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During 2006, I put together a Magnetic Loop antenna for HF operating and I would like to share the experience with you. This web page will hopefully provide sufficient information and inspiration to you if you have been considering the issues of limited space and a desire to operate on the lower HF frequencies allocated to radio amateurs
My main interest in our wonderful hobby is CW operating, particularly QRP on the 30, 40 & 80 metre bands.
As a direct result of life in the Royal Navy, I had to change location regularly and this saw the size of my back garden change too. Some larger some smaller. Bearing that in mind, the environmental change always forced me to use a fair degree of initiative to get the best out of what was available, in order to operate on HF. My move to the Motherwell area after 26 years of service in the RN was no different and I rapidly discovered that I was going to experience a similar problem at the dwelling I now live in. The house is located in a private cul de sac and the back yard is small. Out of consideration for my neighbours, I knew that 25 foot scaffolding poles to the front and rear of the property, supporting a wire antenna was out of the question and "not in keeping with the surroundings". I think that the council planning department refers to it these days as a "visual impact" issue. The simplest answer would have been to move to the countryside with a huge garden, but regrettably, circumstances did not permit that option. Bringing all these features together has driven me to resolve once and for all, the problem of operating on the lower section of the allocated amateur HF bands.
The laws of physics were working against me from the start, so it was time to get the thinking cap on again !!
For many years, I have nurtured a dream to build and, more importantly, use on a regular basis, a transmitting/receiving antenna which would not be the discussion point of my nearest neighbours. Put a piece of wire up in the air and watch the reaction of your otherwise friendly fellow citizen. You will be reproached for every flicker and dither on their computer and television screens. TVI and RFI are with us to stay I'm afraid and the least path of resistance is to blame the local ham. That aside, the idea of a relatively small, lightweight and portable antenna system, capable of being used on a number of HF bands, has until now, been nothing other than a concept bobbing around in my brain. The laws of physics have always suggested to me that a “small antenna” is not going to work with any degree of efficiency on the HF part of the frequency spectrum, unless those frequencies are above 21 Mhz. As already mentioned, I have a particular liking for the lower HF bands so clearly, I was up against it from the outset.
I decided to gather all my notes from filing boxes and drawers up in the loft and sit down in order to work out a new functional antenna system. The main criteria would have to be an antenna that would fit in to my back yard with ease and be transparent to the neighbours.
My story goes back as far back as 1988 when I began to gather information from a variety of sources in a vain attempt to understand what makes this type of antenna work. I guess it’s the dream of at least some radio amateurs, who possess little real estate, to be able to use an HF transmitting/receiving antenna system that is characteristically small, yet has undeniably been proven to perform at or near to, the expectation of the ever popular half wave dipole at 25 – 30 feet.
To build and use an antenna, that I personally believe could consistently perform at or near to that of a half wave dipole, but yet is small enough to sit in the corner of one’s back garden, even at ground level, was my aim.
I thought that this was a very worthwhile project warranting a number of experiments, thus complying with the spirit of the "Amateur Radio Licence", namely quote the self training in communication by radio unquote. The magnetic loop, as far as I am concerned, clearly fitted this category.
The antenna carries a number of mythical tags and I would like to share my findings with you and help to dispel some of those myths.
I owe some of the technical content contained herein to radio amateurs still presently with us and sadly, those who are now silent keys. To them, I tender a grateful public thank you. I do not claim in any way that what I have achieved is altogether original; nothwithstanding, the loop that I built is entirely my own work and does not conform to anything currently laid down on paper, in photographs or on the internet. It is all experimental measuring, cutting and, dare I say, a degree of frustration and fraying temper!! The one other individual who I must publicly thank in all of this is my chum, Tom Reilly (MM3TRZ). He has consistently shown bags of enthusiasm and a very strong desire to likewise learn how these antennae work. He was also a great source for the tools needed to pull the project together. A great buoyancy aid throughout and a consistently reliable friend.
