[Elecraft] K3 Power-hunting on FT8

[Joe Subich, W4TV]

Doppler caused "spreading" is readily identified ... the spreading is
generally in one direction (because airport "patterns" run in one
direction) and often manifests as a second signal offset by 60-80 Hz
(which can be independently decoded).  IMD is generally in both
directions and can not be decoded.

The *most disgusting* distortion is on signals in which the owner
pumps audio into a dynamic mic input and overdrives the mic preamp.
The mic gain control of most traditional rigs follows the preamp
and the preamp is often into clipping - particularly when the "speaker"
or headphone output of a sound card is run into a dynamic mic input
without an attenuator.  Electret inputs typically require a 20 dB
(10:1 voltage - 10 K series, 1K parallel resistors) attenuator while
dynamic inputs call for a 40 dB (100:1 - 47 K series, 470 Ohm parallel)
attenuator.

73,

    ... Joe, W4TV


On 7/23/2023 3:13 PM, Jim Brown wrote:
> On 7/23/2023 9:49 AM, Andy Durbin wrote:
>> It is my belief that many of the wide FT8 signals seen on 6 m FT8 are 
>> caused by Doppler shifted aircraft reflections.  It's very common here 
>> in the Phoenix metro area and does not imply that the transmitted 
>> signal was in any way abnormal.
> 
> YES! It's very common anywhere around airports with much traffic. We see 
> lots of it in the SF Bay area -- N5KO, W6RN, WB6RJH, and I are roughly 
> 20 miles from SJC (San Jose) 30 miles from SFO (San Francisco), a little 
> farther from OAK (Oakland). There are other propagation-related causes 
> for spreading signals.
> 
> 73, Jim K9YC
>