Repeater Desensitization

What is Desensitization?

Maybe you or other members of your amateur radio club or community have seen this two-dollar word and were wondering what the heck it might be. The word “desensitization” sounds like a two-dollar word but, as with most concepts, if we break it down into its components, it becomes trivial. To decompose the word, let’s start by removing the “de” from desensitization so that now we can talk about “sensitization.”

In English grammar (as well as in Chelsea Amateureese), we have a sub-group called “transformational grammar.” In transformational grammar, nominalization refers to deriving a noun from an underlying clause or word. There exist an array of suffixes in English that make this possible. In this case, the operative suffix is “…ation.” The characteristic noun “sensitivity” is made into a noun process with the suffix -ation==>sensitiv-ation.

We, therefore now ask, “What is sensitivity?” This will be getting closer to the heart of most amateur radio operators. This is a radio receiver’s ability to pull in weak signals. Radio specifications for sensitivity are in microVolts (uV). In the case of the Yaesu DR-2X repeater, its receiver is specified to have a sensitivity of 0.2uV. This is pretty much standard for all Yaesu radio receivers.

Let’s now return to our “Chelsea Amateureese” english and add our suffix “-ation” to sensitivity creating “sensitization.” We are going to do something that affects a receiver’s sensitivity. We are going to call it a sensitization process. This “-ation” of sensitization is telling us amateur radio station operators that we will be doing something that makes it better or worse or maybe no different at all. But generally, when we speak of a “sensitization process,” most people will think of improving a receiver’s sensitivity. Therefore, when we speak of “desensitization,” most people (and amateur radio operators like me) will think of processes that take away from a radio receiver’s ability to pull in weak signals.

Therefore, within the amateur radio domain, desensitization is any process that we apply to the receiver’s environment which takes away from its ability to pull in weak signals whether it be from Pakistan or Kalamazoo.

Desensitization in the Repeater Environment

As the reader knows, there are many ways to desensitize a radio receiver or its environment. However, when we talk about desensitization within an amateur repeater environment, we are most likely addressing leakage of transmitter power which finds its way back into the receiver instead of the antenna.

Testing for Repeater Desensitization

It is impossible to block 100% of the transmitter’s power from finding its way back into the receiver’s input. But there will always be a specification whereby that energy will not be significant. There will be a reduction in level by so many dB.

While it is possible to test a radio repeater to see if it is meeting its required dB rejection ratio and thus not having a problem with desensitization, there is an easier test that requires no special equipment. This test is very nicely explained in a document we found on the web. Look for the title in the document “Receiver Desensitization Test.” But the essentials of that document regarding desensitization are found here.

Briefly explained, the RF repeater needs to have its own speaker and the repeater must let you easily disable and enable its transmitter from its front panel. In so doing, you can hear what the repeater hears without repeating it.

Prearrange to have someone predeployed to the periphery of the repeater’s reach to provide a “weak signal” input to the repeater. From the repeater’s front panel, disable the repeater’s transmitter. While the weak signal is coming in, enable and disable the repeater transmitter. If this makes a difference in the weak signal quality, you have a desensitization problem.