After trying various tuning capacitors in parallel with the D plate, I found that one can choose a value that gives 50 ohms load with only a small phase angle, about 15 degrees. The E plate voltage could be shifted 15 degrees with a large, low reactance series capacitor. The space loss at each plate was assumed to be 377 ohms. All the networks I have tried had high Q, low loss properties. I believe that radiation loss should show up in the experiment as a reduction of Q somewhere.
Radiation is, after all, just another form of dissipation of energy.
It doesn't matter how it leaves the system. Second, reconnecting the adjustment capacitor in a parallel resonant circuit excited by the D plate will give a narrow band, high Q adjustment. When this condition, Figure 2, was tried, it behaved exactly that way. I conclude that the E plate doesn't radiate, at least for the configuration of Figure 3. It is easily adjusted with only one tuning capacitor.