Aug 1, 2008 12:00 PM, By Harold Kinley

Their design and construction require tradeoffs between insertion loss and selectivity

Table 1 lists the percentages of loss versus decibel loss for various degrees of insertion loss. Figure 18 shows three bandpass cavity filters connected in cascade between the 1000-watt transmitter and dummy load. Suppose each cavity has the coupling loops set to 1 dB. Referring to Table 1, 1 dB corresponds to 79% of the signal passing from input to output of each cavity. This means that 790 watts appears at the output of cavity 1 and the input of cavity 2. Thus, cavity 1 has to dissipate 210 watts. Cavity 2 also has a 1 dB loss, but only 790 watts appears at the input of cavity 2. Meanwhile, 624 watts exist at the output of cavity 2 (0.79 × 790). This means that the power dissipated in cavity 2 is 790 – 624 = 166 watts.

At the input of cavity 3 there are 624 watts and at the output of cavity 3 there are 493 watts. The power dissipated in cavity 3 is then 624 – 493 = 131 watts. Because each cavity has a 1 dB insertion loss, the total insertion loss (neglecting connector and cable losses) should be 3 dB. Thus, the total power loss between the transmitter and the load should be 3 dB below 1000 watts, or 500 watts. Adding the power losses in this example yields 507 watts — an error due to rounding off in the intermediate calculations. If only a single cavity were used, with the coupling loops set to 3 dB, the cavity would have to dissipate 500 watts — probably leading to an unpleasant situation!

Some very important bare facts regarding cavities are presented here.

• Setting the coupling loops on bandpass cavities to a higher value will improve the selectivity but will increase the insertion loss.
• When connected to a transmitter the cavity must be rated for the amount of power dissipation that its insertion loss will cause.
• Better selectivity can be obtained from multiple cavities cascaded than with a single cavity with loops set to higher insertion loss. That is, 2 cavities with the loops set to 1 dB provide better selectivity than a single cavity with the loops set to 2 dB. Insertion loss will be the same but selectivity is much better with 2 versus 1 cavity.
• A larger diameter cavity provides better selectivity because of its higher Q resulting from the higher volume.
• Bandpass cavities can provide benefit over a wider frequency range than notch cavities.
• Notch cavities only provide a benefit at a single frequency or narrow band of frequencies.
• Notch cavities provide more rejection than bandpass cavities at a frequency near the desired pass frequency.
• Cavities also respond to odd multiples of the fundamental frequency, e.g., third harmonic.
• Interconnecting cables are of critical length. Never use random-length cables when interconnecting cavities.
• Always use double-shielded cable — especially in duplexer applications.
• When using stubs to achieve a particular effect, such as reducing the insertion loss at the pass frequency, opt for the shorted stub versus the open stub. This reduces radiation.
• Minimum return loss (maximum VSWR) should always be at or near the frequency of the maximum notch depth or reject frequency.
• Maximum return loss (minimum VSWR) should always be at or near the pass frequency.

The resonant frequency might shift when coaxial cavities are subjected to a wide range of temperature change. However, with modern construction techniques and materials that have a temperature coefficient near zero, the drift is negligible over reasonably wide temperature variations. Applications engineers at companies that manufacture coaxial cavities are ready and willing to assist in the proper choice of cavity for almost any need. While the intent of this article is to present the basic points the subject of coaxial cavity resonators runs very deep and can’t be fully covered in the scope of a magazine article. Hopefully, readers of this article will be encouraged to pursue the study of coaxial cavity resonators by finding other materials on the subject. To get started, contact the manufacturers for additional literature on the subject. Meanwhile, the September issue of Urgent Communications will present information on how to use cavity filters in the mitigation of typical interference problems.

Until next time — stay tuned!

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