Re: K3 APF Adjustable Q

Posted by Kok Chen on Oct 31, 2010; 6:43pm
URL: http://elecraft.85.s1.nabble.com/K3-APF-Adjustable-Q-tp5688670p5691931.html

On Oct 31, 2010, at 10/31    10:05 AM, Byron Servies wrote:

> Naieve newbie question, because im missing something here: if the  
> signal you want is that hard to receive, what are the chances the  
> other station will be able to hear your reply?

Since I use simple antennas, and even though I always run barefooted,  
I *always* assume a DXpedition to an isolated island can copy me when  
I can barely copy them above the noise.

Although Reciprocity Theory states that if both of you use the same  
power, the received power at the two antenna terminals will be the  
same independent of what antennas are at each end, other things  
determine if he can copy you better than you can copy him.

The obvious factor that influence whether he can copy you is therefore  
obviously the amount of power that you use compared to the amount of  
power that he uses.

However, another factor, arguably more important, is the antennas'  
directivity and where the directivity are aimed at.  If his antenna is  
more directive than yours by 3 dB, all else being equal, he has a 3 dB  
of SNR advantage.  If the arrival angle of his signal at your antenna  
is 10 dB below where your antenna actually peaks, while your signal  
arrives at his antenna where the response of his antenna peaks, you  
have yet another 10 dB disadvantage, etc.

The same SNR argument applies to how noisy his local RF environment is  
compared to your environment.

And of course, how optimal and easy to use the filtering at his end is  
to pull your signal out of the noise, which is what the APF is all  
about for CW (and correspondingly, the use of matched filters for  
digital modes).

73
Chen, W7AY

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