[Lowfer] Still looking for a capacitance dissapation factor test

[Bill Ashlock]

Paul,

How about reducing the tuning range of your variable capacitor so as to make 
the tuning less critical?  I use a 50 to 700pf mica trimmer on the Lowfer 
pipe loop. The total capacity is about 7000pf so the tuning portion is only 
about 10%. Since the Q is higher by about 4X for the Medfer loop I plan to 
reduce the tuning range to about 3%. This will be only about 20pf. What is 
needed to bring the loop to the initial resonance is a VC with a much larger 
range (like you have been using)  BUT once you are close, use you LCR meter 
to find what the bank of fixed capacitors should be, once you switch to a 
20pf VC.

To find the Q of the loop/capacitor combo I use a Wavetek sine wave 
generator set to ~1700. I place a series resistor between the primary of the 
transformer and the generator such that the generator output Z (50 ohms) 
plus the series resistor is the desired load on my transmitter for 100mw. 
For a 12v Vcc final the resistor comes to something like 130 ohms (total R = 
180 ohms). I connect the scope to the transformer primary and adjust the 
turns in the transformer so that at resonance the voltage dips to 1/2. I 
then calculate the what the Rac load on the secondary must be by dividing 
180 ohms by the square of the number of turns. For example, if the turns 
count is 30: 180/900 = 0.2 ohms. The combined Q is the XL of the loop (or Xc 
of the capacitance) divided by 0.2.

Make Q or Rac measurements with different types of resonating capacitors. Be 
sure and try a bank of 6 or so paralleled COG monolithic ceramics. I think 
they will easily beat your micas.

Actually I suggest building a test loop having a diam of about 8" for the 
bench using 1/4" tubing, hanging it from the ceiling. The number of turns 
will be about 8. Sure a lot easier to experiment inside these days!

Bill A

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