Re: question for psk users

Posted by gm3sek on May 02, 2012; 8:05am
URL: http://elecraft.85.s1.nabble.com/Re-question-for-psk-users-tp7517322p7519383.html

Jim Brown wrote:
>> Because with the close spacing of signals in the various PSK  sub-bands, a
>> signal of more than about 50watts will completely annihilate all the other
>> PSK signals and make that sub-band unusable for everyone else.
>
>This is urban myth that keeps getting propagated and eventually
>everyone believes it as gospel.
[...]

The reason why a strong signal will "annihilate all the other PSK
signals" is that most users are trying to receive an entire 2.5kHz
bandwidth in one gulp. This means that even one strong signal within
that bandwidth will activate the AGC and drive all the other signals
down.

Regrettably most PSK users don't understand what is happening, so they
have to rationalize it into simpler terms like "It's a crime to be
loud."

>There are MANY factors that contribute to signal strength, including
>antenna efficiency and gain, antenna pattern, propagation, and yes,
>transmit power.  Of those factors, transmit power is the SMALLEST.

And propagation is by far the BIGGEST factor in making someone's signal
loud, for the most innocent of reasons.

>What is FAR, FAR more important to prevent interference is making sure
>that your signal is CLEAN -- no sidebands produced by audio distortion,
>or by distortion in a poorly adjusted rig or power amp.

IMD is certainly another important factor. Naturally, IMD will show far
more clearly on a strong signal than a weak one so once again this gets
rationalized into "strong = IMD".

But PSK's big, unacknowledged 'elephant in the room' will always be the
habit of receiving in a 2.5kHz bandwidth. Unfortunately this is deeply
engrained in the PSK culture.



--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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