MAGNETIC LOOP SYSTEMS SIMPLIFIED
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1 MAGNETIC LOOP SYSTEMS SIMPLIFIED By Lez Morrison VK2SON Many articles have been published and made available on websites recently. Unfortunately they have tended to make construction sound complicated and difficult. This is not really the case as I intend to take some of the mystery out of being able to build a workable antenna. Many of us engineers fail to communicate at a level which is understandable to amateurs. Please note:- our hobby is called "Amateur Radio", not Professional Radio. For those of us that are familiar with mathematics, physics, etc, I have included a limited amount of formulae which will enable those so inclined to investigate these antennae to a greater depth. Many articles quote all sorts of complicated reasons for loop diameters and materials, such as... Should the diameter be one eighth, one quarter wave or some other size. Will silver plated copper perform better than ordinary plumbers copper tube, or aluminium? The loop antenna is only a very high Q series resonant circuit. I 2 R losses are often quoted by purists as the end-all of usable antenna designs. How many of us take into account I 2 R losses when putting up a dipole, vertical or other type of antenna? Do I 2 R losses really mean anything to the average amateur? All of us have seen and used less than efficient antennas with no thought to losses other than what is seen on an SWR meter (most of which have little bearing of what the antenna is doing.) How many transmitters are only heating up feeders and non-resonant antennas? but still radiate a signal? 1
2 I won't go into the subject of how to transfer energy from your transmitter to the antenna, other than to indicate that which I have used in my prototype antenna. I leave you to investigate practical methods using your knowledge, try-it and see methods. We all know that "Q" is degraded by circuit resistance, and that bandwidth is increased by circuit resistance, a compromise may sometimes have to be accepted, but good techniques will still achieve an acceptable result. When it comes down to radiator resistance versus your wallet you will quite often accept a transmitted or received signal down 2 "S' points when compared to a better type of antenna. This is always in the mind of beginners to our hobby or those of us on a limited income. If money is no problem, then why bother to make an antenna when one "off the shelf" will most likely do everything that you require? If you are expecting the same performance from your "home brew" antenna as that obtained from professional multi element arrays, then pay particular attention to materials and methods of reducing losses. In my investigations I have found that "ex-ww2" capacitors will work well, but be careful of how they are mounted and that low loss leads and soldered joints are used, when testing magnetic loops or any high "Q" circuit be aware that dangerous RF voltages are present! 2
3 Always use an antenna analyser to check for resonance, once resonance has been achieved move away from loop before applying RF energy to the antenna. Should an antenna analyser not be available, a "dip" meter can be utilised, or with just a watt or two from your transmitter through an "SWR" meter to the antenna you will soon find resonance. (Note) if using this method make sure that you do not cause interference to other stations by checking the frequency first. Pick a time that few people are using your particular frequency, late at night or during times that your choice of band is "dead". As can be seen from the diagram my loop is 1135mm diameter, constructed from 3/4" soft copper tube. This type lends itself to simple forming of a circle. The straight length of tube should be approximately 4 metres long, a bit more or less is not critical. Should you want the loop to cover 40 and 30 metres, the length should be reduced to 3.7 metres, always remembering that less inductance will entail more capacity and a higher "Q", thus making the bandwidth less at this frequency. If you have a piece of chipboard about the size of your intended loop, draw a circle then knock in a 50mm nail every 100mm around the circle, gently bend the copper tube against the nails, encouraging it to lay flat on the board. The toroid will have a space between it's inner diameter and the copper tube of about 2mm. Wind 7 turns of "hook-up" wire, (Insulated single strand) onto the toroid core and apply insulating tape to hold it in place. I made a 50x5Omm piece of Perspex with a SO239 socket screwed to it to terminate the balun wires, ensure that all four screws holding the socket have a wire connecting them with solder. Alternatively a piece of single sided PCB can be drilled to take the socket and the outer 3
