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Fig 5.92. The circuit and layout for a 100W 145MHz slab-line filter. The strips are 1 x 1 /16 in (25 x 1.5mm) sheet copper, offset at 45° to avoid overcoupling. Input and output lines are 6 1/2 in (165mm) long;

the centre resonator is slightly shorter to allow for the greater length of C2 and a rib in the cast box. C1 = 50pF, C2 = 60pF, C3 = 4.4pF

gives dimensions for band-pass filters for the 144MHz band [27]. The connections between the copper fingers and the die-cast box are at current maxima and must have the lowest possible RF resistance. In the prototype, the ends of the strips were brazed into the widened screwdriver slots in cheesehead brass bolts. The 'hot' ends of the strips are soldered directly to the stator posts of the tuning capacitors. After the input and output resonators are tuned for maximum power throughput at the desired frequency with the centre capacitor fully meshed, the latter is then adjusted to get the desired coupling and thereby pass-band shape (Fig 5.93).

For the highest Q at UHF and SHF, and with it the minimum insertion loss and greatest out-of-band attenuation, coaxial cavity resonators are used. Their construction requires specialised equipment and skills, as brass parts must be machined, brazed together and silver plated. The Microwave Handbook (RSGB) elaborates on these techniques.

Fig 5.93. Performance curves of the 145MHz slab-line filter as affected by the setting of C2. Note that the insertion loss is only 0.6dB. Also, compare the -10dB bandwidth of this filter with that of the filter of Fig 5.89:7.7MHz for this filter when tuned to 'new curve', 13MHz for the four-section filter with small, lower-Q coils

standard for transporting RF energy between 'black boxes' and antennas. The desired match is valid only at or near the design frequency. The calculations for L, pi and L-pi circuits are given in Chapter 7.