About Radio 67 Magnetic Loop Antenna for 17,20 and 40 metres

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• published: 03 Dec 2012
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In this video I show a Magnetic Loop Antenna that's cut for use on 17 metres. I've also use the loop on 20 metres and 40 metres although it's not very efficient on 40 metres it has allowed me to get out with a very small antenna where it would have been impractical to put up a temporary wire for 40 metres. I built back in 2009, so 3 years ago, and although it hasn't had a lot of use as I only use it portable it works well and could be a good solution for someone with restricted working conditions. The main reason it hasn't had a lot of use was because of the hassle of manually tuning the capacitor but I've recently developed a little remote mechanism to allow easy tuning. I've made a bunch of videos about that mechanism and I'll put the links below. At the end of this video I have a brief contact with Barrak 9K2UU in Kuwait, he's 5 by 9 with me but I only receive a 5 by 6 but given that I have the antenna on the floor at the time it's not too bad a report. The antenna is made from 22mm OD copper pipe. The inside dimensions of the loop are 1.3 metres by 1.3 metres. Each of the 4 corners are joined with a 90 degree elbow. The gap at the top where the capacitor goes is 45mm. The gap is maintained by a sheet of clear insulated material that's 135 x 90 x 4 mm, all non-critical. The bottom section of the loop is joined with a modified 22mm straight pipe coupling (see video) The gamma rod is made from a 600mm long length of 6mm OD copper tube. The input end of the gamma rod, where the centre coax lead connects is held 100mm above the bottom element of the loop by a piece of insulated material. The adjustable gamma link fixes into position with two bolts and wing nuts, the length of the adjustable arm is such as to hold the gamma rod 100mm above the bottom element of the loop. The variable capacitor is a two gang 43pf unit with 1.5mm air gap either side of the moving vane. With the two gangs in series this gives 21.5pF total and this covers both the 17 and 20 meter bands. To operate on the 40 metre band I use an additional 95pF capacitor in parallel with the variable capacitor. The 95pF capacitor is made out of 8 lengths of thick coax, all of the screens are connected together, the ends of 4 cores are connected together to for one side of the capacitor and the other four cores are connected together to form the other side of the capacitor, there is no outgoing connection from the screens. I've only ever run the loop at 100 watts and I've had a bit of fun with it. Do remember that there's quite a high voltage on this antenna even at 100 watts and it will give you or anyone else an RF burn if you get too close while transmitting. By the way I added the bit with the QRZ.com information about 9K2UU as I thought it was interesting to see how from a bit of old copper pipe it opened up a tiny window into another man's world. This is my first contact with 9K2UU but if the band had have been properly open I would have liked to have spent longer talking to Barrak. Thanks for watching and remember I like to hear your comments and feedback. Kind Regards ... Andy GWØJXM Links to other appropriate videos of mine The idea for a drive system started in about Radio 61 http://youtu.be/Fd6n-iVL0Xw?hd=1 I build a the first drive unit to work with a domestic radio capacitor in About Radio 62 http://youtu.be/GXex2112b_E?hd=1 I give more details about the first drive unit in About Radio 63 http://youtu.be/8cqIl0Ei0G0?hd=1 About Radio 64 shows the first part of the construction of a drive mechanism for my magnetic loop antenna, here's the link http://youtu.be/seoYj0OeKIE?hd=1 About Radio 65 update on Magnetic Loop Antenna capacitor tuner http://youtu.be/y7Jf36r2RSQ?hd=1 Below is a link to W2BRI's site where he has a Loop Design program from KI6GD http://www.standpipe.com/w2bri/software.htm