[Alexandria Radio Club Reflector] Fw: [TenTecEagle] Adjust for Optimum Receiver Performance

[Richard Bunn]

All,
    I have a TEN TEC Eagle and it is not a very sensitive reciever but the advice below makes a BIG difference.
73 Rick Bunn
N4ASX
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From: TenTecEagle at groups.io <TenTecEagle at groups.io> on behalf of Bob McGraw - K4TAX <rmcgraw at benlomand.net>
Sent: Thursday, June 18, 2026 1:27 PM
To: TenTecEagle at groups.io <TenTecEagle at groups.io>
Subject: [TenTecEagle] Adjust for Optimum Receiver Performance


Here is something I had written some time back.  I pass along for your consideration.  FYI -  It works quite well.

73
Bob, K4TAX








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 antenna noise to be about 10 dB above the noise floor of the receiver.   In a recent 160M CW contest in a quiet QTH location with the TS-990S, the use of 18 dB attenuation was needed to attain this 10 dB "sweet spot".  With the IC-7300 about 20 dB of attenuation was being used.

If the receiver noise floor is 10 dB below band noise, the receiver is contributing less than 0.5 dB of the total noise. The formula   P = 10log(P1/P2) shows a 10 dB difference in the two power levels is 0.347 dB.   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.

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.

I've actually encountered hams using the preamp on 80M and the attenuator on 80M.   I view this much like pressing on the car brake peddle and accelerator peddle at the same time.   It just doesn't work correctly nor as expected and certainly does not produce any results or enhancement to vehicle performance.

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



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