Index
17.3
Adjusting the shift in a single C-notch-cavity.

17.3.1 Solder the 1/2 wave testcables at the in- and output links. Do this directly at the mid-point of the strips, at the LINK-side (not at the stripside!), directly on the link-wire (See "cn.gif").

17.3.2 The cavity must be pre-tuned to abt. the pass-frequency. Measure the frequency-difference between the minimum output at the notch-frequency, and the minimal =VSWR= at the pass-frequency (NOT minimal insertionloss !). This is called the "shift". It must be exactly -600 kHz.

17.3.3 Adjust and check the shift repeatedly. Increasing the value of C will make the "shift" smaller, but the notch less deeper.

17.3.4 It is possible, that the pass-frequency is moved a little, when the notch-frequency is adjusted. This is called "pulling".
Therefore, allways measure BOTH pass-frequency (min. VSWR) and notch-frequency (min output) to obtian the shift.

After the shift is correctly adjusted, NEVER adjust Cn again! Lock Cn. The notch deepness should be 30 dB or more.

cn.gif 17.3.5 After adjusting the shift, tune the cavity exactly as possible to the repeater-Tx-(downlink)-frequency for lowest VSWR (tune with the tuning-rod). Keep the fine-tuning bolt near the "3 turns in"-position to allow easy fine-tuning later.

17.3.6 Check again for correct shift.

17.3.7 Remove the testcables.

17.4 Drill (two) 3mm holes where the coaxes must pass the sidewalls of the cavity (See "topvuw.gif").

17.4.1 Connect the definitive 1/4 wave teflon cables at exactly the same points where the testcables were connected.

17.4.2 Solder the braid directly to mass. Do not use braid-pigtails! Try to avoid crossing of input- and output cables.

17.4.3 Solder the braid on several points to the topplate "A", and on both sides to the holes in the sidewalls. This prevents unwanted coupling-currents on the outside of the coax, and mechanically stabilises the position of the cables.

topvieuw.gif

17.5 The out-going link of the last C-notch-cavity must be connected to the antenna-recepticle through a ***1/2 wave*** cable.
The length of this cable is critical. Calculate it from the wavelength (notch-frequency) and veloctyfactor (abt. 70cm for 145.6 MHz and teflon cable). Try to avoid crossing of input- and output cables.

17.5.1 Solder the braid on several points to the topplate "A", and on both sides of the hole in the baseplate "H".

17.5.2 Solder at the antenna-side the end of the cablebraid directly, and as short as possible, to the outside of the connector.

17.5.3 Solder the inner conductor of the cable AS SHORT AS POSSIBLE to the central pin of the connector.

17.5.4 The 1/4 wave cable, comming from the last cavity of the L-notch part of the filter, will later be connected here in the same way.

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