RF Preamp setting

John, G3WKL

On the topic  "FT8 Receive Meter Control" Tim, W4TME usefully reminded us of Gerald's help topic on setting the RF Preamp gain.  

We've got some Es today so not having been on 10m for a while, and as there were some signals for SNR assessment, I thought that I would follow Gerald's step-by-step process for setting the RF Preamp Gain.  This is what I recorded (Flex 6700, Butternut HF6VX vertical, and about 90m of RG213 feeder): 

No antenna (XVTR): -135dBm

RF Gain (dB)       Noise in 400Hz BW (dBm)
0                          -119
+10                      -127
+20                      -130
+30                      -132

So, following the quoted article, +10dB of preamp gain would seem to be optimum as it gives an 8dB increase in the measured noise floor. 

The reason for posting this question is why isn't the gain of +30dB better, as that gives only a 3dB increase in the measured noise floor, i.e more RX sensitivity.  Is it sub-optimum purely from the aspect of dynamic range?

73 John

Steve-N5AC

John, can you provide the same measurements with the antenna disconnected?

John, G3WKL

Ah, I get it now!  

0            -119
+10        -126
+20        -136
+30        -143

So the +20dB setting gives 6dB and +30 a +11dB increase.  So the +30dB gain is closest to Gerald's optimum setting

Thanks Steve

Steve-N5AC

No, you were correct the first time (take measurement with antenna on) -- I just wanted to be sure everything was OK with your radio before we got started here. I'm going to provide a bit of a dump on how all this works and feel free to ask questions.

Noise can come from a lot of places, but what we're most concerned about here is the noise in your receiver (due to design) and noise that is propagating to your location (atmospheric noise). Noise is measured as power inside a specific bandwidth. A "perfect" noise figure is 0dB and is -174dBm/Hz read as "minus one-seventy-four d-b-m per hertz" which means a power of -174dB below a milliwatt in a 1Hz bandwidth. Since your measurements are in a 400Hz bandwidth, we must convert to that. The formula for this is 10log(new bandwidth / old bandwidth) or 10log(400/1) = 26dB. So this must be added to the -174dBm/Hz to give us -148dBm/400Hz. This would be a 0dBNF.

Now, first let's look at your receiver using the disconnected antenna. With the +30dB preamp on, your noise figure is -148 - -143 = 5dB. This is a very respectable 6m noise figure and is appropriate for a rural area. With +20dB we have a 12dBNF and with +10dB we have a 22dBNF.

With the antenna connected, now, we can see how much atmospheric noise you have. The difference in the measurements shows the contribution of the atmospheric noise to your noise floor. These numbers are:

+30: 143-132 = 11dB
+20: 6dB
+10: 1dB

You generally want the atmospheric noise to raise the level of your noise by around 6dB or more in order to ensure that your receiver noise floor is below the atmospheric noise. In your case, I would say the +20dB setting is fine. +30dB is also OK provided that you do not have strong signals -- you will reduce your noise by another 2dB using that setting, but your strongest signal handling will reduce by 10dB. If there are no strong signals around, the +30dB is fine also. If you have strong signals near you, use +20dB.

The noise heard in your receiver is also affected by your coax and at VHF this is not an insignificant thing... you should also calculate the loss in your coax to determine if you might want better or shorter coax. This is a common problem in VHF stations -- a good radio and antenna are purchased and then a 100' run of RG58 is used (3.2dB) instead of LMR-400 (0.9dB) or LMR-600 (0.5dB). The coax loss numbers add directly to your noise figure.

Finally, antenna selection is important. For most 6m work (other than FM) you will want a horizontally polarized antenna like a loop or a beam. The butternut will generally pick up a lot more noise. I have used the M2 6m HOLOOP (https://www.m2inc.com/FG6MHOLOOP) with great success and some folks stack these. My guess is that a beam or loop will drop your noise by several dB.

John, G3WKL

Steve,

Thanks for the detailed reply, which I've just scanned as my slot for amateur radio has gone...  just quickly I should remind you that this is 10m and using a Butternut HF6VX vertical and about 90m of RG213 feeder.  In a "residential" ITU type environment...

Catch up later

Regards

John

Steve-N5AC

OK terrific, John -- not sure where I got 6m. I had probably just read something else about 6m!

John, G3WKL

Thanks, again. All makes sense now. I've got quite a lot of loss in the feeder....

73 John

Ken - NM9P

Another great post, Professor Hicks.