Sensitivity vs Dynamic Range Flex 6400

Fred KE4Q

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

Bill -VA3WTB

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...

Fred KE4Q

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

HCampbell WB4IVF

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.

Fred KE4Q

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.

John KB4DU

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.

Dave S

I, like others in the community are also concerned about low signal receiving as stated above. Are we missing some kind of super secret settings regarding "APF" ???

What is APF?

DX receiving is always very challenging and wondering why others "hear" so much better than I can manage...Please do not tell me how to adjust the pre-amp level again.

73,

Dave KE1AV

Stan VA7NF

In most cases your real receive capability lies in the EMI of your location. This noise is the aggregated collection of all the local switching power (includes variable speed motors found in many appliances and industry), sparks and corona in power distribution; and how that noise enters your receive antenna.

If using coax, much local noise is "received" on the outer coax braid. That noise must be prevented from A) making it to the antenna wire and centre conductor and B) making it back to your receiver chassis which forms half of your receive circuit.

Ohms law comes in here: Make a high impedance to where you do not want the noise and a low impedance path to dump it to ground. A toroid filter outside the coax will make a high impedance between coax braid and the antenna ground side. At the shack end of the coax should be another toroid filter making another high impedance going into the shack. Just BEFORE the shack should be an RF ground (ohms law again) making a very LOW impedance to ground that will take all that noise off the cable and very little will reach the receiver via both centre conductor and shield conductor.

Another, not so much recognized, is mixing products which CREATE noise AT THE RECEIVE FREQUENCY. To bring this into perspective, consider the VHF repeater operation where it receives 600Khz away from the transmit carrier while transmitting. A local AM broadcast station at 600Khz may mix with the VHF transmit power in, for example a rusty bolt or aluminium flashing creating a phantom repeater receive input.

The same happens that any diode on a wire (e.g. rust) will mix every HF signal with every other HF signal and CREATE receive frequency signals (noise) that no amount of filtering can remove; it IS the received signal. These sources must be found via direction finding as the RF source is the mixing location that may just happen to be a rusting joint on your tower.

So your actual noise sensitivity is both the quality of the antenna and the elimination of local noise generators.

TMI (Too much information)? but may be useful to some.

John KB4DU

For a test, maybe try a low noise receive antenna. I have been experimenting with a “Loop on Ground”. Really cheap and easy to make, there are a few articles online. It does reduce the signal strength as well as the noise, but in many cases the signal to noise ratio improves enough to copy an otherwise uncopyable signal. No external preamp required,the Flex has plenty of gain to compensate for the lower signal level. Mind is just speaker wire on top of the grass in the front yard looped around some tent stakes at each corner. It attaches to the rx in port so the tx power doesn't feed it.