Selecting the appropriate receiver for your radio feed is crucial to avoiding conditions like desense, third order intermodulation, and overload. Some of the more important receiver specifications include sensitivity and selectivity. Sensitivity is the receiver’s ability to hear weak transmissions, while selectivity is the receiver’s ability to withstand strong adjacent transmissions. There are other receiver figures like Intermod Distortion and Dynamic Range that are also important, but for now, we’ll stick with these two.
To the casual observer, simply using a receiver with the best sensitivity might seem like the obvious choice. While this may work in some situations, such as those rural settings with little other RF traffic to contend with, this increased sensitivity can often do more harm than good as it essentially lowers your defenses against those strong adjacent transmissions in exchange for trying to hear slightly weaker transmissions.
This brings us to selectivity. The goal for a high performing radio feed is maximizing the Signal-to-Noise Ratio (SNR), or the strength of the received signal above the noise floor. While a bit of an oversimplification, look at sensitivity as the receiver’s ability to hear those signals, while selectivity is the receiver’s ability to hold down the noise floor. Even if you have a receiver with the best sensitivity on the market, it won’t matter if its selectivity is not enough to hold down the noise floor below the strength of the signal you’re trying to receive. And once that signal is down into the noise floor, it is lost forever.
Any two-way radio professional will tell you that, as far as radio reception goes, the best offense is a good defense. In the presence of nearby strong adjacent transmissions, a receiver with an excellent selectivity figure will help hold down the noise floor without impacting the received signal – thus maximizing your SNR. A radio with a better sensitivity figure, but poorer selectivity figure will allow that noise floor to creep up, reducing the SNR – perhaps to the point that the noise floor swallows the transmission entirely.
So, if going for the maximum amount of sensitivity isn’t necessarily ideal, then how much sensitivity is enough? While there isn’t a one-size-fits-all answer to this question, the general idea is “enough to hear your target transmission.” It is always better for a transmission to come across a little weak due to slightly reduced sensitivity, than not at all because it was swallowed up by a rising noise floor. Most commercial-grade receivers specify a sensitivity of around 0.25 µv (-119 dBm), although they often beat that spec in actual performance – but combined with a good selectivity figure, this will still be enough to still hear those weak signals.