Bluebell Loop Antenna

Greetings Everyone,

Guess I finally “nailed” it. I built a couple of the mark two varieties, and not only do they work well, they also have survived a “drop test”, as they are a lot more robust! They are also smaller… This project is for a 2 metre “magnetic” loop. It works very well for direction finding, and you can transmit on it.

Figure 1. Completed loop. Mark 2, mounted on top of half a BNC plug.

Parts list

A small piece of double sided printed circuit board, 28mm x 14mm. This must be the standard 1.6mm thick type, on a fibreglass substrate. If you substitute different materials, the size required will change. This PCB is being used not only as a mounting for the capacitor, but is acting as a capacitor because it is two sheets of copper, separated by fibreglass. If it is thinner, the capacity it gives will increase, and if it is PTFE PCB (you got money to burn? ) it will produce a different value. You will then be on your own, unless you give me some of what you want to use, and I make some measurements, and can then tell you what size to make it… Note: Paxolin board is NOT suitable.

131mm of 2mm silver plated wire. (1.5mm is fine. Dave, G7PPB is experimenting with 4mm copper brake pipe. That will work great, but good luck on bending and soldering that!).

75mm of 1mm silver plated wire. (This fits BNC plug centre pins, check what size you need for the plug you are going to use.).

A variable capacitor. I bought a stock of these, and will sell them to you for £1.25, from which I make ½ a penny profit!

A plug to fit the top of your radio, although you could mount this on a BNC plug and use a patch lead to get to the radio.

About 10mm of PTFE sleeving. Free gift from me. (So THAT’S what my ½ p profit went on! )

70mm of any old wire that will go through your BNC plug (with the pin removed), and that can be soldered. This is used as a jig, and is not part of the finished project.

Construction

First off, bend the 2mm wire into a loop of approximately 52mm external diameter. You will discover that the wire is not long enough, and you should have a 20mm gap between the ends. “Roundness” is only visually important, but the 20mm gap is very important. See figure 2. The gap is not seen, but easy to visualise, what is on the reverse is the same as seen.

Cut the circuit board to size, this is also critical. Not only does the circuit board provide a nice mounting for the variable capacitor, it also forms a substantial part of the (rather small) capacity needed to bring the loop to resonance. A junior hacksaw does this well, but dress the cut ends with a file, and ensure that there are no small slivers of copper shorting the two surfaces together. “tickle” all copper edges with a file to get rid of any sharp spots.

Prepare the plug. This is best done by removing the back end of it, (I can turn this off in my lathe for you). The idea is that the main loop solders across the back of the shortened plug. A brave, impatient soul could hacksaw it off, I did that before I owned a lathe!

The capacitor is supplied set to MAXIMUM capacity, even though it appears to be only half meshed. It is at minimum when the moving plates are fully engaged with either of the two sets of fixed plates. Set it so that it is Just away from minimum. (See “tuning notes”). Note that the capacitor has three connections, the ones opposite are used, the centre one is not. The capacitor is what is known as a “differential trimmer”. Normally all three connections would be used, but for our purposes the centre one remains unused, and should stick out horizontally from the body of the capacitor.

Ensure the PCB is clean both sides, (“Scotchbright” pads do a good job here, and also for cleaning the capacitor tags, and the silver plated wire, just don’t let the XYL discover what you used it for, Hi Hi!), then twist the tags so that they lie flush on the board surface, one either side. Solder in place. Ensure that the centre boss (underside of the capacitor) does not make contact with the PCB. This should be impossible as the ceramic body of the capacitor should rest on the top edge of the PCB. See figure 2.

Figure 2.

Mount the “jig wire” to one end of the main loop, do not “over solder” Do this such that the wire leads down through the centre of the loop, and threads through the centre of the plug. The idea here is that this “jig wire” holds the plug in place at the bottom of the loop, and in the correct orientation such that you can then solder the main loop to the back of the plug. I have the scars to prove that trying to hold it all together and solder it is not a good idea!

At this point, you should be able to remove (unsolder) the “jig wire” from the end of the loop, and slide it out of the plug body. This wire may now be discarded.

