successful wire antennas PRELIMS

1 1 successful WireAntennasEdited byIan Poole, G3 YWXandSteve Telenius-Lowe, 9M6 DXX, KH0 UNRadio Society of Great Britain RSGB 20122 Published by the Radio Society of Great Britain, 3 Abbey Court, Fraser Road,Priory Business Park, Bedford MK44 3WH. Tel: 01234 832700. Web: 2012. Radio Society of Great Britain, 2012. All rights reserved. No part of this publica-tion may be reproduced, stored in a retrieval system, or transmitted, in any formor by any means, electronic, mechanical, photocopying, recording or otherwise,without the prior written permission of the Radio Society of Great : 9781 9050 8677 1 Cover design: Kim MeyernDesign and layout: Steve Telenius-Lowe, 9M6 DXX, KH0 UNProduction: Mark Allgar, M1 MPAP rinted in Great Britain by Latimer Trend of PlymouthPublisher s Note:The opinions expressed in this book are those of the author(s) and are not neces-sarily those of the Radio Society of Great Britain.

2 Whilst the information pre-sented is believed to be correct, the publishers and their agents cannot acceptresponsibility for consequences arising from any inaccuracies or omissions. RSGB 20123 ContentsContentsContentsContentsContents Foreword, by John D Heys, 5 Editor s 61 antenna 72 Feeders .. 1647 End-fed 1818 Impedance Matching and Baluns .. 2009 antenna Masts and Rigging .. 227 Appendix .. 237 RSGB 2012 successful wire ANTENNAS42 DipolesDipolesDipolesDipolesDipoles3 THE wire DIPOLE is a very effective antenna that can be constructed andinstalled very easily and for only a small cost. The half-wave version of thedipole has become the standard against which other radiating systems arejudged and it remains as perhaps the most effective, yet simple, single-band an-tenna, and one which can virtually be guaranteed to perform well even when used infar-from-ideal the name suggests, it contains two legs or poles.

3 The most common form isthe half-wave dipole, which (not surprisingly) is an electrical half-wavelength basic format for a half-wave dipole along with the voltage and current waveformscan be seen in Fig The voltage rises to a maximum at either end and falls to aminimum at the centre, whereas the current is at its minimum at the end and itsmaximum in the centre. Its feedpoint in the centre forms a low impedance pointsuitable for many sorts of dipole does not have to be a half-wavelength long. A three half-wavelengthversion can be seen in Fig Again the points of voltage maximum are at either endand at a minimum in the centre. Likewise the current is at its minimum at either endand maximum in the LENGTHSA resonant half-wavelength of wire will be somewhat shorter than its name energy in free space (electromagnetic radiation) can travel at the speed of light,but when moving along a conductor it travels more slowly.

4 At HF (between 3 and30 MHz) wires exhibit skin effect, most of the RF energy flows along the outersurface of the conductor. A practical half-wave antenna made from wire needs endsupports; each end usually being terminated with an insulator. The capacitancebetween the ends of dipole and its supports, even when the supporting material isnon-metallic, gives rise to end effect. This effect additionally loads the wire capaci-tively and contributes towards its shortening from the theoretical half-wavelength.(Left) Fig : A basic half-wavelength dipoleantenna with the voltage and current waveforms.(Above) Fig : A three half-wavelength dipole. RSGB 2012 CHAPTER 3 DIPOLES43 The theoretical half-wavelength may be calculated from the expression:Theoretical half wavelength (metres) = 150 / f (MHz)orTheoretical half-wavelength (feet) = 492 / f (MHz)To take account of the end effect and the use of insulators, the length may becalculated by using either: antenna length (metres) = 143 / f (MHz)orAntenna length (feet) = 468 / f (MHz).

5 When using nylon rope it has been suggested that no insulators are required. Inhis book HF antennas for All Locations (published by the RSGB), Les Moxon, G6XN,suggests that when no insulators are used a half-wavelength can be found by usingeither 478 / f (MHz) feet or / f (MHz) further factor which influences antenna resonant length is the diameter of thewire used for that antenna . The formulas above are for typical wire antenna lengths for the amateur bands from 160 - 10m, both when usinginsulators or nylon rope, are shown in Table IMPEDANCESA half-wave transmitting antenna , when energised and resonant, will have high RFvoltages at its ends with theoretically zero RF currents there. This means that theends of a half-wave dipole in free space will have an infinitely high impedance, but inpractice in the real world there will always be some leakage from its ends and intothe supporting insulators.

