Stealth Antennas

1 IPublished by the Radio Society of Great Britain,3 Abbey Court, Priory Business Park, Bedford MK44 3WH, UKFirst published 2010 Radio Society of Great Britain, 2010. All rights reserved. No part of this publicationmay be reproduced, stored in a retrieval system, or transmitted, in any form or by anymeans, electronic, mechanical, photocopying, recording or otherwise, without theprior written permission of the Radio Society of Great s noteThe opinions expressed in this book are those of the author and not necessarily thoseof the RSGB. While the information presented is believed to be correct, the author,the publisher and their agents cannot accept responsibility for consequences arisingfrom any inaccuracies or AntennasbySteve Nichols, G0 KYA RSGB 2012 Contents1 Introduction12 Electrical and other safety issues213 Using your roof space294 Stealth Antennas for the loft375 External antennas756 Over-the-roof Stealth antennas817 Examples of external Stealth antennas998 Let s get really stealthy123 9 Commercial Stealth antennas13910 Avoiding interference and EMC issues16111 Maximising efficiency177 Appendix 1 Glossary and abbreviations184 Appendix 2 Other RSGB antenna books197 Index201 RSGB 201237 THE SIMPLEST (and probably cheapest) antenna you can build is a loft-mountedhalf-wave dipole (Fig ).

2 This is simplycut for your band of choice according tothe chart and fed with 50 HALF-WAVE DIPOLEA half-wave dipole is cut to lengthaccording to the formula: total length (infeet) = 468/f, where f is the centre frequency in MHz. The metric formula istotal length (in metres) = Typical dipole lengths for the bands from40m to 6m are given in Table I was first licensed I strung up a half-wave dipole for 10m in my worked a treat with a very low SWR across most of the band . What s more itworked stations with many contacts being made across Europe and the about any type of insulated wire will work and you don t need endinsulators just use a staple gun to tack the wire to the wooden rafters and letthe final four inches or so dangle free away from the wood. This is a safetyfeature and prevents the antenna arcing to the woodwork, although that is veryunlikely to happen if you keep the power levels down.

3 I usually leave my powersetting at about 25W and seldom use more than 50W. Do not use a linearamplifier!A so-called chocolate block electrical connector will let you connect theends of the dipole toyour 50 coax, whichcan then be ledvertically away atright angles to thedipole to prevent RFpick-up and help keepthe antenna possible use a balunor choke as you haveinstalled the antenna4 Stealth Antennas for the loftTable : Half-wave dipole (MHz)Total length (ft in)(metres) : The half-wave dipole is a simple antenna ,but very effective. RSGB 201238 Stealth Antennascheck the SWR in the centre of the band . Now check it at the band edges too. Ifthe SWR is better at the lower edge of the band than the top end this means it istoo long. If it is better at the top end of the band it is too short.

4 If it is too longdon t cut it just fold the ends back on themselves and either twist the wirestogether or use nylon cable clips. Electrically it will appear shorter to yourradio and have the same effect as cutting t be surprised to find that you have to shorten the antenna slightly onceyou have it in position. The PVC covering of the wire and capacitative endeffects will make it appear too long electrically and it is easier to shorten anantenna than lengthen you have the lowest SWR at or around the centre of the band , stop aslong as it is less than about 2:1 you will be fine. In reality you should be able toget it to :1 or may struggle to get the 30m and 40m versions in your loft they maybe too long. If that is the case see the section on the zig-zag dipole later on. Justabout all of the others should fit within an average-sized is best to mount the centre of the antenna as high as you can in the apex ofthe roof.

5 This helps keep it away from electrical wiring and copper water tanksthat might otherwise affect its tuning. It also helps with your radiated signal asit is the current portion of the antenna that does all the work and the currentmaximum on a half wave dipole is in the dipole legs can either be tacked along the central apex with a staple gunor brought down in an inverted-V configuration - see next half-wave dipole is strictly speaking a monoband antenna , although a40m (7 MHz) version can be used on 21 MHz (15m) albeit with a slightly higherSWR. You can use a dipole on something other than its design frequency, butyou will need to use an ATU and the performance will not be very good. But asa get you going compromise it will work. As explained in our later chapter onmatched and unmatched losses using a long run of coax with a high SWR willresult in high losses and is not you really want to use a single dipole in the loft on a band other than thatof its design frequency you are better off feeding it with 300 or 450 ribboncable, but this is less easy to handle than coax and must be kept away frommetallic objects.

