25 August 2015 | f5rds | Leave a comment When summer is back, I often like to go outside for HF portable operations. Personally, I do not like to carry too much equipment so I find the QRP approach quite appropriate to me. Operating QRP requires to select an efficient antenna. Sometimes, light wire antennas can be easily erected when natural supports are available around such as trees or when carrying light fiber pole. However, a magnetic loop still represents a good candidate in terms of compactness, as far as designed to be folded for easy carrying. Multiple commercial versions exist, especially the famous “Alex Loop” of a very simple design, so quite easy to reproduce. So I decided to resume my experiments of magnetic loops that I conducted several years ago and here is the version that I built this summer. Opting for operating during the day, I set as specifications to cover the bands from 40 to 10 meters. The compromise between a good performance and a good compactness oriented me on a main loop with a diameter of 1 meter (more modestly, let’s say that I agree with the choice of Alex, PY1AHD :-)), that I made with a coaxial with outer shield diameter of 9 mm which should theoretically provides efficiencies of 91% on 10 m, 78% on 15m, 46% on 20 m, 21% on 30m and 7% on 40m (calculations done using from my personal spreadsheet). For the outer loop, to avoid adding an horizontal support, the coaxial cable must be rigid enough to maintain its circle shape by its own. As a result, coaxial cable using multi-wire cores are unusable so I opted for the POPE H100 that I had on hand (quite light and single wire core). For the coupling loop (diameter 1/5 of the main loop, so 20 cm in this case), the RG58 is quite suitable. According to my Excel spreadsheet, to cover the bands from 10 m to 40 m, the variable capacitor must be able to cover from 10 to 170 pF. As always with magnetic loops, in order to limit the losses, it is necessary to avoid going through a frictional contact in the variable capacitor so the best is to put in series the two cages of a double cages variable capacitor (the ideal is the butterfly type – quite difficult to find – but a classic double-cage variable capacitor is just as good). In addition, setting in series the cages increases the insulation voltage which makes it possible to use the variable capacitor of an old BCL in transmit (while keeping reasonable powers anyway but one can go up to 30 or 40 watts in CW on 40 m which is well beyond the necessary specifications in QRP !). I had in hand a 2 × 540 pF or about 12 to 260 pF measured with my capacitor meter once the two cages are put in series. Not exactly what I needed because with this variable capacitor, I can cover only from 15 to 40 meters bands and maximum capacity is unnecessarily too high so I will have to replace this variable capacitor in the future or I should blow out some blades ! The advantage of this variable capacitor is that it has already a gear in place on the cage, which greatly facilitates the tuning. The supporting tube is made up of three short sections of gray electrical tube of 25 mm external diameter (these tubes naturally fit one end into another so you just have to buy two to make the three sections). The upper fastening collar is thermomolded with a hot air gun from a fall of these same tubes and attached to the coaxial with rislan collars. The tubing and collar were spray painted in black. The variable capacitor box has been thermoformed from a can of cat litter deodorant (it’s cool, the antenna smells super good but the painting does not hold too much on this kind of polypropylene, one must scratch the surface first with sandpaper and then put a coat of primer before passing the final layer, it will be for later). In QRP, given the low power, we can reasonably stand close to the loop when tuning it manually, one hand on the TX and the other on the antenna (the tuning is of course very sharp, which confirms the good Q). As usual, we start by tuning in the receiving mode for a maximum signal and then transmit for the lowest SWR. On all the bands I can achieve a SWR of 1:1 (in outdoor, because in indoor it is very dependent on the surrounding metallic masses). During the first test (France, Allier-03) with my Yaesu FT-857 set to 5 watts and powered by a 12V 7Ah lead gel battery not very young, I made successively 3 QSO in CW on 30m with a G4, an OZ1 and an HB9 then a phone QSO on 15m with an M0 who was operating mobile in the suburbs of London and that passed me 55 to 59! The possible next steps envisaged are 1) the modification of the variable capacitor to be able to go up to 10 m while keeping the 40 m and, 2) if I find some time and courage, a new version with a stepper motor driven by an Arduino for remote use during the winter days. In summary, you probably have everything you need on hand or not far, it’s quick and easy to do and it works every time so if it’s not already done, try to build one! Wishing you a nice building and good traffic. Olivier, F5RDS