Introduction
Most of the commonly used wire aerials adapted to transmit rf energy are electrical or E field devices, ie, they generate a large electrical field when excited by RF. The larger the electrical disturbance, the more efficient the antenna. If you recall from your RAE learning processes, where there is an electrical field, there has to be a magnetic field too in order for electromagnetic waves to propagate across free space. The laws of physics quite clearly state that EM waves are made up of electrical and magnetic properties. In the case of the loop shaped antenna, a stronger magnetic field is generated because a very heavy RF current is induced through the radiating element. When this excitation takes place, a corresponding electromagnetic wave is formed and propagated through free space. Because the magnetic element tends to be much stronger on EM waves generated from the circular loop, due to the very high RF current, the antenna is sometimes referred to as a quote Magnetic Loop unquote. Contrary to popular belief, this type of antenna does not only radiate the magnetic component of an EM wave nor does it just receive/react to the magnetic component of an EM wave striking the loop element. It propagates both elements, but, as previously stated, being fed with a strong RF current, the magnetic component is shall we say, more predominant.
For a radio wave to propoagate across free space, it must and does contain both electrical and magnetic components; the magnetic loop is no different in this respect.
I hope this has now demystified the antenna !
Calculations
An excellent Mag Loop calculator is available at http://www.standpipe.com/w2bri/software.htm. I am not saying that this is the be all and end all, but it is a very good indicator when trying to establish a set of rules around which you can build your first loop. I personally found it invaluable. It reinforced many of my notes. The W2BRI website is a great inspiration and if you have absolutely no idea about this particular type of antenna, by the time you have surfed this website, you will know a great deal about building, tuning and operating the magnetic loop.
Materials
I decided right from the start that instead of using a butterfly, mechanically pressed capacitor, I wanted to try and obtain a vacuum variable device. These are glass encased, capable of withstanding extremely high voltages and, in general, very very robust devices. They are also an absolute feat of engineering and beautifully constructed. I never really knew an awful lot about them but, during my time in submarines, I sat alongside the tuning cabinet of the Vanderheim power amplifiers wherein these devices lodged. The whirring and clicking was, in actual fact, the vacuum variable capacitors being rotated in and out for loading purposes.
As luck would have it, I managed to pick up an absolute beauty from Ebay, all for the princely sum of £12 plus £5 Postage & Packaging. For that, I am greatly indebted to Brian, who advertised it for sale. Believe me, vacuum variables can come very much dearer than this. If you've surfed ebay, you will know exactly what I mean. The price could be a stumbling block if you are not prepared to pay the going prices as advertised on http://www.ebay.co.uk Having said that, you can be lucky and pick up a capacitor for a price that will not break the bank. All I would say is do not be deterred if you are serious about putting this antenna on the air. There are cheaper alternatives like a capacitance stub but this will greatly restrict your operating because you may only gain a few Khz either side of the frequency to which the stub is cut.
The vacuum variable I purchased has a limited capacitance range (16 – 80 pf) but it has enabled me (just) to operate on four HF bands. It is rated at approximately 4 Kilovolts. From what I understand, this voltage rating is based on a 75% safety margin therefore the device will handle much higher RF levels. I have not been in a position to put this to the test as I can only generate 120 watts out of the TS530S. This particular capacitor takes seventeen turns to go from having the plates completely meshed to fully open and vice versa. This is good because that, coupled with reduction drive/slow turn rates of the remote tuning DC motor, greatly assists in getting the high Q point spot on. Remember, bandwidth is very narrow when using this antenna and requires retuning as you “cross the band”. This only really applies to the transmitting side of things. I can comfortably tune across 40 metres without having to rotate the vacuum variable, however, if I wish to transmit, the VSWR does need adjusting. The efficiency on various frequencies is markedly different too so you need to be able to get the “tuning spot” exact. Taking this in to account sways many amateurs from getting involved with the mag loop. I remotely retune the loop now and do not give it a second thought so far as this feature goes. The other reason amateurs may not indulge themselves in this antenna is because they generally have sufficient real estate to at least raise a half wave dipole or a quarter wave vertical in their back garden. The need for this type of antenna rapidly diminishes under these circumstances. Notwithstanding, portability and miniaturisation coupled with inquisitiveness do draw a certain number of our fraternity to the loop, so let’s carry on. The one constant and supportive statement I will reiterate throughout is that this antenna is very forgiving and a great deal of fun to use, as you will undoubtedly find out if you decide to build and operate one.
