But unlike some, I like MFJ stuff and this is the typical sort of easy thing to do with their equipment. Why MFJ don’t just add the toroid themselves, for the cost of a Dollar, I don’t know. I now have an ATU that covers everything again. Then I put some quality RG213 on it and Top Band was not having any of it. When I got it, I used 50ft of RG58 as always seems to be recommended – just lossy cable to hide the real VSWR. Took seconds to solder it in and the job was done, quick test before screwing it back up The coil does a good job of holding the toroid in place, so I might just get a double sided sticky pad to keep it down – the ATU will only be used at home.įor me this was great, I have a G5RV. That meant I could get a pair of tin snips (for cutting chicken mesh wire) on it and the job was done. Eventually I got some fine nosed pliers and pulled the part of the coil to be cut, in towards the middle. I didn’t have a pair of long thin nosed snips. Stuck it on the little Chinese LCD component checker that I have, and it came in bang on 10uH.Ĭutting the coil was the difficult part, it’s tougher than it looks, you don’t have a lot of room, and have to be careful not to damage things. It was about £4 delivered, had a lot less turns than yours, but thicker gauge wire. I bought a toroid from ebay, I know it’s cheating, but after I had a stroke last year, some things aren’t worth worrying about. Thank you sir, did the toroid mod today and I can have a 1:1 match all the way down to 1.843 (don’t use the lower frequency modes). I hate to imagine what the result would be if you put the ‘rated’ 200w into this thing and touched a control! One problem with pulling the knobs forward a little is that then the nuts themselves are more visible, although anything is better than an RF burn! 73 de M0UKD. This can give you RF burns whilst you are tuning, even at 5 watts. Most of the time they are hard to come into contact with being tucked away behind the control knobs, however, on the MFJ-971 the knobs must be pulled forward a little otherwise the metal inside the knob comes into contact with the metal shaft causing the grub screws that hold the two control knobs on to be at a high RF voltage. Please let me know if you perform any of these modifications, I’d be interested to know what you think.There is one small problem remaining and that is RF BURNS! The nuts that hold the variable capacitors in place on many MFJ tuners are at RF potential. The completed, modified, much improved MFJ-971 is seen below. The toroid is then fixed with a nut & bolt and suitable insulation washers either side of the toroid. The wires cross to keep any magnetic fields circulating the same way between the 2 inductors. Now, when ‘L’ is selected, the whole original inductor is in use, plus the extra inductance of the toroid, making it suitable for use on 160m! Of course, the ‘K’ setting still uses the whole original coil (less 2 turns) so nothing is really lost. I cut the wire just after the ‘L’ tap and inserted the extra inductance. The ‘K’ inductance setting is 2 turns up from this point. The inductor switch picks up its ground from the bottom of the inductor and this is also the ‘L’ setting, meaning the whole coil is in use when ‘L’ is selected. The MFJ-971 inductor has 17 turns, 43mm long and 66mm diameter which is about 17µH.Īdding on the extra inductance means we have a max inductance of around 27µH which allows matching on 1.8MHz.Īir inductance coils can be calculated here and the T130-2 toroid here. Therefore, most models cannot be operated concurrently, and are only used for a certain transceiver.The MFJ-949E inductor has 27 turns, 75mm long and 75mm diameter which is about 37µH. Because of its detection circuit, the control circuit and coding method and interface do not have a universal protocol. Most of the finished products adopt the closing of automatic control switches to achieve the purpose of tuning. In fact, almost all antenna tuner structures and principles are the same. It can be seen from the figure that the structure is very simple. Schematic Diagramįigure 1 shows the schematic diagram of the antenna tuner circuit, and Figure 2 shows the actual diagram. This is because it is a tuning transformer, which improves the RF selection of the receiving device. In addition to increasing the strength of the signal, the antenna tuning device can reduce the spurious response of the receiver. Due to different antennas and operating frequencies, the impedance presented by the antennas can vary greatly. The impedance mismatch caused in this way will cause great loss to the signal transmission, and even cause the transceiver to malfunction or be damaged. The antenna tuner plays a very important role in short-wave radio communications.
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