Sensitivity vs Dynamic Range Flex 6400

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As I review the Sherwood table on the 6400 I noticed a sensitivity number of 4.0 uV (I presume with no preamp) and .63 and .22 uV with 16 db and 32 db, respectively, preamp on. The narrow space dynamic range of 100 db is with 16 db preamp on. From what I have read the recommended use of the preamp is balanced against maximizing dynamic range. I also understand that the use of the preamp will amplify any noise present. However, under general conditions (no contest or in the presence of nearby strong signals) is there any value in using the preamp, say on 20 meters, to hear weak signal that are barely copyable without the preamp or am I already noise limited and I am not going to gain anything? I typically do not use the preamp below 15 meters. I have experimented with the preamp on with a little improvement in signal strength but I would not say with regard to S/N. Wonder what others have experienced. I noticed that many superhets have sensitivities in the .25 uV range. I consistently have stations that hear me better than I can hear them. Thanks for your thoughts..Fred WD5F
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Fred Nassar

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Posted 3 months ago

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Bill -VA3WTB

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How do you know they hear you better than you hear them?
Depending on the radio they are using, yaesu, Kenwood, Icom, their S meters are only an estemate of your signal strength, they dont relflect your accual signal strength very well.
Your Flex on the other hand work just like an lab instrument, extremly accuate.
That may acount for different readings.

I find in the feild the Flex to be very sensitive regardless of some bench testing numbers...
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Fred Nassar

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Thanks for the reply. My example above was just a relative set of numbers based on the Sherwood chart. It is not really an S meter issue. My experience is with the number of people who are responding to my CQs that I can hear are there but not well enough to get their call sign. The receiving station is clearly hearing me well enough to get a call sign and try to carry on a QSO. An example of this was when trying to work C21WW on 20 meter CW. I knew he was there based on the spotting network and I could hear him well enough to get a partial call. Of course I called him anyway and he clearly responded to me on the second call in a pile up. I had no expectation he would hear me based on my copy. I have had this experience many times. Generally, I run without the pre-amp, and an AGC-T of around 30. On CW using 250 hz with NR off and APF on. Just wondering if there some other things I can try to tweak the RX...Tnx
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HCampbell WB4IVF

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Below are Flex suggestions for setting preamp gain. Sherwood recommends the same I believe.

Howard

https://helpdesk.flexradio.com/hc/en-us/articles/204923669-How-to-determine-the-amount-of-RF-Preamp-gain-to-apply-for-band-conditions

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 reception 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.

  1. Here is the process to optimize receiver sensitivity for your given operating conditions:
    Find a clear frequency on your VFO. 
  2. Set the slice receiver mode to CW and the receiver passband filter to 500 Hz.  Turn off any noise reduction, APF, RX EQ that will add gain to the receive audio.
  3. Disconnect the antenna - You can select an antenna input with no antenna on it like RXA or XVTA.
  4. Note the dBm reading with no antenna connected.
  5. Connect the antenna.
  6. Note the reading in dBm. This is where an accurate dBm calibrated S meter really counts. Ours is truly 6 dB per S unit and measures the actual receiver sensitivity in the selected bandwidth.
  7. If the noise goes up about 8-10 dB, you have the optimal RF preamp setting for the noise on your antenna for the given time of day and propagation.
  8. If the noise with the antenna connected goes up significantly more than 10 dB, you have too much preamp gain, which will limit dynamic range for large signals. Reduce preamp gain.
  9. If the noise goes up much less than 8 dB, increase preamp gain to get it closer to the 8-10 dBm increase.

If you are not working weak signals near the noise floor, then less sensitivity is better because you want to use the least amount of preamp gain that allows you to hear the signals of interest. Even attenuation can be helpful on the lower bands. This maximizes large signal dynamic range.

You may wonder why an 8-10 dB signal (noise) increase is the target range for adjusting the RF preamp. 

With a 10 dB difference, the receiver noise figure adds only 0.1 dB of noise, which on HF has no impact on the received signal whatsoever. What you are trying to accomplish with this setting is to match the receiver noise figure with propagated noise coming from the antenna so as to maximize dynamic range for given band conditions. An 8-10 dBm differential is about right if you are working ultra-weak signals right at the antenna noise floor. Otherwise, you should turn the gain as low as possible while still maintaining the appropriate signal to noise ratio for the stations you are contacting. In other words, don't apply RF preamp gain unless you need it.


(Edited)
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Fred Nassar

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Thanks, Howard. I am aware of this and used this procedure to set the gain for each band. My gain for each band mimics the table provided in the article.  I also found a Flex community thread that was posted several months ago questioning the 6400's sensitivity and the experience in that thread are very similar to mine. I'll keep plugging away.
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John KB4DU

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You may find that your local noise floor is noticeable higher that the noise at the other end of the QSO, affecting some of the difference between you and them. I live next to an Army post. The RF noise limits who I can contact.