To Preamp or Not to Preamp


Every radio receiver and receiver site has an electromagnetic noise environment. Not only does it go with the territory, but it goes with planet Earth. There is a noise element that is present in the environment even if you are in the middle of a secluded desert island in the South Pacific where the nearest piece of electronic equipment may be a thousand miles away. Further, on that island you have no electricity. You are truly…in the sticks.

The preamp question can be further subdivided into 0 to HF and VHF and higher frequencies. Preamplification is generally not needed on the lower frequencies of HF.

Bringing in the Electricity and Technology

But now we spice up the equation giving you electricity and a top-of-the-line radio transceiver optimized for amateur radio. The transceiver has a fine-quality S-meter that has been factory calibrated for precision. You also have an antenna (which configuration is irrelevant for now).

You apply power and turn on the transceiver. Note that you have not yet attached the antenna to the receiver input port. You might fiddle with the controls a little bit turning off any AGC, set the receiver passband filter to 500 Hz, turn off any noise reduction, and maybe set a mode to CW if applicable.

S-meter readings are linked with dB gain. A change in S-meter reading of one notch (i.e. moving from 3 to 4) means a 6 dB increase in signal strength.

A Base Reading

You look at the S-meter and note its reading. Let us say that you noted an S-2 reading. It would be fair to call this an “atmospheric noise floor.” This reading will be a sum of whatever man-made environmental noise exists for that site together with a uniform noise that exists in the universe which goes back to the Big Bang. A member of the Chelsea Amateur Radio Club has written up a web page (To Preamp or Not to Preamp) describing in great detail what these two noise sources are.

The Antenna

Now you connect the antenna feedline to the receiver input together with a preamp designed to deliver 10 dB gain. We shall assume that the vector impedance appearing at the feedline is 50+j0 Ohms. Again, you fiddle with the controls a little and tune the VFO for a frequency where you cannot hear any station activity.

A Balancing Act

It is critical to recognize that there is a balancing act that goes on when we add preamplification to the receiver input. A preamp will amplify everything appearing at its input. It can possibly pull in a weak signal that was not previously detected. But at the same time, signals that are especially strong will be made stronger. This doesn’t hurt anything unless it saturates the output stage resulting in distortion.

Receivers have a dynamic range within which they must work. Installing a preamp where it is not necessary will almost certainly degrade a receiver’s dynamic range. This therefore increases the risk of distortion while delivering no benefit. More will be said about this very important consideration later.

The good news is that signal overload and its distortion is usually not called into question, so there is some slop to work with. But all of us want to get it right.

A Test

Let us suppose there were five different conditions and five different readings. Recall that earlier (with no antenna attached) you measured S-2. The S-meter readings were:

  1. S = 4.5ing
  2. S = 3.5
  3. S = 2.5
  4. S = 2 — base reading, no antenna attached.
  5. S = 1.0

What Does S=4.5 Tell Us?

At this point, we need to introduce the subject of dBm. This is an absolute measurement whereas dB is relative. Please see a web page where a member of the Chelsea Amateur Radio Club has explained these distinctions.

A reading of S=4.5 tells us that the antenna has added 2.5 S-units which is 15 dBm. When the noise goes up beyond 12 dBm, this suggests is too much preamp gain or too much local man-made noise. A preamp here will limit the dynamic range for large signals. With a reading of 15 dBm, it is clear that we need to reduce the preamp gain getting it closer to the optimal 8-10 dBm increase. But certainly, if there is no preamp already in the circuit, you don’t want to put one in.

What Does S=3.5 Tell Us?

A reading of S=3.5 tells us that the antenna added 1.5 S-units which is 9 dBm. This suggests that any preamp gain would be unnecessary and possibly hurting us on the overload gain (but unlikely).

What Does S=2.5 Tell Us?

A reading of S=2.5 tells us that we are getting a half-S-unit gain which is 3 dBm. Preamplification would likely be beneficial in this case.

What Does S=1.0 Tell Us?

A reading of S=1.0 tells us something went wrong. Better take all of the measurements again. You cannot decrease noise by attaching an antenna unless there is a powerful local source active.

What is the Objective?

This is a match game. Can you match the receiver noise figure with the environmental noise floor? In doing so you maximize the receiver’s dynamic range while pulling in signals that are above the environmental noise.