I have been fortunate in that a choice of materials were available to me; RG213 coaxial cable, 15mm or 22mm copper piping. In order to produce a template, I sensibly started with the cheapest option – RG213 coaxial cable.
Based on the calculator recommendations and my own figures, the first “RG213 coax template” was cut to a length of 13.5 feet. I then cut another length one fifth this size, ie, 32.4 inches. The second smaller length was to be used to form the coupling loop; the coupling loop directly links the transmitter to the main radiating loop element. Please note that there is no Aerial Matching Unit (ATU) used in this set up so do not try to connect one in series as it will not produce the results you might otherwise hope for.
I peeled back the outer rubber coating ends of the large loop coax, cut back the dialectric and exposed the centre core. I exposed around 2 inches on either end, sufficient to allow it to be directly connected to the vacuum variable capacitor. I then soldered both the braid and centre core together. Please bear in mind that using coaxial cable for experimental loops does have a draw back. If you induce very high levels of RF energy in to it, a terrific amount of RF current is generated so much so that it may melt the dialectric and, in the worse case scenario, cause a fire.
As you would expect, because of the inefficiencies of the loop elements, a percentage of the induced RF ends up being dissipated somewhere and the end product is generally HEAT. Remember the fire triangle HEAT – FUEL – OXYGEN. You have all the makings of a rather unpleasant fire. Please, do not induce any more RF power than is necessary for tuning/operating purposes whilst using coax. If you intend to use coax in a long term project, minimise the amount of RF you will regularly use, especially if the loop is to be installed in your loft space. YOU HAVE BEEN WARNED.
Loop Coupling methods - Inductive Loop or Gamma Match ?
Inductive Loops
Essentially, there are but two ways to couple the transmitter output to this antenna. Either the inductive loops method or Gamma match. I started off with the inductive loop purely because I did not understand enough about how the gamma match was physically designed or how it worked. Let us go through the design process.
This can be marginally tricky ! The inductive loop which, as I said earlier, is approximately one fifth the size of the main loop, can be put together in around 15 to 20 minutes, dependant on your cutting, measuring and soldering skills. You must spend time getting this right in order to present a reasonable VSWR to the transmitter output stage. More relevant I guess if you are going to use a solid state output PA.
I have managed to get the VSWR on my loop flat and that is on all operating bands (7, 10, 14 & 18 Mhz), believe it or not. Although a completely flat VSWR reading is not the be all and end all, it is a reasonable indication of a satisfactory loading condition between transmitter and the antenna feed system.
Take a look at the diagram below (Fig 1) This will give you a hint or two as to how the inductive loop is formed.
Figure 1
I think that in order to benefit from having to carry out the minimum amount of soldering, try not to cut and break the length of coaxial cable you are going to use at the point which is marked in red. When I made my first inductive loop up, I actually cut it at the point coloured red and, of course, it immediately weakened the structure and, despite soldering at that point, it never quite worked out. You may need to practice a couple of times to get it right but, perseverance is the name of this game.
Just like “ Blue Peter”, I made up five small inductive loops. One an inch longer and another two inches longer than the optimum size (1/5th of main loop). Similarly, I made one an inch shorter and likewise two inches shorter than the optimum size. This then gave me lots to play with when it came to matching feedpoint impedance. You might want to try a gamma match. Since completing this project I have had more time to experiment with various types of wire, copper pipe etc and have inserted a new section regarding gamma match feeding. It is not as mysterious as I used to think.
The inductive loops has a PL259 plug attached and is directly coupled to the RG213 feed via a double female connector all the way back to the transmitter. The length of the coaxial run does not seem to impact on VSWR in any way. I’ve tried both short and long runs without any serious implication. RG213 is, of course, relatively low loss which, if you are only driving 5 watts or less into the loop, is important in terms of efficiency and transfer of power.
Funny story………I comfortably worked a G3 on the South Coast of England recently with 5 watts, thinking that I was actually transmitting 50 watts. When I realised what I was doing, I pressed the right power level button on the VSWR meter and stepped up the power accordingly. What a terrific difference it made. He was able to slump back into his comfortable armchair after that. I need new glasses I think and I also need to understand my VSWR meter’s functionality !!
