[Kenwood] help with qrm/qrn

[Dr. Gerald N. Johnson, electrical engineer]

No HF radio mode other than a little 10 meter FM has the noise
suppression of the VHF FM radio. Any radio that has adequate sensitivity
for copying weak DX signals has the gain to produce copious noise on SSB
and CW modes. That's true whether its at 30 KHz, 3 MHz, 30 MHz, 300 MHz,
3000 MHz, or higher. Some of the current rice boxes for VHF/UHF
multimode are short on gain on SSB.

Some of the audio DSP modules are effective at reducing the ordinary
background noise for more comfortable copy. I know my Timewave DSP-59+
does that well, though if I toggle the AVC option on, the noise level
comes back up but signals that were just detectable without it are now
copiable. Audio processing is most effective when the receiver filters
don't ring on noise to make it more prolonged. For that reason I prefer
the filters of the Tentec receivers, from Corsair and Corsair II (superb
strong signal handling rarely matched from overseas) through the Omni
VI+. The VI+ does include an audio DSP, though I've not tested it.

An important consideration is the detected Signal to Noise ratio. Some
receivers with all too many mixing stages and too many IF stages after
the filter's selectivity have a relatively poor S/N at audio. Receivers
with fewer conversions such as some earlier Tentecs and maybe the FT101
should do better for S/N at audio though that can depend a great deal on
the quality of the IF stages.

You can often reduce noise by backing off on the RF gain of the
receiver, though you will begin to neglect the weaker signals in the
process. Some receiver's RF gain sets a threshold. I often run my
Corsair II with the RF gain backed off for less noise. And my 75S3B RF
gain control is effective at that also.

The vertical antenna will be optimum for picking up the most locally
generated noise.

I sometimes use a passive low pass filter in the speaker circuit to get
rid of all circuit noise. I have one that I can switch the cutoff
frequency down to 440, 940, 1800, or 3600 Hz, and the lower cutoffs are
really nice for bringing out a low pitched CW signal. The audio DSP
boxes fail to filter the wideband noise of their output audio amplifier.

I think Yaesu sells a passive speaker filter for just such occasions.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.