Эксперименты на 8.97 кГц

[R3TNE Алексей]

Вот что получилось. При ооочень пристальном рассматривании можно предположить наличие следов трасс.... Попробуйте посмотреть непредвзято.

Александр, а нет у тебя возможности по мониторить за RSDN-20. Интересно, как они у тебя проходили бы? И как минивип справился бы с этой задачей. На 8 - 9кГц наверно и станций то нет путных, в смысле мощных.

RSDN-20:
14.880952 kHz
12.648809 kHz
11.904761 kHz

[rn3aus]

Вот скриншот что у меня видно на СДВ. Прихожу к выводу, что с 10 кГц и ниже - завал частотной характеристики...

[rn3aus]

Продолжил попытки принять на 8970. Пока днем сидел слушал, сделал запись RDL, где звучат занятные позывные.

[R3TNE Алексей]

Пользуетесь програмой SAQ Panoramic VLF Receiver v0.6 при приёме?

[rn3aus]

Нет, спектрумлабом.
В ней можно загрузить набор установок File-> Load Settings-> "VLF stations"
Есть возможность курсором наводить на интересующий сигнал на спектре и слушать его в наушниках. Можно также его и записать: File-> Save output as wavefile (примерно так называется).

[rw3adb]

Вот скриншот что у меня видно на СДВ. Прихожу к выводу, что с 10 кГц и ниже - завал частотной характеристики...

есть чуть-чуть.
по моим наблюдениям на 137 в длинных модах-это не суть важно. DK7FC принимаемый на скате фильтра выглядел гораздо лучше, чем посреди полосы пропускания.

Продолжил попытки принять на 8970. Пока днем сидел слушал, сделал запись RDL, где звучат занятные позывные.

контору палим?=)

[rn3aus]

С этого, 2014 года, частоты 8300-9000 Гц решением wrc распределены службе мониторинга грозовой активности. Видимо сдв-активность наших европейских друзей переместится в район 8270-8280 Гц.

[rn3aus]

Сигналы этого СДВ диапазона переданы через Атлантику:
W4DEX tx - Paul Nicolson RX , 6194 km, 8971 hz


At 2014-06-01 10:00 UT W4DEX began a long transmission of GPS
locked carrier at 8971.000 Hz from 35.273N,80.371W.

Between 2014-06-02 00:00 and 06:00 a carrier was measured
at that frequency here in Todmorden 53.703N,2.072W with flux
density around 0.05fT. It was not very significant so I
combined the H-field and E-field receiver outputs (correcting
for their known phase and amplitude response) to produce a
uni-directional antenna response. This brought the signal up to
a significant level.

The signal bearing was roughly west. W4DEX is bearing 285 deg
from here. The S/N is max on a bearing of 315 degrees which
puts the prevailing south-westerly background nearer to the
side of the antenna response.

This gave the signal a respectable 12.5dB S/N in 46uHz bandwidth.

  http://abelian.org/vlf/tmp/w4dex_140602a.gif

This was the strongest peak in a span of over 6500 Fourier
bins so is quite significant.

The carrier was also visible during the daylight path in 23uHz
bandwidth with average strength of around 0.02fT and with
similar S/N thanks to the lower day time noise and reduced
bandwidth.

At 2014-06-02 12:00 Dex altered the TX frequency for a blind
confirmation test. Six hours later the carrier (in 46uHz)
had vanished and a new signal appeared at 8971.100 Hz with
the same strength.

An email response from Dex confirms 8971.1 as the new frequency.

Range W4DEX to Todmorden UK is 6194 km.

Ratio of day/night flux density agrees well with LWPC
predictions for this path. LWPC predicts strongest night
signal between 00:30 and 04:30 and a sliding spectrum window
agrees with this.

LWPC requires ERP of 150uW to reproduce the observed flux
density. This is in the center of the ERP range estimated
from W4DEX antenna dimensions and measured antenna current.

