp9-19.
3) Ferrite bead balun: "The W2DU Balun" ch 26, p21-23.
"Building and Using Baluns and Ununs", Jerry Sevick, W2FMI. CQ Communications 1994. ISBN 0-943016-09-6.
Chapter 1 on the 1:1 Balun is highly recommended.
Book is available from Amidon Associates:.
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