|Inside of the Mark V matching network. Notice the larger|
capacitor in the back upper right, and the new inductor,
both for 80m.
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.
|New 80m inductor.|
|New cap and inductor for 80m.|
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.