Circuit implementation, attention to RF design practice, the device selected, and electrical operating bias levels all contribute to noise generated in a preamp.
In general, you should look for a unit with a low noise figure. How Low? There are many schools of thought on this subject. Some believe that a noise figure of 1.5 – .5db is fine for terrestrial weak signal operations (144 – 432mhz), and a noise figure as low as possible for EME applications on all bands.
We like to think that the noise figure of a preamp also indicates the relative technology level
or “quality” of the preamp. Older types may have a noise figure well above 1db. Current preamps with a high technology device operating in the 100-250Mhz range will have a noise
figure of around .5db or better.
Noise figure is important when you are attempting to receive a weak signal. To improve
the signal to noise ratio, the amount of noise the preamp produces can make the difference between good, poor, or bad reception.
What makes a cavity preamp better than a standard L-C preamp?
If our goal is to narrow the bandwidth of the preamp front end, there are many ways to accomplish this task. The problem is that most of the methods that decrease bandwidth, increase front end losses. This loss, when incorporated into the preamp front end, will increase the noise figure. At some point, depending on the application, the noise figure may rise to unacceptable levels. An input cavity can improve performance without a substantial increase in noise figure.
When properly implemented, a cavity input circuit can decrease bandwidth and lower input losses at the same time. This allows low noise figure and decreased bandwidth, both of which are valuable when all things are considered. The primary draw-back assiciated with a cavity preamp is the size. In most cases the cavity body will be much larger than a typical L-C preamp, at lower frequencies.
Will I be able to hear more stations, or weak signals with a preamp?
That is a tough question to answer, without knowing all of the particulars of a receiving system. Factors such as: Antenna type, antenna gain, antenna polarization, antenna height
above local obstructions, antenna height above sea level (ASL), feed line type and length,
preamp location (at antenna / at receiver), system losses (cable, connectors, switches),
receiver sensitivity, receiver front end noise figure, and RF pollution level in the area.
The easy way to gauge receive performance is to compare your receive performance against similar installations in your area. If you are constantly missing weak signals, a preamp might improve system performance.