On Feb 5, 2014, at 12:38 PM, Bill Turner wrote:
> What am I missing?
Let me explain.
The keying sidebands of a continuous phase FSK signal is down only about 50 dB,
even when 1 KHz away from the signal. A waveshaped AFSK signal or a K3 FSK
signal will not be as poor, and a phase coherent FSK signal will be much worse,
but lets assume that a lot of people in the pile are using continuous phase FSK
(what most DDS superhet rigs today use).
With my SDR, I see lots of loud stations that are 60 dB over my panadapter's 6
Hz equivalent noise floor. I can usually copy clear frequency DX that is 6 dB
above this noise floor.
Lets say I call the DX and he comes back to me. However, people around me
don't know that he has come back to me, so they keep transmitting while the DX
is transmitting (this is the weakest aspect of the
DX-calls-you-where-you-are-transmitting method).
The above does not even consider receive IMD, which will make the problem
worse. Check out VA7OJ/AB4OJ's Noise Power Ratio NPR measurements, many
transceiver NPR numbers are in the mid 70s dB.
http://www.ab4oj.com/test/docs/npr_test.pdf
(The NPR method measure the rises in noise floor of a notch in the presence of
noise outside the notch -- think of the notch as a hole in the pileup and the
pileup being the noise level of the NPR measurement.)
That means that under usual operation, because of the FSK keyclicks, you will
need to be in the middle of a 2 kHz hole to even hear the DX come back. If you
have a directive antenna, the keyclick QRM problem still exists, but on
average, perhaps 10 dB to 20 dB better than with a omni directional antenna
like a vertical.
Due to that, I'd rather take my chances with the current methodology, where the
LIDs and frequency cops are rare compared to the constant FSK keyclicks from
strong stations.
That explains my "vertical" comment.
Now to my 100 watts comment.
For decades now, the only way I can work a heavy RTTY pileup is to call the DX
right at the previous QSX and hopes that 1) he does not spin his know right
away (pretty much the behavior of the DX even today), and 2) a stronger station
does not decide to do the same.
Because of that, I built up my station around two receivers, waterfalls and
what I called the click buffer in cocoaModem (which copies a signal that has
stopped transmitting) so I can find the QSX more easily. If you can find the
QSX faster than other people can, this method works wonders as long as the two
conditions I mentioned in the previous paragraph holds.
Also because of my weak signal, just like the reverse direction, I need a wide
hole so the DX can hear my signal through the keyclicks. Because of that, the
hole method has never worked for me.
The pounce-on-QSX method used to work really well before Flex Radio came out
with the SDR-1000. But not much worse (you can count SDR-1000 RTTY ops with
the fingers in one hand).
When Flex Radio released the Flex-5000, there was a sudden increase in the
"mini-pileup in a pileup." And today, with the P3 and such instruments, the
QSX method is even harder, but still better than transmitting where the DX is
not listening.
Before my success with the QSX method, I had to either 1) find the DX before he
is spotted, or 2) wait for propagation to attenuate the rest of NA signals to
the DX (but I then usually have to compete with a JA wall instead)..
73
Chen, W7AY
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