Finding resonance on linear

[AB2EZ]

Rich

As a comment on the design shown in the article... I would suggest that L1 be oriented perpendicular to L2. If L1 is oriented parallel to L2, then... when all of the turns of L2 are shorted (end-to-end) by the band switch, a portion of the magnetic flux traveling along the axis of L1 (produced by the RF current passing through L1) will also pass through the axis of L2. This will result in an extra load on the tank circuit that will disturb the behavior of the tank circuit.

Stu

[W4AMV]

I had never heard the ceramic plate caps could cause such additional capacitance, seems counter intuitive.

Rich  

The motivation for the statement is due to surrounding the bare metal of the plate cap with ceramic whose dielectric constant is about 10. So it is not counter intuitive, however it does beg the question, where are the set of metal plates creating such a shunt C on the plate. One is the plate itself, but where is the 2nd plate. Again, I suspect this is not the issue in your case, however if you are trying to chase stray C you begin to question all possibilities. The original article looks straight forward. Rich, you may want to reduce your troubleshooting to the most basic circuit elements. Hence, just consider the PI network only terminated in the total Rp of the tubes alone, 4K I believe. And see if that is straight forward to match to 50 ohms. For a Q of 12-15, it is easy to determine the presetting of the two C's at 21.3MHz. Then attempt to find a tap point on the PI net inductor... if possible, that would be L1 I think.

[W4AMV]

Along Stu's suggestion, remove L1 all together. Is L2 just to large to begin. The mutual coupling present even after "shorting" all of L1 is effectively still increasing L2. 

[flintstone mop]

Went out and bought myself an MFJ259.  I was able to find good coil tap points for 80m and 40m but trouble on the higher bands.  I found a tap for 20m but it was right at the end of of L1, the tubular coil.  I added a 500pF padder on the output side and that moved the tap back a few turns.  I know you can calculate preset positions for C1 and C2 but I can't find any combination what would work.

Not sure what to do now.  I have a 1500 ohm resistor from the plates to ground (tubes in place).  (2500V / 0.4A * 2) / 2   (two tubes)

Rich

It's nice when there is a way to get all this sorted out without stressing the electronic circuits.