Gamma Match coupling
Having read up on what a gamma match was, I ended up tweaking the mag loop for performance by changing the feed mechanism to that of a Gamma Match. In the end, it was done for no reason other than curiosity. I decided to have a go at making up a couple of these gamma match devices and see if they would be as effective as the inductive loop. I tried, as shown on the W2BRI website, a heavy length of wire to start with. I cut an 18 inch length of hard drawn copper wire and formed it in to a semi circle. I attached a crocodile clip on one end and screw fastened the other to the copper radiating element of the loop 180 degrees opposite the vacuum variable. W2BRI states that the more efficient the loop the closer in the "crocodile end" of the gamma match will be. I started about 16 inches out and induced a little RF in to the loop element. The VSWR was reasonable. I then moved the croc clip further in and ended up with a 1:1 VSWR. I tried the other bands and got the same results. I had established the fixing point on the copper tubing. In this instance it was 8 inches from the SO239 socket to the point I brazed the copper pipe on to.
Getting the gamma match tubing formed in to a perfect semi circle was rather difficult and it is not as cosmetically pleasing as I would have liked, however, it is functional. I ended up polishing the copper, (takes me back to my days of polishing copper piping onboard submarines hi ! hi ! How did I braze it to the main radiating element ? Just bought a cheap and cheerful blow torch out of B & Q. You will know the type; replaceable container which screws on to the base of the torch assembly. I flattened both ends of the gamma match and bent it until I ended up with an 8 inch gap between the two ends. I simply clamped the end to the main copper loop and let the solder do the rest. Incidently, the SO239 socket was fixed onto the copper gamma match by first drilling out a couple of holes and then nut and screwed on to it prior to installation. I also soldered a small length of hard drawn copper wire to the input side of the SO239 and left a tail of about 2 inches. This was then secured around a brass screw and the screw driven through the radiating element. This is hopefully clear in the picture below.
Fixing the gamma match to the main supporting strut was done by using copper clamps.
........another couple of photos of the end product
In the end you may consider that the gamma match is not for your particular loop. Looking back, although I swore by the 1/5 sized inductive loop made from RG213 coaxial cable, I have now come down very favourably on the side of the gamma match. It really is a case of experimentation. Thankfully, I have been looked upon favourably by the Gods and have managed to get mine to works very effectively on all four bands, (7, 10, 14 & 18 Mhz).
The Main Radiating Element
Back to the beginning again. I mentioned earlier that in order to get a working template, I decided to use RG213. I mounted my vacuum variable from a rafter in the garage and then coupled the main loop (RG213) to the contact points on the capacitor. I secured the temporary connections by wrapping/soldering the centre conductor/braid of the coax, via heavy duty copper strap cables, to the tap points on the capacitor. I wrapped a bit of masking tape around the joints and that was it. I then connected a piece of oval shaped plastic conduit to the stem of the capacitor in order to be able to manually rotate it, whilst under load conditions. As I mentioned before, rf burns hurt and the potential developed across this device is considerable. The coax forming the radiating element was “spread out" in to a circle by tying light string to set points and pulling it out. Again, you can see this arrangement in the photograph below
Here is a photograph of the very first prototype loop hanging up on the garage door. Looking at this afterwards, I concluded that I had not been particularly clever because there is copper pipe running round the door and the aluminium composition of the door itself had the potential to adversely affect tuning. As it turned out…....….it did not have any bearing on its functionality whatsoever !! I got it to load up beautifully without any difficulty.
The Vacuum Variable capacitor is literally taped up in situ !! Note the white plastic conduit hanging down. I used this to turn the cap and tune the loop with 10 watts of RF. "aaah the joys of yellow electrical tape" !
This particular coax loop worked for me and I managed 21 Stateside contacts during the ARRL CW contest earlier this year. This was operating both 7 & 14 Mhz. Reports………….599 both ways. I am, however, mindful that everyone gets a 599 during CW contests don’t they. Nonetheless, it worked well and signals were bounding in across the pond. This did nothing but increase my desire to get the project completed.
The next photograph shows how the inductive loop was connected for test purposes. I took my old Sommerkamp FT250 out to the garage and, once tuned to the appropriate frequency using a dummy load, I connected her directly to the inductive loop.
