Re: LF: VLF traces

[Andy Talbot <andy.g4jnt@gmail.com>]

Can I point out that a carier that appears at a known time and on a known frequency IS modulated.   It is on off keyed.  (Well...off/on anyway :-)    The only way that a carrier can be unmodulated is if it has been there since the beginning of  (RF)time and is never switched off, ever, never, before the end of (RF)time.   Its appearance or disappearance is information;  what that info means depends on how it has been coded.

 

Any switching, at all, introduces modulation and widens the bandwidth, and therefore counts as a means of identification if the meaning of the on/off is pre-arranged.  

 

In comms-theory-speak  If G9BOF makes the announcement "I'll be transmitting on Sunday at 1300z on 8910.0004Hz" he has now generated a codebook.   The appearance of the carrier at that point in time indicates a single bit of information, which by reference to the code boook means G9BOF is sending a '1'.  No carrier detected,he is sending a '0', or there has been a bit error.   In other words, no error correction.or detection has been performed.   If G9BOF now makes another announcement that "I'll switch it off at 2358z" he has added another entry to the code book.  If the carrier really does go off then,  the two bits of information  together make a dual-redundant pair, the minimum needed for error detection.  

 

If G9BOF also makes the statement that "my signal is stable and accurate  to 1mHz" and another signal appears in the passband that is 2mHz away, or wobbles by 1.5mHz, then is is not G9BOF, and error detection will flag it as not valid.   If a receiving station cannot detect this error, and incorrectly assesses the carrier then the Rx has used the wrong codebook, and therefore cannot be deemed to be listening to G9BOF at all.  

 

All a rather extreme and somewhat petty  example of coding theory and error detection,  but its exactly the same as simple Hamming codes all the way thorough to reed Solomon.  And...  I'm not sure how G9BOF getting his frequency or time wrong would fit into the coding theory.

 

So, conclusion, a carrier as stable and on the exact frequency as stated, at the right time is going to be valid.   If you have no way of proving it really is as stable as quoted, or in the right place, you've no right to question its validity.

 

And anyway, how many spurious signals can even give any pretence to being stable in these terms - if you think they are, get a better receiver.   We are talking about 0.1ppm over the duration of a signalling element as a minimum frequency stability requirement for most serious VLF through LF signalling - which means a quite good TCXO, an average to middling OCXO or ideally a locked source.   If you can't manage this, do not cry 'foul' when others do.

 

Andy

www.g4jnt.com

 

 

 

 

On 11 March 2011 21:30, Markus Vester <markusvester@aol.com> wrote:

Dear Mal,

with all due respect, I very much disagree here.

Whenever I see a clear trace appear at the right time and within a milliHz
of a confirmed and correctly calibrated frequency, I have hardly a doubt
about the validity of the reception. Even more so if the transmission can be
verified by comparison to a calibrated grabber in another location.

It seems that the G3KEV antenna does have excellent sensitivity at VLF. It
would be fantastic if you could augment that with a stable, properly
calibrated, and narrowband reception system. SpecLab has made that so easy
now - throw out the wildly drifting "receiver", connect your antenna
directly to the soundcard instead, lock to one of your MSK neighbours (GBZ
or GQD), and your in the game.

Best regards,
Markus (DF6NM)


From: mal hamilton
Sent: Friday, March 11, 2011 5:57 PM
To: rsgb
Subject: LF: VLF traces


I can see a couple of vy weak traces around 8970 but this is no guarantee
who it might be because there is no ID. Frequency alone is not a VALID
report- it could be anything.
G3KEV