Calculation of the QBL 5/3500 amplifier
Anode compartment dimensions
An impedance of 73 Ohm was originally used to calculate the lecher strip line.
The calculation is quite cumbersome. Fortunately on the Internet a site was found where the calculation could be performed on-line.
With the following input : w = 140, height 2 = 60, height 1 = 180, t = 3 an impedance of approx. 73 Ohm was found.
The anode compartment can be made lower, but this will have an influence on the impedance, hence the tuning. Given the fact that de-tuning effects may occur when the top cover is put on / off or bent inwards (causing impedance change) a height of approx. 25 cm was chosen to minimize this effect.
Lecher length and tuning capacitor
Length of the lecher : λ/4. In practice 240 mm measured from tube midpoint to ground.
The value of the necessary capacitance in order to obtain resonance at 144 Mc is given by :
C-1 = 2π. f . 73. tg (2π.- l / λ ). For 144 Mc and 24 cm this yields : C-1 = 2π.144 E6.73. tg (2π. – 24 / 208 )
Giving a total C of 14 pF. According to the specifications the Cak of the QBL tube is 8.4 pF.
Therefore the total C for tune and load is 5.6 pF. A smaller value of C is not recommended, because proper loading can not be achieved anymore.
Loading capacitor / output strip line
The loading capacitor consists of a variable "flapper" connected to a 50 Ohm transmission line leading the rf power to a 7/16 connector..
The width of the horizontal part of this capacitor/line combination is 45 mm, plate thickness is 1 mm, mounted 10 mm above the grounded middle plate.
With these figures an impedance of 52 Ohm is achieved, close enough. See this picture for details.
The width of the flapper at the "capacitor part" is 67 mm.
High voltage decoupling capacitor
The capacitor consists of a brass plate of 240*180 mm, insulated form the ground potential of the rear panel by a sheet of teflon of 0.3 mm.
The capacitance can be calculated as follows : C = εo . εr . A/d.
With εo = 8,85 E-12 Teflon εr = 2,1 A = 0,0432 m2 d = 0.0003 m a C of approx. 2,7 nF is obtained. This is more than adequate for proper rf decoupling of the high voltage input.