The picture below shows the inductive loop connected via a double female plug and hooked up to my old FT250
As I mentioned before, you need to take your time making up inductive loops; it will save you a lot of pain and frustration when it comes to coupling up to the main loop and obtaining an acceptable VSWR.
Let's introduce the copper
I spent a considerable amount of time ensuring that I had the best match on 7, 10 & 14 Mhz. This was achieved by varying the inductive coupling loop and the physical length of RG213 coaxial used. With the assistance of the loop calculator program I also increased the lengths of coaxial cable used for the main loop element.
In the end, I opted for the following:
Main loop element = 186 inches or 15.5 feet.
Coupling loop = 37.2 inches or 3.1 feet.
Diameter = approximately 59 inches
So, having done the template testing, it was time to cut to the copper tubing. I fortunately found a couple of lengths of 15 millimetre copper pipe up in the loft. I joined them both together using a straight 15mm cooper connector. The connectors are available in B & Q or any plumbing outlet and, if you are not too good at the soldering, like me, get the pre soldered ones and “go to town” with the blow torch. Thereafter, I ended up with a length of pipe something in the order of 16 feet. I cut off the excess and and ran it through a set of pipe rollers kindly loaned to me for this project by my mate Tom (MM3TRZ). I am not sure how effective a pipe bender kit might be. You may end up kinking the pipe. My engineering skills are questionable, as can be seen by the end product, however, the operating result is functional and that is all that matters. If you dont have rollers/pipe benders to hand, then just make up a hexagonal or square loop. They are marginally less efficient, according to the calculator, but I do not see this as an issue.
I used a piece of 3 by 2 inch timber, approximately 7 feet in length to mount the 15mm copper loop on to. I drilled out two 15mm holes at the top of the supporting timber and filled it with mastic sealer. Slotted the ends of the loop in on each side and left this to set. If totally permanency is required, why not try using “No More Nails” available from B & Q or any of the standard outlets ? That would ensure the loop would not move. I intend to adopt this stance when I have gotten the loop up in the loft space and mounted on a rotator. I also used a 15mm white plastic clip, specifically designed to hold copper pipe, at the base of the copper loop to increase its rigidity. I left the mastic sealer to set overnight.
The next day I set about mounting the capacitor, remote DC tuning motor and coupling components.
Mounting of the DC motor and brass couplings
For ease of use, it is probably well worth considering remotely tuning the variable capacitor you are going to employ. Apart from anything else, non ionising radiation will not do you much good if you are exposed to it for a considerable period of time. You can use the extended length of plastic conduit to rotate the shaft of the capacitor, and that works very well, but it means that you have to have the loop close by. I do not recommend this. Your wives and sweethearts may also get a bit upset with a rather oversized “Victorian rolling hoop” propped up against the bedroom wall !! You'll certainly have some explaining to do.
I have a choice of two remote tuning motors; one is a barbeque spit roast motor which has been stored away for over 10 years. The other, a beautiful, high torque, DC motor from Maplins. I visited the local model shop in Uddingston and bought two 24 inch lengths of 6mm brass rod. I also bought four 6mm universal jointed couplings. They are absolutely ideal for the job of coupling everything together. .
Below is a picture of the Maplins DC motor and the brass universal coupling
I have since discovered that these motors are more or less the same as those used in motorised drills, screwdrivers etc. Whilst not condoning that you should strip a perfectly good motorised screwdriver, its worth asking around your chums if they have a non working one now which you can use. Failing that, the motors are cheap enough to buy from Maplins.
I cut the brass rod lengths to the necessary sizes and then added the brass coupling end fittings. These are held in place with small Allen key type grub screws. In all honesty, this arrangement is inadequate and all that ended up happening was that the couplings and brass rod started slipping, leaving the motor turning and the capacitor stationary. The little grub screws provided are soft metal and therefore useless for this particular application. The answer is to get to grips with the blow torch, heavy duty solder and flux and braze them together. This arrangement works just beautifully. Once the rod cools, slip the plastic universal joint fitting over the ends. I thoroughly recommend the use of these modelling shop universal joints because they absorb any movement that would otherwise take place if DC motor and or VariCap are not properly mounted on the support strut. You can see the universal joint above in a beautiful shade of red.
