Home brewed Parallel Plate capacitor for L Matching Unit
Added on 2005-08-31
I was planning to build a ATU for feeding my
160m inverted L. The design goals were
I have asked some of my colleagues to look into their surplus boxes and give me a parallel plate capacitor suitable for the purpose. The plate gap should be around 3mm and capacitance should be around 500pf maximum. I couldn't find a suitable one even though many hams offered to help. Therefore I thought of home brewing a parallel plate variable capacitor for my specifications. Since I already had a commercially made variable capacitor with Aluminum plates, I have decided to take the plates from it. Reason is that these plates are made with smooth edges to prevent the sparks from the edges. |
The Schematic of the L network tuner given in ON4UN's Low Band Dxing book. |
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After carefully opening the commercial capacitor, I was able to remove the plates without too much damage. Two separate perspecs sheets of 2.5mm were prepared to hold the thread bar as given in the picture. The 6mm thread bars were attached to one plate both parallel to each other. Careful drilling of holes is required otherwise, it is difficult to fix the parallel plates to the thread bars. |
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The Fixed parallel plates were attached one by one to the thread bars using 6mm nuts. Extreme care should be taken not to over tighten these nuts. Otherwise the very thin Aluminum plates can be broken easily. I have broken 2 of those :( |
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The final assembly of the plates look like this. It had 14 plates which should give about 400pf capacitance according to my calculations. But later I found that this gives only about 350pf. Maybe the overlapping is not 100%. The separation is also not uniform. Welcome to the world of home brewing !. |
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One more picture for you to see the other side of the assembly. |
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Then the moving plates were also fixed to another thread bar. This thread bar
is about 1" longer than the others since this has to come out of the assembly
and get connected to a knob for tuning.
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Neat looking moving plates assembly. Welcome again to the world of home
brewing.
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| The Final assembly of the finished product. Neat Ha !. |
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What I learnt from the capacitor construction
It is not easy to obtain the calculated capacitnace due to many factors
It would have been better if I didn't assemble the boarder plates closer to the platic plate. If I did so, it would have been easier to align the mechanical movements of the capacitor.
Rather than using many plates of smaller size, it would have been better to use few plates of larger size. Then again the alignment would have been easier (or maybe not ?)
Try to use higher separation of plates and more area than otherwise.
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The Construction of the Inductor was not a magic. I had an old single pole 6 way switch removed from some old junk. This was perfect for 6 tappings of the coil . The coil was wound on a PVC 4cm diameter piece of pipe using 18 SWG enameled copper wire. The coil dimensions were calculated using the formulas given in the ARRL handbook. I don't have inductance measurement equipment so I didn't bother to verify the design. It works at my power levels, so it is good enough for me for the moment. |
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The Final matching unit was constructed on a homebrew Aluminum box using dead
bug style to see the performance. It works fine with my inverted L and also
with the T-loaded vertical for 160m . So the next phase is to water seal the
box and place near the feed point of the antenna. I am sorry for not having
pictures beyond this point.
- 73 es Good home brewing DE Kamal 4S7AB |
