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), if an improvement happens, 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 (SNR) 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.  

There is a caveat to adding RF preamplification; it reduces the dynamic range of the radio. If you enable the preamp (or increase gain by n-dB) and the noise floor in a FLEX-6000 goes down, the preamp will likely improve the SNR because it is exposing signals that were below the noise floor, which was NOT the atmospheric noise floor -- it was the noise figure of the preamp preventing optimal SNR. If the noise floor does not drop or it goes up, this means that you were already receiving at the atmospheric noise floor and you just (a) added unneeded gain and noise and, (b) decreased the dynamic range of your radio by the gain of the preamp.

For each 10dB of RG gain you add, it decreases your overload on the "high end" (large signal) by 10dB. If the noise floor only goes down by 1dB when you enable an additional 10 dB of RF preamplification, you decreased your dynamic range by 9dB.

In general, no RF pre-amplification is usually needed on most HF bands due to environmental and atmospheric noise at most QTHs versus the radio's internal noise.  If you add 10dB of attenuation and your noise floor does not rise, it means that your noise figure is well below the atmospheric noise floor. By adding 10dB of attenuation, you just added 10dB to your overload point and therefore an additional 10dB to your dynamic range. 

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

  • 1
    Avatar
    Tim Ellison

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

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