Hi Top Band lovers
The emotion should be aside , lets dig into the technology and ask why things
are that way nowadays.
Computer is not only part of the radio but the radio itself is a computer
today.
The reason FT8, or JT9 and others digital modes dig signals we can't hear can
be understood in 3 different levels.
1- The signal do noise ratio, as the name explain itself, is the ratio between
power noise for a nominal bandwidth and the signal level. There are two ways
to increase the signal to noise ratio . First and most common is to reduce
bandwidth, and that's why CW is a better weak signal mode than SSB, CW can be
used down to 100 Hz and SSB 2 KHz. Let's check some number's, assuming the
Noise figure is 5 db the MDS for different bandwidth are.
BW = 1 Hz > MDS = -169 dBm
BW = 2 Hz > MDS = - 165 dBm
BW = 10 Hz > MDS = - 159 dBm
BW = 100 Hz > MDS = - 149 dBm
BW = 500 Hz > MDS = - 142 dBm
BW = 2000 Hz> MDS = - 135 dBm
To copy a signal at noise level or near the MDS the BW has a huge impact.
Narrowing the BW from 500 Hz to 2 Hz the increase in signal to noise ratio, it
would be -142 to -165, it means 23 db improvement.
The JT modes uses the DSP digital data and calculations to get a practical BW
less than 2 Hz!. In order to accomplished that the clock on both PC must be in
sync by few milliseconds. That is not new at all, in the 70's we learned the
coherent CW can provide few hertz of BW but it was very difficult to
synchronize the clock between both transceivers that time without internet.
When you compare 2 KHz to 2 Hz BW the improvement is 30 db!. The JT modes are
designed to narrow bandwidth. Joe could use a "JTCW" mode and decode CW and
play it back on the speaker and let our brain to decode it. Instead JT modes
uses Salomon and other modern algorithms that decode better than the human
brain using multiple tones.
2- The call sigh decode guessing against a list is not much different from a
DX Cluster spot.
3 - JT modes is a weak signal but not a solution for manmade noise, The second
way to improve signal to noise ratio and can be used with the first one is to
narrow the beam width of the receiver antenna. Increasing RDF the signal to
noise ration also increase a 1:2 ratio, based on my 10 years or more measuring
signal to noise ration con different RDF.
Comparing signals using two instance of WSJT n the same computer, one on each
receiver of my IC7800, one signal decode from my TX antenna was -19 dBm, the
same signal decode on my HWF was +1 dBm. The RDF and rejection from manmade
noise can increase another 20 db on the JT modes using a TX vertical.
If a JT node can copy -20 db using a vertical, the same JT mode can copy -40 db
using a high RDF receiver antenna.
The new technology can evolve to synchronize new SDR transceivers using an
internet connection resulting a 2 Hz BW for a CW signal! This can become
reality, think about that!
What concerns me is the fact that the technical level of the new ham is
declining too fast. There is the feeling that "GOOGLE" can explain everything
and we don't need to know things any more, just ask ALESIA,,,
The people going to FT8 are enjoying the contacts but does not understand why?
And if they don’t mind why should us care about it?
Ham radio is beautiful, enjoy all you can the way you can!
The best antenna is the one you have and not the big gun that you don’t have,
and can’t use.
73's
JC
N4IS
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