6 This means that in reality the impedance at the dipoleends is close to 100,000 , a value which depends upon the wire or element thick-ness. At a distance of approximately one-sixteenth wavelength from either end it is1000 , and at the dipole centre, where the current is greatest and the RF voltage islow, the impedance is also Length With insulators Without insulators (kHz)(feet)(metres)(feet)(metres)1850252 ' 11" ' 5" ' 0" ' 1" ' 10" ' 8" ' 9" ' 5" ' 4" ' 10" ' 4" ' 4" ' 2" ' 11" ' 10" ' 6" ' 10" ' 5" ' 2" ' 8" ' 11" ' 5" ' 9" ' 2" ' 8" ' 0" ' 5" ' 9" ' 1" ' 6" ' 10" ' 2" : Lengths of half-wave dipoles. RSGB 2012 successful wire ANTENNAS44If it were made from an infinitely thin conductor wire , our theoretical dipole in freespace would have an impedance of about 73 at its centre.

7 Such an antenna isimpossible in the material world, and a practical half-wave dipole made from wirewill have an impedance at its centre at resonance close to 65 . antennas fabricatedfrom tubing have lower values at their centres, of between 55 and 60 . These im-pedance values also depend upon the height of the antenna above ground, as willbe shown very high values of self-impedance at the ends of a half-wave wire makesend-feeding difficult, and this is why breaking the wire at its centre and connectingthe inner ends so formed to a low-impedance feedline makes a convenient andefficient coupling and match. Suitable feeder is available in the form of twin-lead orcoaxial cable, which both have design impedances lying between 50 and 75 . Thesepresent a good match to dipole exact resonance the impedance at the centre of a half-wave dipole is like apure resistance.

8 At any other frequencies the same dipole will have either inductiveor capacitive reactance at its feedpoint. If the dipole is too short to be resonant thereactance is capacitive and when it is too long the reactance becomes inductive. Ineither case there will be problems in matching the 50 or 70 feeder to the dipole andif the reactances are great, there will be a high SWR on the feeder and considerablepower QA half-wave antenna is something like a conventional tuned circuit where the Q, or Quality factor , is largely determined by the resistance of the coil. Losses in thecapacitor used in the circuit are generally small and are not so significant in thedetermination of Q. A high-Q tuned circuit exhibits very sharp tuning (selectivity) andthis is also the case when an antenna has a high thin wires lowers the bandwidth of a half-wave antenna , but not , short wires that are brought into resonance will exhibit high Q.

9 The shorter thewire in terms of wavelength, the higherthe Q. Small changes in the transmit-ting frequency away from the antennaresonances will give rise to a rapid risein the reactance at the wire will lower the Q, reduceresistive loss and make the half-wavedipole less frequency conscious. It istherefore best to ensure that such anantenna is made from the thickest pos-sible wire consistent with such factorsas the pull on the antenna supports,windage and HEIGHTThe height of a horizontal dipole abovethe ground as a ratio of its design fre-quency is important (see the stand-ard curves of feed impedance againstheight in Fig ). When below abouthalf a wavelength high the radiationresistance at the feedpoint will be re-duced, and down at a height of justone-tenth of a wavelength it will onlybe 25 . This means that a dipole fedFig : Radiation resistance of a half-wave dipole as afunction of height above the ground (reprinted with per-mission of the American Radio Relay League).

10 RSGB 2012 CHAPTER 3 DIPOLES55 THE SLOPING DIPOLEH orizontal half-wave dipoles require two end sup-ports and it is not always possible to providethese in some awkward locations. In such situa-tions a single support, preferably a non-metallicmast or a high point on a building, will suffice,and then the antenna can be arranged to slopedown towards the ground at an angle lying some-where between 30 and 60 (Fig ). The slop-ing half-wave dipole should have its lower end atleast one-sixth of a wavelength above ground,and its feeder should ideally come away fromthe radiator at 90 for at least a quarter of a wave-length. If coaxial feeder is used the braid shouldconnect to the lower half of the performance of a sloping dipole is quitedifferent from one of the horizontal variety and itcan be good for long distance work.