I just had a graphic demonstration of the fact Bob mentioned when I was
just rebuilding an old R-390 receiver a few weeks ago. I was pleased
that I saw an AM sensitivity (30% modulation in a 4 khz BW) of 0.49
microvolts for a 10 dB S/N ratio at 26 MHz. I then realized that I had
the IF gain (an internal pot) set way too high. I re adjusted it to drop
the gain and saw 0.28 microvolts in a 4 kHz BW and 0.14 microvolts in a
2 kHz BW . More isn't always better.
Dave K1WHS
On 10/31/2019 1:03 AM, Bob McGraw K4TAX wrote:
In writing this, I do so, saying that I find the method to be
excellent and reliable and repeatable with regard to HF radio receiver
operation for all modes.
In general it seems that hams have the more incorrect notion or idea
where more gain is needed to hear a weak signal. To some degree in
certain conditions that is true, but not understanding the why and the
how can actually lead to worst receiver performance issues. In my
communications with Rob Sherwood, {Sherwood Engineering, Inc.} both
face to face and in writing, I've learned that most if not many hams
use too much RF Gain with their receivers. All legacy receivers are
typically designed for 10 meters, thus attenuation is desirable on the
lower HF bands. I've adopted this recommend practice and I'm amazed at
the weak signals that I would often miss hearing that I now can hear
and pull out of the noise.
If the no signal band noise is reading upscale on your S meter, add
some attenuation and/or reduce the RF Gain. There is no point in the
noise running the AGC. You would like the no signal band noise to be
about 10 dB above the noise floor of the receiver.
If the receiver noise floor is 10 dB below band noise, the receiver is
contributing less than 0.5 dB of the total noise. Thus more gain means
more receiver noise. So how does all of this work? As an
example; the value taken from the manufactures specifications we find
the receiver noise floor to be -130 dBm. Therefore, the target of 10
dB above the receiver noise floor for the no signal band noise would
be -120 dBm. Band noise, by band, varies as much as 30 dB over the
bands 160M - 10M. In a noisy urban environment it is anybody's
guess at to you band noise level. The point being, you must adjust
receiver gain, and attenuation based on your antenna and your location
to attain be best results. It will most assuredly be different for
each band and antenna to antenna. And with directional antennas,
some directions are noisier than others.
Here's how one is able to determine band noise. On a clear frequency
for that band and with the chosen antenna, with no attenuation, no
preamp and RF gain a maximum, observe the S meter value. For our
example; lets say the S meter reads S-5 on no signal band noise. This
is equivalent to -97 dBm. There are charts readily available to
convert S units to dBm. {see ARRL Handbook} We accept the standard
of 6 dB per S unit. Now for the math; our target of 10 dB above
receiver noise floor is -120 dBm and our band noise is -97 dBm. The
difference is 23 dB. This says we need to use some value of
attenuation, example 15 dB, and 8 dB of RF gain reduction for a total
of 23 dB. This then places the band noise at 10 dB above the noise
floor of the receiver. Depending on your receiver, you may have
different values of attenuation choices. Also we must realize with
directional antennas that band noise will likely vary with direction.
Usually worst case and best case conditions must be observed in order
to resolve a nominal value.
As to the method for adjusting RF Gain, most receivers' S meter will
move up scale as RF Gain is reduced. At 6 dB per S unit, reducing the
RF Gain such that the receiver S meter indicates 3 S units would be
equivalent of 18 dB. This is easily seen when the receiver is
connected to a dummy load.
Another way to assure you have this correct is to connect an AC
voltmeter, one that reads dB to the audio output of the receiver.
Adjust the receiver gain such that the meter reads -10 db when the
receiver is on a dummy load. Then connect the antenna. If the noise
goes up 3 dB that says the the receiver is contributing 1/2 of the
noise. If you can barely hear your antenna connect, you will need
more RF Gain or less attenuation.
As to the use of the receiver preamp, again that is some amount of
gain added to the receiver chain and must be considered accordingly.
As a rule, the preamp should only be engaged on the higher bands, 6M
and 10M and to a lesser extent on other bands. Also one must take into
consideration of the noise floor of the receiver with the preamp engaged.
For further study on the subject, visit Sherwood Engineering for one
of his papers, videos, or PowerPoint presentations for Contest
University. Look under the heading of "Presentations, Tips and How
to, and White Papers" . http://www.sherweng.com/
73
Bob, K4TAX
On 10/30/2019 7:33 PM, Bill Frantz wrote:
There is no reason to always run RF gain at maximum. Otherwise there
wouldn't be a control.
I have always set up FT8 with the gains about 12 o'clock and adjusted
the RF gain so the signal strength meter in wsjt-x is about 60 to 70.
This adjustment gives the maximum level for the weak signals without
overloading the analog to digital converter. The whole idea is to get
every stage in the RF and AF chains in their linear range.
Sometimes when trying to copy a weak CW or SSB signal, dropping the
RF gain so the band noise is barely audible will help.
73 Bill AE6JV
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