Here are a couple of H.F. DTC filters. This filter topology requires that both ports be terminated in 50 ohms in order to achieve the designed band pass response and it's associated stop band attenuation.
To the right is a second order Butterworth band pass filter designed for the 20 meter band. The center frequency is 14.2 MHz and the bandwidth is 0.575 MHz. L1 and L2 are 0.5 uH and are wound on T50-6 toroids with 11 turns of #24 AWG wire. The unloaded Q of the inductors is around 200 ohms. The 150 pF parallel caps are rounded to fit a standard value. C tune should include a variable cap in parallel with a fixed value capacitor to tune each resonator. A suggested value might be a 180 pF in parallel with a 7-40 pF trimmer cap on each side of the 7.2 pF coupling capacitor.
Below the main filter is an equivalent filter using transformers to match the filter to a 50 ohm load. Note C tune is now 250 pF and again a fixed value cap should be used in parallel with a variable trimmer cap to tune each resonator. A 2 turn secondary is wound over the grounded end of the main coil. The 7.2 pF coupling cap can be achieved by placing a 12 pF capacitor in series with an 18 pF capacitor.
To the right is a second order Butterworth band pass filter designed for the 30 meter band. The center frequency is 10.125 MHz and the bandwidth is 0.40 MHz. L1 and L2 are 2.1 uH and are wound on T50-6 toroids with 23 turns of #24 AWG wire. The unloaded Q of the inductors is around 225 ohms. A portion of each C tune should be variable to allow tuning of the filters resonators. Below the main design is a filter using transformers to match the filter to its load with a 3 secondary to make the impedance transformation. The 3.3 pF coupling cap may be achieved by placing in series a 5 pF with a 10 pF or a 3.9 with a 4.7 pF capacitor.
A filter was built and the resonators tuned at the center frequency of 10.125 MHz. The coupling caps were NP0 ceramics (5 pF in series with a 10 pF). The variable cap was a Digi-Key bottom-adjusted (SG20016-ND) 5-20 pF ceramic trimmer cap. In parallel with the each trimmer cap was a fixed 82 pF silver-mica capacitor. After tuning, the coils were disconnected and the total resonating capacitance was measured at 94 pF. This illustrates the fact that you need to be able to tune well under or in some cases over the designed C Tune frequency. Many builders use variable caps as high as 60 - 100 pF for the C Tune trimmer to allow for deviation. The bottom-adjusted type trimmer cap was selected, as the leads are easily bent for use with Ugly construction and tuned from the top.