Transmitter Combiners

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  1. Do I need to terminate the unused transmitter port on a combiner?
  2. My transmitter carrier frequency assignments are spaced closer than the minimum separation specified for your product. What can I do?


  1. It is considered good practice by many but not always necessary. Hybrid combiners do not require termination. Cavity types should be tuned as far off-frequency as possible. This minimizes insertion loss on the remaining channels and makes it unlikely that any reflected power will arrive at the unused port.
  2. The minimum spacing for most cavity type combiners is 125 kHz. If we try to retune the combiner for closer spacing, the published isolation specification will no longer be met. Unfortunately, this is usually the most important feature. We prefer not to build such a a marginal version of our product. Hybrid type combiners offer a solution because they will meet specifications with spacings down to 0 kHz. However, there is a tradeoff because the Insertion Loss of Hybrids is much greater. If you cannot raise the transmitter output power you might need to substitute a higher gain antenna to compensate for some of the loss but the resulting coverage pattern may not be optimum. Sometimes splitting the frequencies between two antennas is the best compromise if there is space on the tower. Twin antennas do offer the benefit of redundancy. Circumstances may require employing two Cavity combiners, each carrying its own set of properly spaced frequencies, joined together by a broadband Hybrid feeding a single antenna.

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