Message-ID: <005101bf6cc3$633678c0$2301a8c0@maly.gmc.net>
From: "Petr Maly" <p.maly@gmc.net>
To: rsgb_lf_group@blacksheep.org
References: <3892A61A.C049A749@virgin.net> <E12Fb5r-00018z-00@mail1.isys.net>
Subject: LF: Re: Transatlantic
Date: Tue, 1 Feb 2000 15:48:06 +0100
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My question was not as much serious as it perhaps seemed. I am sorry for
this misunderstanding. Vice versa, breathlessly I read all those thoughts
and ideas how to cross the ocean. With a thrill I wonder who and when will
be this "first". I admire what fools we are and what all we are willing to
undertake to get the goal. Is it clear now? Let's stop this silly discussion
about sense of our great hobby.

73, Petr, OK1FIG


----- Original Message -----
From: Klaus von der Heide <v.d.heide@on-line.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, February 01, 2000 11:55 AM
Subject: LF: Transatlantic


>
>
> 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
>
>
>