DC Motor - The motor is controlled by a variable DC power supply. I just happened to have such a supply sitting on the bench here in the shack. It provides for variable voltages 1 through to 15 volts. I wired up a suitably rated double pole double toggle switch, (available from Maplins) in order to be able to reverse voltage feed the motor and a pressel switch from Maplins in order to have absolute control over the feed supply. It is a very simple arrangement and, as I say, works just great. I have no reduction gearing in my setup, however, I guess if you added that, it would make tuning slower and very much more effective.
The other device I use to power the variable capacitor is, as I said earlier, a rather dated spit roast motor
On this loop, you can see that I have had to, in the short term, be somewhat imaginative and I used whatever I could get my hands on, including a wood countersinking drill bit, a piece of dowelling rod with two copper jointing fittings on each end and I hooked the whole thing up. Of the two motors, the spit roast motor is my preferred option. It is strong, nice and “torquey” and reduction gearing is already built in to the black housing on the top of the motor. I know it looks pretty ugly, but it works !
Altogether now !!
So, to summarise; I have fixed the main copper loop on to a length of 3 by 2 timber; I have mounted the variable capacitor on to the 3 x 2 timber using heavy duty tie wraps and I have likewise attached the remote tuning assembly, again using tie wraps. I then got hold of some heavy duty hard drawn copper wire and measured out two suitable lengths. I deliberately kept them a little on the long side to allow for adjustment. I then connected one length to the top of the vacuum variable and soldered the other on to the right hand side of the main loop. I then connected the other length of copper wire on to the slip ring on the bottom of the vacuum variable and connected it to the left hand side of the main loop (you can see the yellow wrapping around the copper wire in the picture below).
Although not terribly clear in this picture, I think you get the idea.
Note that I housed the vacuum variable in a type of chassis made out of the wife’s old bread board, a number of long threaded bolts and nuts. How you get hold of the bread board, I leave very much to your own imagination. It cost me an afternoon’s shopping in East Kilbride and a meal at the local restaurant in recompense !!
Incidentally, if you do not have any experience using a vacuum variable; note from the picture above that there is a concertina type shaft/stem made out of pure copper within the glass dielectric housing. Below the chassis you can just about see a slim stem protruding out of a fairly chunky circular base section. This stem is attached up through into the copper stem. This is rotated and that whole copper circular array moves out and in or up and down, if you prefer; this action meshes/unmeshes the circular veins of the capacitor. It goes without saying that good contacts between the capacitor and the ends of the copper radiating tube must be as sound as possible. I worked a German station recently who told me that solder was not a very good conductor so, in addition to the solder work, it might be an idea to use a nut and bolt arrangement first. To date, I have not found my own set up to be suffering in any way ! As I mentioned before, the loop is very forgiving.
The picture below may give a better viewpoint of the vacuum variable inner workings. You can see the copper stem more or less fully closed. This is a side ways on shot. The top of the capacitor is to the right and the base to the left.
At last - the tuning process
The finished article does not weigh too much so it is very transportable. Tuning the loop is great fun. I initially ran some tests with it propped up in the garage and I connected my old faithful Sommerkamp FT250. With the radio tuned to the right band and the PA stage loaded up using a small 60 watt dummy load, I connected the coax run between inductive loop and the input stage of the FT250. On 40 metres, I could hear fairly weak CW signals. I rotated the stem of the vacuum variable until there was a marked increase in signal noise at the front end of the receiver and those weak CW signals were now S9. That was my queue to put the radio in to transmit mode. I induced around 10 watts into the loop referencing this to the VSWR meter and began to rotate the base of the vacuum variable again. The VSWR dropped very dramatically and settled at 1:1 I was thrilled to bits. I have, throughout these test, used a cross needle VSWR meter. It makes life very easy, so give that some consideration. I worked G3JVC, John in Surrey and we exchanged very acceptable signal reports. The same procedure was carried out on 10 Mhz and 14 Mhz too. Although 10 Mhz was quiet, 14 Mhz was absolutely bouncing and in my excitement at seeing this aerial working, I had to try and remember that I was meant to be conducting tests only and not working as many European CW stations as time would allow. I have recently discovered that the loop will also load on 18 Mhz with extremely satisfactory results.
