[Lf] [Fwd: LF: Transatlantic]

[Andre' Kesteloot]

Klaus von der Heide wrote:

> Hello dear LF Friends,
>
> thanks for the many and divergent replies to my post!
>
> My Mother celebrated her 90th this weekend, so I didn't
> have any time to give an answer immediately.
>
> Now, let me quickly try some comments to the different
> points:
>
> 1. Science and HAM Radio
>    =====================
> Thanks to Gamal for his answer to Petr's serious
> question what HAM radio is !
> May be, all discussions and experiments clearly say
> that a forest of 100 m towers, many DSP's and a super
> computer are necessary to get information on LF across
> the ocean. Then, Petr, OK1FIG, please ask again.
> My intention was to start a discussion on ideas that
> may be practicable for radio amateurs. As long as
> there is no answer we have to discuss - just as in
> science.
>
> 2. PSK vs. ASK
>    ===========
> ASK keyes between sinus signal and no signal, PSK
> keyes between sinus and minus sinus. The distance
> between the two signal alternatives measured in
> voltage is doubled with PSK while the noise remains
> unchanged. If on the transmitting end the maximum
> power is limited (not the average power) the gain
> of PSK over ASK is 6 dB, i.e. to get the same SNR
> with ASK four times the power of PSK is necessary.
> If ASK is detected incoherently as usual a factor
> of 2 (3 dB) has to be added to this difference
> because more noise is received. Unfortunately with
> real channels the gain is not as great (see: DPSK).
>
> 3. Long Integration Time
>    =====================
> The ideas proposed by Andre', N4ICK, are good when
> ASK or FSK is used with non coherent reception.
> FSK would be the better choice.
>
> But a synchronization to time ticks of a second is not
> a problem. Following the comments of Larry, VA3LK,
> and Johan, SM6KL, GPS can be used. I prefer DCF77
> or MSF because of its simplicity.
>
> In fact, if no information is sent then the result of
> a very long integration time is the same as integrating
> over many short time slots. The advantage of a one second
> period or even faster is a better averaging of non
> gaussian noise (especially in the case of ASK).
> Using a random bit pattern helps to minimize man
> made noise. Possibly, one should not use the second,
> but 5 minutes divided by a prime number to get away
> from anything that is synchronized to a clock.
>
> If real information is sent, then the transmitted
> bitrate should be considerably greater than the
> information bit rate. This is achieved by heavy
> coding with a low code rate. But that is not a point
> of discussion as long as even the carrier cannot
> be heard.
>
> 4. DPSK
>    ====
> Unfortunately, there comes a loss with PSK when the
> communication channel may vary in delay. A delay
> produces a phase shift, and a varying delay results
> in a changing phase. The symbol rate must be so fast
> that the change in phase from one to the next symbol
> is negligible. 136 kHz is about 14 MHz/100. On 20 m
> the symbol rates of PSK31 and Pactor2 (31 and 100 Hz)
> are in the optimum region of 20 ... 200 Hz. I simply
> guessed that 1/100 of that (1 Hz) should be good for
> LF although the waves may travel quite another way.
> As a consequence of the varying channel, one cannot
> use a constant reference phase. One therefore detects
> the difference between the last and the actual symbol.
> Because now the noisy last phase is used as reference
> instead of an absolute and noise free one this
> differential PSK (DPSK) has a loss of up to 3 dB
> over the absolute PSK. At extremely low signal level
> FSK is as good as DPSK. Therefore FSK should remain
> in this discussion.
>
> 5. Coherence
>    =========
> As mentioned above, coherent detection gives a better
> SNR. But, if the signal is so weak that the time to
> transmit one single information bit (that means
> identification of the carrier) is longer than the
> period of approximately constant phase then the
> 3 dB gain of coherence are lost. In other words: If
> you cannot keep track of the carrier things get worse.
> Using a milliwatt on 20 m it's the same situation and
> 2 m aurora too, the only difference is in the time scale.
> Where are we on LF? Is something known about QSB periods
> on transatlantic LF? If the typical QSB-period is a few
> minutes long or faster coherent detection is impossible.
> I then would try FSK keyed with a random pattern at
> a symbol clock of 1 per second up to 1 per minute.
>
> 6. Phased Array / Synthetic Aperture
>    =================================
> Paul, OH3LWR, is right in pointing out the problem
> of a complex radiation pattern with many "fingers".
> That's the reason why I proposed a mean distance of
> a quarter of the wavelength. Then nearly the maximum
> gain can be achieved with only one main lobe.
> There are many different possibilities for the
> realization of the correct phasing of transmitting
> and receiving antennas. These should carefully be
> discussed. Paul's proposal to use a local BC or TV
> station seems to me a practical one.
>
> 73 de Klaus, DJ5HG