[Elecraft] FT8 and Strong Signals

[turnbull]

Jim,  Thank you for the solid information.
73 Doug EI2CN
PS I understand AGC should be disabled.


Sent from my Samsung Galaxy smartphone.
-------- Original message --------From: Jim Brown <jim at audiosystemsgroup.com> Date: 17/07/2018  18:51  (GMT+00:00) To: elecraft at mailman.qth.net Subject: [Elecraft] FT8 and Strong Signals 
On 7/17/2018 8:52 AM, turnbull wrote:
> I agree but many people are suffering from wide FT8 signals from others with strong signals.

Part of this is perception rather than reality -- a failure to 
understand the waterfall display. Because the waterfall displays both 
signal strength and spectrum, a very strong signal may APPEAR to be 
broader than it really is. I have a half dozen or so neighbors running 
FT8 on 6M within about 15 miles, and all are running a power amp (mostly 
KPA500, but my closest neighbor, about 5 miles, runs 1.3kW. Most of us 
are running K3s. It's not uncommon for one of these signals to SEEM to 
occupy 500 Hz or more, the reality is that I often decode weak signals 
within that 500 Hz.

Another point. K1JT recommends setting audio level into the computer for 
an indicated 30 dB on the bar graph. I think this is part of the problem 
caused by strong signals taking over the AGC and greatly reducing the 
gain. If, for example, a local who's 30 dB over S9, reduces the RF gain 
by 30 dB, an S9 signal would be below the level of the sound card! I set 
audio level into the computer for the highest level that doesn't flash 
RED (indicating digital clip) on strong signals. My bar graph typically 
runs 50-60 dB. With this setting, even when AGC has reduce the gain by 
30 dB or more, weak signals are still well above the "bottom" of the A/D 
converter, and are decoded just fine.

73, Jim K9YC

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