--
Paul Nicholson
--

[rn3aus]

Новое достижение: первая передача сообщения (12 байт) через Атлантику на СДВ с помощью BPSK и самого мощного в истории радиолюбительства помехоустойчивого кода. W4DEX и Paul Nicholson.
Подробности здесь: http://w4dex.com/vlf/8822hz_dec14/

[rw3adb]

мощно.

[rn3aus]

Интересная статья Маркуса DF6NM, из рассылки. Так как в ней есть интересные цифры, решил выложить сюда чтоб потом не потерялась.

Big congratulations again to Paul and Dex!
 
What Paul described in unpretentious and matter-of-fact words should really be regarded as a major achievement. It has been a well-deserved fruit of several months of effort, and there were a number of difficulties to be overcome. As I was lucky to be included in the preceding email exchange, I had the chance to witness milestones and setbacks during the process.
 
Most members of the LF group will appreciate the challenges to Dex on the sub-9kHz transmit side, dealing with large coils and high voltage, realizing accurate GPS-derived frequencies and sub-second bit timing, and last not least being able to leave the signal on air reliably for many hours.
 
The information processing side handled solely by Paul surely presented an even higher hurdle: In the first place, he searched and found a small handful of "good" FEC codes. The search involved extensive simulations on powerful multicore computers hired from the Amazon cloud. Then the soft Viterby decoding of a potential receive signal is also computer intensive, especially for longer messages. There need to be numerous trials while optimising reference phase evolution, bit timing, and antenna weights.
 
Perhaps the most challenging part was guessing appropriate parameters before a transmission, ie. how many characters should be sent in which amount of time. Although some experience had been gained from carrier measurements during previous nights, ionospheric and atmospheric variations make it hard to predict SNR accurately enough if you want to exploit the channel capacity to the last couple of dBs. In the third round on new year's night, Dex and Paul dared to take a bet, and won. Allow me to cite from their final email exchange on Dec 31st:
 
Dex:
Want to test the limits?
 
Paul:
Yes please.
Let's go for broke.
8 seconds is close to the limit, which is what I'd like to see.
 

The ultimate goal of this work has been to take decoding sensitivity close to the theroretical limit. An universal metric for this is Eb/N0, the ratio of the received signal energy per payload bit (Eb in Joules) and the noise spectral power density (N0 in Watt/Hertz, equivalent to "noise energy" in Joules). The Shannon limit for long messages spread to infinite bandwidth is
 Eb/N0 = ln(2) = -1.59 dB,
which (similar to the speed of light) cannot be surpassed by any possible encoding scheme. Paul's and Dex' experiments showed that his codes can come within about a dB of this limit in a real long-distance propagation experiment.
 
To put that into perspective, let's derive Eb/N0 figures for two popular digital modes:
 
WSPR-15 transmits 50 information bits in 15 minutes, ie one bit in 18 seconds. The decoding threshold is -38 dB in 2.5 kHz, or -4 dBHz. This gives
 Eb/N0 = 10 log(18) - 4 dB = +8.5 dB,
ie. about 10 dB above the Shannon limit. Note that although different speed variants (eg WSPR-2) need different power, the minimum energy per bit has to remain the same.
 
Opera-32 carries 28 information bits in 32.6 minutes, ie. one bit in 70 seconds. The threshold is about -39.5 dB in 2.5 kHz, (-5.5 dBHz), referenced to the average power of the 50% dutycycle on-off keying. This gives
 Eb/N0 = 10 log(70) - 5.5 dB = +13 dB
or about 14.5 dB above Shannon. Note however that for LF / VLF transmissions, the limit will often be antenna voltage and peak power rather than average power, which can result in a further 3 dB disadvantage for Opera against frequency- or phase-modulated techniques.  
 
The opds correlation decoder can go about 9 dB lower than Opera. But of course it can only find the best match from an a-priori defined list of callsigns, and doesn't attempt to decode any message.
 
However we must recognize that the amateur modes spend a significant part of their energy to provide a reference for synchronisation, so not all of the Eb/N0 difference is due to less efficient encoding. The "nude" FEC-PSK mode doesn't contain any such overhead. So it can only work when the link has a stable phase (like on VLF), and the decoder has been given accurate information on carrier frequency and symbol timing.
 
