Date: Thu, 5 Oct 2017 09:40:42 +0200 (CEST)
From: Jacek Lipkowski <sq5bpf@lipkowski.org>
To: rsgb_lf_group@blacksheep.org
In-Reply-To: <7737095b-edc9-8b5f-ebf5-2a6b2884d425@abelian.org>
Message-ID: <alpine.DEB.2.11.1710050933380.734@mailn.lipkowski.org>
References: <15ec7c794b3-bff-66@webjas-vac136.srv.aolmail.net> <alpine.DEB.2.11.1710022138560.734@mailn.lipkowski.org> <59D2BD65.4080909@posteo.de> <59D4B109.27652.1415B4FA@mike.dennison.ntlworld.com> <7737095b-edc9-8b5f-ebf5-2a6b2884d425@abelian.org>
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Subject: Re: VLF: 300 mA on 6470 Hz
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maybe Stefan could qsy a bit?

for example i'm 19.8 wavelengths from Stefan (and Paul is almost the 
same). shifting 81 Hz either way would make the path 1/4 wavelength 
longer/shorter. if there are some multipath propagation effects then i 
should get a stronger, or weaker signal. the offset should be the average 
of offsets that would lead to a 1/4 wavelength path difference for all 
stations that currently receive Stefan.

VY 73

Jacek / SQ5BPF


On Wed, 4 Oct 2017, Paul Nicholson wrote:

> Date: Wed, 4 Oct 2017 12:49:28 +0000
> From: Paul Nicholson <vlf0403@abelian.org>
> Reply-To: rsgb_lf_group@blacksheep.org
> To: rsgb_lf_group@blacksheep.org
> Subject: Re: VLF: 300 mA on 6470 Hz
> 
>
> Stefan wrote:
>
>> And for some reason, the signal was much stronger last night
>> at SQ5BPF. Did you see it? What could be the reason?
>
> I think this is typical of short ranges.
>
> When I model the propagation using ray tracing, it needs the
> first dozen or more rays (ground wave, 1-hop, 2-hop, 3-hop and
> so on) to produce something like a realistic diurnal.  At some
> ranges, 2 or 3 rays have significant contribution for much of
> the time amd small shifts of relative phase of the rays can make
> large changes to amplitude.   Maybe we are seeing that.
>
> It gets simpler at longer range: the contribution from higher-hop
> paths is much reduced.
>
>> i'm changing to:
>> Coding 8K19A
>
> OK.
>
> The table shows also that for longer messages - convolutional
> payload of 36 bits or more such as 4chars+CRC12 or 3chars+CRC18,
> it is always worth using a rate 1/16 code, ie 16K.
>
> 8K or 4K codes have symbols twice and four times the duration,
> so would only be better if symbol timing was a potential problem,
> eg with fast signaling at LF or MF.
>
> You can see also that longer constraint lengths are more
> successful with larger payloads.  Eg no point in using more
> than K21 with payloads between 18 and 36 bits, and limit to K23
> which payloads up to 72 bits.
>
> The convolutional payload is always 6 * number_of_chars + CRC size.
>
> --
> Paul Nicholson
> --
>