The next thing to do is mount the PCB at the top of the main loop, as shown in figure 2. It is important to place it as shown, with the ends of the loop in the centre of the PCB. you will notice that in the picture, the bottom corners have been removed. You need to have room to get the side cutters in here and maybe clip a corner off. You could also file a groove in the bottom of it, either way, what you are doing is reducing the value of the capacitor the PCB provides. On my second one, I found that with the trimmer at minimum value, it actually worked straight away, but the idea is to use the trimmer as a fine tune. Unless you are using more than minimum capacity on the trimmer, you will need to “adjust” the PCB, then increase the trimmer. See “Tuning notes”, and figure 4.

Now, make a loop of the 1mm wire. This is the “coupling” loop. It needs to be 15mm INTERNAL diameter. Make it so that it looks like a “lollipop on a sick” Use 75mm of wire and thread the sleeving on the “stick”.

This loop is not continuous, make it with a very small gap between the end, and the other end where the stick is. The very top of the “stick” will need to be bent to allow the coupling loop to lie within the same plane as the main loop. If you don’t get that right, you are likely to get a loop whose directional accuracy will suffer. See figure 3.

Take a guess at mounting this loop on the plug pin. It will be too high, so measure how much you need to remove. When trimmed, solder to the pin, and assemble into the plug body.

Figure 3 coupling loop installation.

Ensure that both loops are in the same plane, and solder the end of the coupling loop to the main loop, where that connects to the plug body. Not too much heat here, if you overdo it, the main loop may detach from the plug body, and profanities are not part of the construction notes, Hi!. (Been there, done that!). Figure 3 shows how to place and connect the coupling loop.

Double check your construction, and set your handheld to LOW power. Tune to 145.450Mhz. Connect the loop via a suitable (i.e. can work properly at 2 metres) meter, and then check the VSWR. You should be able to achieve less than 1.5:1 by tuning the capacitor.

Tuning Notes

Tuning is quite sharp. Tune for 145.5 Mhz, and the VSWR at 144.5 Mhz will be bad! I suggest tuning it to 145.450 Mhz, we can go up or down a couple of channels if need be with no dire results, Hi.

Tuning tip. With the capacitor set at MINIMUM ((moving plates fully engaged with one set of fixed plates, bring your hand flat up to the loop. If you can “tune it” (lowest VSWR) by moving your hand to and fro, you need to start adjusting the trimmer. On the other hand, if it will not “tune” in this fashion, you need to clip a SMALL amount off the corners of the PCB (or file a groove). Again, with the capacitor at minimum, use your hand, and if you can “tune” it, it’s time to adjust the trimmer. If you have made everything according to plan, and clipped the corners off the PCB, you could expect the capacitor to be set as shown in figure 4, when you have minimum VSWR. The more PCB you clip off, the further the capacitor will be rotated clockwise. In figure 4, the screwdriver slot is at 9:30. Maximum capacity occurs at 12:00, and if you are near there, you have clipped off too much PCB, and will probably have to start over…

Figure 4. Capacitor setting on prototype, at 145.45 Mhz for VSWR 1.3:1

Troubleshooting

Now if you can’t get it to tune, what could have gone wrong? Well if you don’t get the gap between the ends of the main loop at 20mm, then if too big, you will be clipping a lot off the PCB, and if you make it too small, it will not tune, even with the capacitor at maximum. ( Half into BOTH sets of fixed plates).

Now if you want a great dummy load at 2 metres, you only have to buy the cheapest co-ax you can find, and have a fair length of it. If that is what you have used to connect the VSWR meter to the loop, then you will most likely not see a “bad” VSWR, however the loop is tuned. Short cables are important, although a long cable between the radio and the VSWR meter is helpful if you can’t set your radio to low power, it will just result in lower meter readings overall, both forward and reflected.

Now if you got this far, imagine the fun you could have with a loop for HF. It probably will fit in your garden BUT…. The capacitor won’t be £1.25. More like £200!

Good luck, we are going to have a lot of fun with these, chasing all over Bluebell Hill and it’s surroundings! (Working off some of Mickey’s dinners!).

73 de Stan, G4EGH

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