All the best,
Markus (DF6NM) 

From: Paul Nicholson
Sent: Sunday, January 04, 2015 1:13 PM
To: rsgb_lf_group@blacksheep.org
Subject: VLF: Transatlantic messages at 8822Hz


W4DEX achieved another 'first' recently by sending a series of
messages across the Atlantic at 8822 Hz which were successfully
copied at Todmorden UK, range 6194km.

Transmissions used coherent BPSK signalling with ERP of around
150uW.   The modulation encoded the messages using a rate
1/16 terminated convolutional code with constraint length 25,
cascaded with an outer error detection code.

The first message was received at 2014-12-30 03:00, a 4
character message 'EM95'.  Eb/N0 was -0.8dB using 9 second
symbols.

A second test the following night managed 12 characters 'PAUL
HNY DEX' using 14 second symbols giving Eb/N0 of +1.0dB.
Conditions were good and we could have used shorter symbols
and a longer message.

The third test and best result so far was a 25 character message
'8822HZ 2015 JAN 1 TA TEST' sent from 2015-01-01 00:00 using
8 second symbols.  This was received with Eb/N0 = -0.1dB.

In the 0.125 Hz bandwidth of a code symbol, the S/N was -13.2dB.
That corresponds to -56dB S/N in a 2.5kHz audio bandwidth
after sferic blanking.  Before the blanker the S/N would be
around -76dB.

The source encoding uses 6 bits per character to produce a
payload of 150 bits.  An outer code adds a 16 bit CRC and the
convolutional encoder expands the message to 3040 signal bits.
The effective code rate is therefore 150/3040 = 1/20.27.

Of the 3040 signal bits, 1153 were demodulated incorrectly
but the FEC was able to fix them all to reveal the message.

Received signal was around 0.12 fT and it was necessary to
combine H-field and E-field receiver outputs to obtain a
sufficient S/N to decode.

The decoder is a soft Viterbi list decoder.  The signals are too
weak to reveal a reference phase by the usual method of summing
the squared complex symbol amplitudes.  Instead the decoder has
to do a brute force trial and error search.

The information rate in the 3rd test was 24.6 bits per hour
which is 80% of the channel capacity.

--
Paul Nicholson
http://abelian.org/
--

[rw3adb]

Интересная статья Маркуса DF6NM, из рассылки. Так как в ней есть интересные цифры, решил выложить сюда чтоб потом не потерялась.

молодец, что выложил сюда, не все там читают, у меня не хватает времени там каждый день читать. так бы и прощёлкали событие, как ФЦ на Аляску попал, к примеру, если б я тогда его случайно не заметил...

цифры - это, конечно забавно, но на практике опера всё же капитально сливает вспр-15 и все об этом хорошо знают.

[rn3aus]

В журнале Радио 1/2015 опубликовали мою статью "Знакомимся с диапазоном сверхдлинных волн".
К сожалению, опубликована лишь примерно треть от исходного объема статьи, в частности, исключены схемы и печатные платы МШУ, приемника и т.д.

[rw3adb]

В журнале Радио 1/2015 опубликовали мою статью "Знакомимся с диапазоном сверхдлинных волн".
К сожалению, опубликована лишь примерно треть от исходного объема статьи, в частности, исключены схемы и печатные платы МШУ, приемника и т.д.

оформи в виде пдф-ки или там док и выложи в свою тему полную версию.
будут гуглом искать- найдут по названию, оно же совпадает.

[rn3aus]

В журнале Радио 1/2015 опубликовали мою статью "Знакомимся с диапазоном сверхдлинных волн".
К сожалению, опубликована лишь примерно треть от исходного объема статьи, в частности, исключены схемы и печатные платы МШУ, приемника и т.д.

оформи в виде пдф-ки или там док и выложи в свою тему полную версию.
будут гуглом искать- найдут по названию, оно же совпадает.

Обновил страничку http://136.su/index.php/topic,199.msg9575.html#msg9575
Там можно посмотреть упомянутую статью.
Полная версия будет позже.