What is bandwidth?
The bandwidth of a preamp, or audio amplifier for that matter, is the frequency range
where the preamp will produce a specified amount of gain or power level. Usually the point above and below a specified frequency where the output decreases by 3db. The bandwidth may not even be listed or specified for many non-commercial preamps.

For most LC preamps the bandwidth at –3db is on the order of 50 mhz(+-25Mhz). For a 144 mhz
Preamp having gain of 23db, it may provide gain of 20db (23-3) at 120Mhz, and 20db gain at 170Mhz. The gain extends much wider than the 144 -148Mhz that is required to cover the whole of the Amateur band. Even more important, is the fact that the preamp may provide
gain of 10db all the way down to 90Mhz, and up to 230mhz !

It is easy to see that the signals above and below the 144 – 148Mhz band can use up your preamp’s power to amplify signals that you can’t, or don’t want to hear!

In an areas with high RF congestion, a low bias LC preamp is just not going to work for you.
The same applies to many older radio front ends. They just can’t handle strong out of band signals that we see in today’s crowded spectrum.

Now again let’s visit the Home Stereo example. We have a 30Khz signal (that is too high for
us to hear) drivng our amplifier so it is producing 100 watts output. Since our amplifier is
rated at 100 watts, and even though we can’t hear the 30khz signal, we can’t amplify our music. We can of course, replace the amplifier with a larger unit, but that is very costly. An easier way to solve the problem would be to put a filter in line with the input, that will filter out the frequencies we can not hear! By doing so, the 30 khz signal is blocked, or decreased. Now we have nearly all of our 100 watts back! This is what happens when we place a cavity, or other type of narrow bandpass filter in front of our preamp. Nearly all of the amplifiers capability is now available to amplify in band signals! Now you are able to hear weak signals, and the “quality” of the receive signals will improve. No matter the band, audio, or 144, 222, 432Mhz, keep the gain in band !

When we talk about the “one-two” punch of an LNA Technology “NB” Cavity, or other narrow band preamp, this is what we are talking about. First, a narrow band front end, to limit out of band signals from using up the capability of your preamp, and second, we operate the high technology PHEMT device at a high bias level, resulting in up to a 10db improvement in power handling! Some of our units go even further, filtering the output for additional protection.

Check your area for commercial, FM broadcast and other out of band signals. Then look
around your installation and think about all of the digital equipment, other radios, and various items that emit (authorized and unauthorized ) signals. This may be why you have problems with weak signal reception.

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