4 flange of socket can be soldered to it. Makes for a better job, this also makes for easier portable use. Solder capacitor leads to each end of loop using copper strip or soldered braid. This will hold loop and stop it from springing apart! Alternatively cut a length of 1 inch (22mm) PVC tube in half length ways and fit either side of open loop using brass nuts and bolts, the PVC can be made to fit neatly by heating it with a hot air blower or a hair dryer. If a capacitor of small size is not available, mount the capacitor on a piece of insulating material keeping the length of leads as short as possible, ( I found that 50mm not to cause problems) The length of copper tube or brass strip for 80 Metres was 9 metres long, for 80 mettres 40 Metres = 4 Metres long 30 Metres = 3.7 Metres long 20 Metres = 2.6 Metres long 18/15/12/10 Metres =2.4.Metres long. Feel free to experiment with loop diameters. The above worked for me. For metres a capacitor of 10-60pf is sufficient to resonate loop. For 80 and 40 metres pf will be required. Although a vacuum variable is the best choice I found that old ones could be used provided that they were protected from the weather, even changes in temperature can vary resonant frequency by as much as 100 khz. If the larger loops are too mechanically unstable to handle without warping, mounting them on a circle of plywood with plastic saddles will ensure rigidity. When testing and using the magnetic loop make sure that no person will come into contact with it, the loop can be 4
5 made safe by fitting the copper tubing through soft plastic tubing before fitting balun and capacitor, I am sure that you will decide on a method that suits you. Equivalent circuit of series resonant loop WHERE L = INDUCTANCE IN uh a = CONDUCTOR RADIUS IN INCHES b =CONDUCTOR LENGTH IN INCHES 5
6 ln =NATURAL LOG = x COMMON LOG (base 10) I 2 R LOSS (D.C. RESISTANCE) OF 40 METRE LOOP = 50mΩ 10 AMPS = 0.5 WATT In practice I found turning the capacitor extremely touchy. I therefore used a reduction drive which made tuning more acceptable. I later used a Jaycar 12 volt reversible motor connected to a plastic rod to turn the capacitor. The motor assembly had to be placed at right angles to the loop or it would de-tune the loop. Any suitable method may be used to install the loop. I firstly hung the loop by a length of nylon cord from the top of the loop to a post 3 metres high, so that the lower side of the loop was about 1 metre above the ground. Unfortunately a step-ladder was required each time a change of resonant frequency was required. Mounting the loop upside down seemed to cause no effect, so I could easily tune capacitor when it was at bottom of loop. Note that balun can be positioned anywhere around the loop to within 30cm of the tuning capacitor with no apparent effect on the SWR or performance of the antenna. My first on air test with 20 watts allowed me to make contacts on 40metre with VK3, VK4 and VK5 stations. I compared signal reports using a 40metres inverted V antenna at 15metres high and was surprised to find 2-3 "S" points between the antennas. As an antenna for restricted QTH's and portable use I think many of you will be pleasantly surprised with results achieved. Don't expect this antenna to outperform yagi or similar types, but do expect to be able to use your radio in adverse locations. The antenna is quieter than a wire dipole and is less prone to electric wave inference (lightning, power cables) and can be rotated to limit some kinds of interference. 6
7 Mechanical construction of 40metre loop. Loop should be mounted vertically(radiation in plane of antenna) Antenna can be tuned from Mhz with a capacitor of 10pF- 100pF. Diameter of loop 1135mm. Material used:- 4Mts X 3/4inch soft copper tube. 10pF-100pF variable capacitor. Jaycar Toroid type cat No mm hook-up wire. S0239 UHF socket. Dimensions of loop not critical,i have used 25mmX3mm brass strip. Loop diameters from 1.00mt-1.5mt work. "ex WW2" capacitors are large and will need to be supported on insulating material such as Perspex.Keep connections short,i found 50mm not to cause problems. The Toroid balun can be mounted on a piece of PWB then a S0239 socket can be soldered to it allowing the coax cable to be disconnected for transport. Any suitable insulated support may be used to hold loop 1Mt Or more above ground, I used a length of 90mm PVC tube. You may even want to mountloop on an insulated pipe on a rotator. 7
8 8
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