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Suggestions for the construction of cavities |
Fig.5 shows the construction of a 70cm cavity. The dimensions D and d have influence on the characteristic impedance of the cavity, and hence the Q.
For optimal Q in circular cavities and resonators :
| D/d = 3.59 |
This results in an impedance of 78 ohm.
The value S is of influence on the temperature coefficient of the cavity. S should be equal or greater dan D. When S is smaller, the design is more influenced by temperature variations, due to the non liniar capacitance variation between the end of the resonator and the lower lid. This "end effect" influences the resonant frequency of the cavity.
The shortening-factor of such a design is about 0.95, and the "mode of operation" is T.E.M. (transfers electromagnetic).
These data also can be used to design a cavity for the two metres band. The fixed part of the resonator will be 48 cm. The moving part must allow a variation of the total length of 50cm +- 2cm. The outer length of the cavity will be 56cm.
In both cases, the contact between upper lid and outer cilinder and inner conductor must be very good.
| Pa0nhc: HF-currents are high, as the impedance here is near 0 ohms. Some risidual resistance degrades the Q of the
cavity. Even very small spaces between the lid and the outer cylinder can lead to radiation of HF, causing HF-leakage between cavities! Good bye deep notches (experience
with very expensive German cavities).
Do not electrically interconnect toplids of cavities for the same reason. It can degrade notches to -70dB ! Currents running through the top-screws are big, so even a few milliohms internal resistance generate HF voltages which can degrade the filter performance. |
At least 16 screws are needed for connecting the lid to the outer cylinder. At the lower lid, impedance is very high, and HF currents small. Less screws are here needed.
The diametre of the tuningrod is not critical. It must be stiff enough, the thread must be fine.
| Pa0nhc: This rod can best be made of "Invar"-steel. In stead of thread, a clamp can be used to fix the rod in position. |
Silverplating inner surfaces is essential.
| Pa0nhc: Cleaned inner surfaces can be covered with CRC 5.56 anti oxidant oil. The pi3rtd duplexfilter is not silver plated, only CRC 2.26 is applied. It serves well for more than 5 years until German professional cavities were available. |
The couplinglink in fig. 5 has about the correct dimensions. It needs change according to shift an frequency band.
The couplinglinks in a bandpass cavity must be adjusted for correct 3dB bandwidth
| Pa0nhc: I prefer to adjust for just critical or little undercoupling, and a in- and output VSWR of 1:1.0 to 1:1.2 . This ensures low insertion loss, small bandwidth, and good match. When cavities are connected together, less influence on each other can be expected. |
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