How to determine the amount of RF Preamp gain to apply for band conditions

Content provided by: Gerald, K5SDR

When do you need to use the RF Preamp?

On HF - almost never. 

Before the instructions on how to set the preamp, a description of how signal strength is read and the effect of RF preamp gain on signals is warranted.

FlexRadio transceivers measure the signal strength at the antenna terminals in the same way that a spectrum analyzer does. If you change the RF preamp settings (turned on or to a higher gain value depending on the transceiver), the noise floor goes DOWN and the signal level stays the same.  Remember that when you engage the preamp, you have not changed the signal level at the antenna terminal. You have, however; improved the signal to noise ratio with the preamp and therefore the noise will go down in reference to a fixed signal level. The previous statement holds true in all cases except one. That is if the radio is antenna noise limited.

In general, no RF pre-amplification is needed on most HF bands due to environmental and atmospheric noise at most QTHs versus the radio's internal noise.  An important phenomenon that most people don't realize is that even in the quietest locations on earth, noise propagates with the band opening.  In other words, noise is RF and RF propagates just like intentional signals. This is easily observed when pointing an antenna away from the opening causes the antenna noise to drop.  Sometimes it will drop below the radio's internal noise floor. When an antenna is pointed towards an opening, the noise floor will rise, sometimes more than 8 dB above the receiver noise indicating that an RF preamp was not needed on that band. 

As a rule of thumb, you want your antenna noise to show an increase in your S-meter by 8-10 dB and no more. When this condition is met, it means that your receiver is not adding additional noise to the signal and that you have the correct RF preamp gain to maximize performance and optimize your signal to noise ratio (SNR). If the noise goes up more than 10 dB with the RF preamp and antenna, you have too much RF gain and the SNR is degraded.

An easy way to check is to do the following:

  1. Turn the RF preamp to 0 dB (off).  Turn off any RF preamps that are in line with the receive signal path too.
  2. Set the receiver bandwidth to 500 Hz
  3. Tune the receiver to a segment of the band where there are no signals, just noise.
  4. Disconnect the antenna and note the signal strength in dBm from the S-meter.
  5. Connect the antenna and read signal strength in dBm from the S-meter. 

 

  • If the signal strength increase is less than 8 dB, increase preamp gain.
  • If the signal strength increases more than 10 dB, decrease preamp gain or add attenuation.
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2 Comments

  • 0
    Avatar
    Don Niles (K2PMC)

    I did your test on 20M. With preamp on and no antenna -126db. Preamp on and antenna connected -106. This would suggest too much preamp. However, with antenna connected and preamp off the reading is -86db. And, it is apparent when looking at signals on the panadapter they are much more readable with the preamp on. Am I missing something?

    Don
    K2PMC

  • 0
    Avatar
    Tim Ellison

    Perform the test with the preamp off and look at the difference in dBm.

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