Friday, November 20, 2015

Mark V Shunt Feed Matching Network (80m)

Inside of the Mark V matching network. Notice the larger
capacitor in the back upper right, and the new inductor,
both for 80m.
I wrote earlier that I was having trouble with my 80m shunt-feed matching network. I managed to identify the problem using an enormous variable capacitor, but that was only temporary. As it was, the Bread-slicer obtained several spots of rust for the week I left it outside. I needed a permanent solution.

Having nothing in my junk box that was suitable, I found a 80 pF variable cap with reasonable (4 kV) plate spacing on eBay. It would fit into my NEMA box, but just barely.

In my earlier hunt for the 80m issue, I had already re-wound the 80m inductor on a T200-2 core with insulated wire.

While I was doing this, I realized I had no more T200-2 cores. I was giving idle thought to going to 2 T200-2 cores, as I had for the 160m inductor. Double cores would allow me to use fewer turns, so I could use a larger gauge wire, increasing the inductor Q.

T200A-2 core in center, cover in
fiberglass tape on left, and a
finished 80m coil on a T200-2 core
for comparison on right.
While I was shopping on-line, I came across a new product from Amidon. It is variously labelled T200A-2 or T200-2B, but it is essentially a T200-2 core that is 1 inch thick -- the same as stacking two cores together. Needless to say, I bought two.

New 80m inductor.
Using the larger core, I went from 38 turns on the T200-2 to only 28 turns on the T200A-2 core. The most difficult part of this exercise was finding suitable wire in the junk box. I ended up using a bright orange 16 gauge wire that had much thicker insulating than I needed -- but it is what I had. The resulting coil was a bit difficult to wind, and I used a couple of wire ties to keep the ends from unravelling.

New cap and inductor for 80m.
Shoehorning the capacitor and the larger inductor into the NEMA box took some doing. I had to move the relay just to get the capacitor in the box. And the wiring had to be re-done.

I used two 100 pF 6 kV disc ceramic caps in parallel with the 80 pF variable to get enough capacitance to match.

All assembled, it tunes up nicely around 3800 kHz with a 1.1:1 SWR. And the SWR doesn't change at all when going to 100 watts. Perhaps with the new cap and inductor, I could put a few hundred watts through it with no problems. I'll have to wait to fix the AL-80A before I can test that.

Next step will be to get the 160m network to tune better, it's a little off lately, I think I need more capacitance.

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