Since conducting those initial tests, I have continued to refine the loop. By that I mean I have changed the copper strappings connecting the main loop to the vacuum variable. I cut two lengths of microbore copper pipe and used that instead. (more copper pipe for me to polish) and I tried changing the remote tuning motor and use the Maplins motor as standard now. I also changed the inductive loop to that of a gamma match. I use a length of 15mm guage copper pipe initially but settled for a nice workable piece of microbore copper piping. Because it is quite pliable, its very easy to bend and shape. These have all been very positive improvements to the loop's performance.
The plain and simple facts are that on all four bands, I am experiencing extremely low VSWR and, running a variety of power levels and modes, I have achieved some very satisfying results.
These results include:
SSB on 20 metres (30 watts PEP) – Lahore in Pakistan and Crete
CW on 40 metres (50 watts) New Iberia, a suburb of Los Angeles.
I regularly listen around 2300 hours at night and, with the loop pointing shortpath toward a 220 degree bearing, hear Venezuela and Brazil. These signals are 599 +20db. The very low background noise is just superb and its an absolute joy to listen. This feature, apart from anything else, does help to reduce tiredness. In recent days I worked a number of stations in South America with remarkable results. Once again I harp on about the low background noise but it greatly helps when winkling out DX.
The stations mentioned above were worked with the loop at ground level. I did raise the loop up on to a 15 foot pole but was rather disappointed at the variance in VSWR. I now keep it on the ground. It either gets moved out on to the small lawn at the back of the house or remains in the garage if its wet and windy. The results are consistently the same..............very strong reports in both directions inter G and near/middle Europe on 40 metres and the similar with DX on 20 metres. I am not kidding anyone when I say that all of this is very very dependent on good propogation conditions. No D layer, no communication !!
I have recently acquired a huge chunky butterfly type capacitor and have been experimenting with it; I swapped the Maplins DC motor back over and am currently developing a new loop using 22mm copper piping. Here are a few more photos that I took more recently. In order to prevent a series of heavy rain showers soaking the tuning cap and motor, I “installed” a golfing umbrella !! It had no impact on VSWR hi hi !!
You will note the use of a " state of the art rotator". In this instance a North Lanarkshire District Council “wheelie bin”.
I mentioned that I use remote tuning for the DC motor. The modified control box (started out life as a 3 amp regulated power supply). Note the red pressel switch on top of the box. I control the motors movements with this. The yellow circular button is a Maplins supplied voltage regulator that slows the DC motor down dramatically and uses PCM to control the motor turn rate.
Two Copper Prototypes
They are a bit ugly but, they work just fine. The one on the right hand side proved to be less efficient on all bands between 40 & 20 metres and, as a result, was abandoned.
The finished article - a plastic coated loop
I bought some 22mm plastic piping from B & Q recently and disassembled the 15mm copper piping from the stand. After much puffing and groaning, I managed to "persuade" the copper inside the plastic and, below is the result. It greatly strengthened the loop and has had absolutely no adverse effect at all on operating.
.........and the final finished magnetic loop
I am determined not to carry out any more modifications to this loop !
You can view the latest magnetic loop that I have built at the following link : http://www.magwill.co.uk/page10.html
........and finally, I cannot forget to mention the best friend I have in the world. My XYL Margaret who demonstrates terrific patience and understanding in what she refers to as my mad, "nutty professor" world of radio. I publicly pay tribute to her for her love, loyalty, support and assistance without which, none of this would have taken place.
…………and so my story ends. If you have any questions or you want more in depth detail regarding any aspect of the loop build, either ring me on 01698 341126, email me at wb48@blueyonder.co.uk or write to me at the address given in http://www.qrz.com I have made a number of CD ROM disks up so if you want one, just ask.
Thank you for taking the time to check out my website. If you wish to link this web page to another site, please feel free to do so. I hope that you will find some of the content of this web page to be an inspiration.
PLEASE NOW RETURN TO http://www.magwill.co.uk
Good luck.
de Will/GM0HKS
Site last updated at 2330 on 3rd March 2007 