Re: K3S NR

Bob McGraw - K4TAX


You might give the ATTN and RF Gain some work to do.   With information provided by a highly respected ham and one that evaluates a lot of radios, in a conversation and discussion we had at the Huntsville hamfest a couple of years ago,  he suggested that most hams use too much RF Gain for typical band conditions.   In his words, for legacy receivers such as the K3 series and other like radios the optimum receiver performance occurs when the no signal band noise is some 10 dB above the receiver noise floor.

How does one accomplish this in as much as we have no control over receiver noise floor?   The solution is then to attenuate the signal and noise arriving at the antenna input.  The attenuator and RF gain is used as a step in this direction.  

To do a bit of math;  the noise floor of the K3 series radio is typically about -127 dBm.  This is equivalent of S-0.   Now at our QTH with the antenna of choice the no signal band noise is S-4 or -103 dBm.  The difference between the two values is 24 dB.  Now to have the noise appear 10 dB above the noise floor is should be at a level of -117 dBm.   The optimum level is then a difference of -117 dBm {RX noise floor + 10 dB}  minus -103 dBm or 14 dB.  Thus 15 dB of attenuation brings the configuration to within 1 dB of the optimum receiver noise floor to band noise. 

In some cases, additional signal / noise reduction is needed in the case where the no signal band noise is higher.   Again an example:   Noise floor is -127 dBm,  No signal band noise is S-7 or -85 dBm.  Thus the different is 32 dB minus our desired 10 dB.   Here we need RF gain reduction of 17 dB or about 3 S units, plus 15 dB of attenuation.

Once the method is applied, we then find that signals pop out of the noise.  And in this case the actual receiver noise only adds about 0.5 dB to the total noise value.  I've tried it on several receivers including my K3S, my Tentec Eagle, my Tentec Omni VII and my KW TS-590S.   It really does work.   It is just difficult to get past the mental notion that more gain is needed to hear weak signals.   Where in practice, less noise is required to hear weak signals.


Bob, K4TAX

On 8/25/2019 8:06 PM, Jim Denneny wrote:
Thanks Bob.  I will stick with TXDATA & NB
Jim K7EG

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