[RTTY] KJ4QF North American QSO Party, RTTY

[Kok Chen]

>> I seem to have an asynchronous antenna. (I think I made that up).  
>> I can
>> receive well, but not everyone could receive me well. Strong  
>> stations did
>> not answer my call even when nobody else was calling them. I think  
>> my sig
>> was nice and clean so I suspect I have some antenna work to do.
>>
>
>    I must have the same "asynchronous antenna", I experienced the  
> exact same
>    symptoms.

On the surface, communications systems obey this thing call the  
Reciprocity Theorem.

What it says is that if each of you is transmitting the same power,  
then no matter what antenna you are using (dummy load) and what  
antenna he is using (a 500 foot dish), each of you will measure the  
same received signal strength at the two antennas' terminals when the  
other is transmitting.

This seems to imply that things should be symmetrical.  But that is  
true only if you don't take sky noise and QRM into consideration.

Where the asymmetry can come in is from the Signal To Noise ratio,  
which is probably more important to copying an FSK signal than is  
signal strength.  There is a very steep threshold between where RTTY  
is copied virtually perfectly, to a state where it is virtually  
garbage.  With FSK it could be as little as 4 dB or 5 dB that  
separates a signal with good copy to no copy (i.e., good modem,  
Gaussian noise case).

Just because he is loud to you does not mean that he is receiving you  
with a good SNR.  The most common case is when he is beamed away from  
you; he now hears more noise and QRM from the direction he is beamed at.

It is worse if he has set some kind of squelch in an attempt to get a  
"clean" display on his screen -- this is why I always run my modems  
wide open (no squelch) and my speakers turned up :-).

Where the detection threshold sits also depends on the two modems.   
His modem's SNR threshold could be 10 dB worse than yours (not  
uncommon, really).   Here is where having a good modem is very  
frustrating -- if you have a better than average modem, there will be  
more stations that cannot copy you, when you can copy them perfectly.

Some of the things that you can do when you can hear them and they  
can't hear you are (i) get them to point their antenna your way --  
this improves their SNR, (ii) raise your transmit power -- this  
improves their SNR, (iii) convince them to get a better modem -- this  
moves their SNR threshold, (iv) ask everyone else to QRT -- this  
might improve their SNR, (v) go to PSK31 -- which requires a lower  
SNR than steam RTTY :-).

73
Chen, W7AY