Preamplifiers are often touted as an easy solution to improve one’s reception, and in ideal conditions, they can indeed help. In practice however, they often cause more problems than they fix, so care must be exercised in their use.
Preamplifiers (also commonly referred to as Low Noise Amplifiers, or LNAs) tend to amplify everything, whether you want them to or not. They may sometimes create desense or bleed-over in cases where it did not exist previously. In situations where RFI is already an issue, they will only exacerbate the problem.
This isn’t to say that they don’t have their uses, but care must be exercised in their deployment. A site survey with a spectrum analyzer to see how busy the radio spectrum is nearby the target frequencies will tell you a lot about whether or not adding a preamplifier to the system is even viable. The site must also be extremely clean with regard to RFI from non-radio sources (such as LED lights, appliances, etc.), as even RFI that is inaudible without a preamplifier can become a nuisance when one is added to the system.
As with anything else in the two-way radio world, you get what you pay for. Cheap Amazon preamplifiers, broadband scanner preamplifiers, and TV preamplifiers are all examples of low quality products that exemplify the expression “garbage in, garbage out.” High quality PHEMT preamplifiers with a low noise figure and a high IP3 figure will ensure maximum performance. Noise figure refers to the noise floor of the preamplifier, with lower being better (ideally less than 1.0 dB); the IP3 rating relates to the linearity of the preamplifier, which is basically how well it can reject intermod & strong adjacent frequencies, with a higher number being better (ideally 40 dB or more).
While it may be tempting to crank up the gain in using a preamplifier, you will quickly reach a point where the receiver gets saturated and the noise floor starts rising again. Remember, the goal is signal-to-noise ratio, meaning how far above the noise floor your target signal is. The rule of thumb that has been well established by industry giants like Motorola, is 6 dB gain is about the most you would want to use. If your preamplifier has more gain than this, use an attenuator to bring down the gain to a reasonable level (hint: variable attenuators are a great way to dial things in on your system before ordering a permanent attenuator of a fixed value).
Whenever using a preamplifier in your antenna system, you should always place filtering ahead of it to only allow transmissions of interest to make it to the preamplifier. Bandpass filters (also known as preselectors) are perfect for this task, and help ensure that transmissions outside of your target spectrum don’t interfere with what you’re trying to listen to. If you look at most commercial receiver multicoupler systems, you will notice that virtually all of them include a bandpass filter ahead of the preamplifier for this very reason.
Getting back to basics though, first and foremost, ensure that you are using a high quality, commercial grade receiver from the likes of Kenwood, Motorola, etc. Preamplifiers became prominent in two-way radio in response to relatively deaf receivers of decades past; but with many contemporary receivers having excellent sensitivity and a robust front-end, there aren’t nearly as many scenarios in which a preamplifier would be useful in today’s antenna systems. That isn’t to say there is never a time and place, but in most cases, you’ll get as good of performance as you can get by using the right receiver without needing a preamplifier.
Now for the biggest rule of all: antenna height is the single most important factor in reception. Even an antenna height increase of 10′ can make a world of difference sometimes. Trying to cheat by using a preamplifier instead usually won’t work, and should be near the bottom of the list on things to try to improve reception. Focus on maximizing antenna height, using a good quality gain antenna tuned to the correct frequency, low-loss coaxial cable, and a quality receiver first and foremost.