>> 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
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