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Multiband Fuchskreis EFHW matching unit (just finished)


#1

Hi all,
just finished constructing my first Multiband Fuchskreis EFHW matching unit from QRP shop in Berlin:

The one I have just finished is actually for a friend, I thought is would be a good way to gain experience by building one for someone else first. The results are OK:

The wiring inside could be simplified somewhat, and the lead lengths shortened, but it works fine. I know it’s nothing special inside, so I won’t be at all offended by any comments suggesting improvements. It just goes to show how forgiving this kit is. It is fairly straightforward to build and could be done in a morning by most competent hams. I learnt quite a lot while building it, so the next one should be much better. There is a set of English instructions available for this kit:

http://www.qrpproject.de/UK/multifuchs.htm

I found them to be very comprehensive and well translated, however, it is important always to refer to the circuit diagram to make sure everything is exactly as it should be before soldering. I had to rewind one of the toroids, so now I have to wait for more enamelled wire before I can finish off the next one. The variable capacitor has changed since the manual was written; the new one needs two of the tabs to be connected together in order to get enough capacitance for the low bands. This was the only thing that was not translated into English, so a working knowledge of German certainly helped here. One very interesting thing I learned from G3UNA whilst visiting England was that the rotary switches used in these kits are in fact made by Grayhill; approx cost per switch is normally over €35, so someone picked up a job lot somewhere along the line:

Part number: 56D36-01-1-AJN

With the total cost of the kit at €37, these little units are incredible value! Can’t wait till the MTR3b gets here; the KX3 is nice, but it’s a bit heavy if you are planning on camping overnight. Ultralight SOTA here we come!

               73 es 55 de OE6FEG / M0FEU
                                                     Matt

P.S.
Looking up the switch by part number on the internet showed a unit price of €9. Including VAT and shipping the total cost for a pair came to €45, so G3UNA must have had a similar switch in mind. Still, they’re nice switches.


#2

I finished mark II (the unit I will keep) this evening:

I bought a set of helping hands to do this version, and I think it shows in the finished article, which is a lot better that the first. I may rework the first one for my friend as I feel a bit guilty seeing the difference between the two. I actually wired the coupling toroid differently than in the photo, although there is not much difference visually. In the photo I had copied the wiring (with 3 extra turns) that another friend had done that he said gave a better resonance on 80m. When I tried it out, I found I couldn’t get below 2:1 on 20m, so I took it apart and rewired it exactly according to the instructions. It now tunes all bands better than 1.3:1 and I get 1.2:1 on most. So follow the instructions seems to be the lesson here.
I found it way easier to put together if the taps are soldered to the switches outside the box and then the rotary switches installed one at a time. After that, the BNC goes in; then the variable capacitor and all the remaining connections are made: dead easy!
One modification my friend made that I did decide to keep was the use of a blue or white LED instead of the supplied red LED. A blue or white LED is much easier to see in daylight and completely obviates the need for the coupling capacitor in the first version. I used a super-bright Nichia LED that cost another €2. In some ways it works a bit too well, as it shines even when the tune is way out. It’s not so easy to decide when it is at its brightest. Perhaps someone with more knowledge of LEDs can find a more suitable one; blue may be a better option. I’ll only be using it with the new MTR3b, so the right switch positions can be easily marked in white paint or something; perhaps luminous paint for when operating at night.
The other modification I made was to move the variable capacitor further back towards the banana plug, which is allowed to protrude more out of the enclosure. I went about 42mm from the front of the enclosure where the BNC is instead of dead centre at 36mm as shown in the instructions, but other variations are possible. The reason for this is that with the variable capacitor further back, there is more room around the various knobs and switches, which makes tuning easier.
Now it’s just a matter of waiting for the MTR3b to make it through customs where is has been languishing since the 20/05!
73 de OE6FEG / M0FEU
Matt


#3

I have reworked the English instruction manual to reflect both the recent changes in some of components and my own experience in making the Fuchskreis.

https://1drv.ms/f/s!At7lkKhflujXhRap7hAiFURE1Y8-

Hope people find it useful. QRP Project will be making the file available on their own website in the near future. For those interested in the weight, the finished unit weighs 95 grams.

         Cheers
                    Matt

#4

Hi Matt,

Thanks. I’m not a fan of EF wires but will build the ATU and add a wire for use as a back-up.

Instructions saved.

73
Ron
VK3AFW