6 Your dipole then becomes a doublet. If you do want toexperiment with this, cut your dipole for the lowest frequency you want to useand then use it with open wire feeder on this and higher are other coax-fed multi- band alternative that you can readabout INVERTED-VThe dipole doesn t have to be laid out in a straight line. You can, in fact,arrange one as an inverted-V with the centre at the apex of your roof and thelegs coming down at an angle. In fact, there are a number of advantages indoing this. The first is that an inverted-V loses some of its directional RSGB 201239characteristics. A dipole usually has its maximum radiation atright angles to the wire with little radiation off its ends, butwhen arranged as an inverted-V it becomes slightly moreomnidirectional, as shown in Fig will find that as an inverted-V a dipole will need to beup to about 4% shorter than if it is in a straight line.

7 Don tworry about this. Cut it according to the chart and fold it backonce installed to bring it to as close to a 1:1 SWR as you , the impedance of a half wave dipole in free spaceis more like 70 - 75 , not the 50 our radio expects. This canresult in an SWR of about :1 :1. By narrowing the anglebetween the legs the impedance changes and the SWR is reducedto closer to 1 , and it is a big but, the angle between the legs of a dipoleshould not fall below about 70 - 80 . For best results with thistype of antenna , the apex angle should be kept between 90 and110 . Less than this and the radiators start to become parallelto each other and signal cancelling will start to occur. Aboveabout 120 the antenna starts looking like a standard dipole,minimising any of the feed impedance and shortening optimum apex angle is 90 - 120 , but it isn t that pitched roofs are actually very steep and you shouldcheck to see what the angle is on yours.

8 If you wish to fit aninverted-V you might be better off putting each leg across theside of the roof rafters to reduce the angle. That is, install it anangle with the dipoles for several different bands can beconnected to the same feeder, as shown in Fig to provide amulti- band Stealth Antennas for the loftFig (a): You can see thatwhen erected in a straightline the dipole is : This five- band loft-mounted antenna has been used at two locationswith great (b): But when erectedas an inverted-V you gain acouple of S-points (10 - 12dB)in the directions in which thedipole wires point. RSGB 201240 Stealth AntennasMULTI- band PARALLEL-FED DIPOLES WITH ZIG-ZAG (NON-INDUCTIVE) LOADINGThis antenna has nothing new about its design, but it does bring togetherquite a few useful ideas and characteristics. These are: Fully no-tune antenna system for five popular HF bands; Suitable for SWLs, QRP, M3 licensees and PSK31 operating plusoccasional use up to 100W, but watch out for RFI at high powers.

9 Do notuse a linear! Uses non-inductive (zig-zag) loading for actual mileage may vary depending on the type of loft you have, thetile type and the amount of metalwork in your attic. I have used this antenna attwo different modern semi-detached and detached houses and it has alwaysworked well. What surprises me is just how well it does work so well, in fact,that I have yet to find another experimental indoor antenna that can beat it,including magnetic loops, crossed field loops and EH has been compared with the following: 85ft W3 EDP end-fed that went up to the top of a 60ft oak tree in mygarden with a 17ft counterpoise the multi- band dipole beats it handsdown on 20m and up and offers much lower noise level. On 40m theyare fairly even. Capco and MFJ magnetic loops mounted in the loft and outdoors at aheight of four metres the multi- band dipole was consistently eitherequal or 1 - 2 S-points better.

10 Commercial five- band , no counterpoise, vertical at 30ft the multi-banddipoles beat the vertical on 20m by 1 - 2 S-points. But the vertical beatthe dipoles on 17m on 15m by the same amount. They were very similaron 10m, although the vertical also tuned 12m. The SWR was flatter onthe multi- band dipoles. While trying to read weak CW from the 3Y0 XPeter 1st Island (Antarctic) DXpedition signals were inaudible on thevertical and S1 - S2 on the dipoles on antenna consists of separate half-wave dipoles for 40, 20, 17 and 10m(as shown in Fig ), cut to a length of 468 (feet)/frequency. Only the resonantdipole will accept current at the desired frequency the others are effectivelyignored and offer a high impedance. These are fed via RG58 50 coax at acommon feedpoint at the very apex of the centre of the loft just buy an electricalchocolate block connector.