The AM Forum

Good day.

I have been doing a restoration on an old Heathkit AT-1 transmitter. An interesting observation has occurred regarding the linked coupled output tank of this old relic. The design from 1951 is reminiscent of the MOPA. In essentially all MOPA schematics the plate tank and the link are connected so that NO phase shift occurs. The primary (PLATE) and secondary (ANT) output voltages are in phase. And in the AT-1 the coil connections; plate tank and link are done in precisely that way. See simple schematic rendering.

Is there a reason for this? What if I reversed the plate tank winding wrt to the link? So ideally the antenna output voltage would be shifted 180 degrees wrt to the plate tank voltage.  Any issues?

Note in both cases the winding sense is always maintained so the mutual coupling is maximum.

Thanks!
Alan   

RF is just AC and unless you are using the link as a feedback path, it will make no difference. Try swapping it temporarily. Wire it back original to the instructions for historic reasons, I guess.     

Thanks for the reply!

"the link as a feedback path"

Exactly and that was my concern in the AT-1 transmitter.

The plate tank coil that is historically used has the hot side of the RF link
facing the hot side of the plate tank. There would be no phase shift of the plate output to the antenna port.

I have constructed my first version of this output tank reversed in that the link output is facing the cold side of the tank. My concern was RF feedback to final grid in phase and hence a power oscillator... At this point I tried this arrangement and it appears not to be an issue.

I need to sort out the frequency multiplication next as 40 meter output requires 80 meter LO. The AT-1 design forces you to double.