Repeater antenna isolation test

The isolation between the receiving and service antennas is a crucial parameter in the installation and operation of a repeater system. It is imperative to ensure adequate isolation to prevent unwanted interactions between these two components. If the isolation is inadequate, it can lead to a problematic scenario where the service antenna and the receiving antenna form an unintentional closed-loop amplification system. This closed-loop system can cause the repeater to enter a state of cyclic amplification, resulting in the power amplifier becoming unstable and ultimately self-exciting. Such a condition can significantly impact the repeater's performance, causing it to malfunction or fail altogether.

In the past, the approach to testing this isolation involved the use of an analog signal source and a spectrum analyzer. The setup typically entailed connecting the analog signal source to the service antenna, which then transmitted a signal towards the receiving antenna. The receiving antenna, in turn, captured this signal and fed it back into the repeater system. Meanwhile, the spectrum analyzer was connected to the system to monitor the frequency spectrum and detect any anomalous behavior.

Specifically, the analog signal source was tuned to emit a signal at a specific frequency, representing the intended operating frequency of the repeater. This signal traveled through the air, passing through any obstacles or obstructions that might be present in the environment, and was ultimately captured by the receiving antenna. The received signal was then processed by the repeater's circuitry, including the power amplifier, which was responsible for amplifying the signal to a sufficient level for transmission.

However, if the isolation between the antennas was insufficient, the amplified signal from the power amplifier could leak back to the receiving antenna, creating a closed-loop amplification system. This loop would continue to amplify the signal, leading to a runaway condition where the power amplifier would become unstable and self-excite. The spectrum analyzer would detect this anomalous behavior by observing an abnormally high signal level at the output of the power amplifier or in the frequency spectrum being monitored.

To address this issue, careful consideration must be given to the physical placement and orientation of the receiving and service antennas. Proper antenna spacing, shielding, and filtering can help improve isolation and prevent the formation of a closed-loop amplification system. Additionally, the use of modern digital test equipment, such as digital signal generators and digital spectrum analyzers, can provide more accurate and efficient isolation testing, allowing for faster identification and resolution of any potential issues.

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