[Fwd: LF: SLow CW between the LORAN lines]
Sat, 23 May 1998 18:16:41 -0400
Looks to me like a job for the DSp Wallahs...
Peter Martinez wrote:
> >From Peter Martinez G3PLX Kendal Cumbria.
> Following some discussion on the 80m net about how to avoid the
> LORAN spurii for very low-signal working, I thought I would dig out
> some calculations I did a while ago and feed them into the group.
> The LORAN signal is centred on 100.000... kHz (as many zeroes as
> you like). The one we hear splattering across the 136 kHz band
> radiates pulses with two different repetition rates. The RF waveform
> in the pulses starts at exactly the same point in the RF cycle on
> each pulse. This means that the carrier frequency must be a
> whole-number multiple of the pulse repetition frequency and that the
> spectrum consists of two arrays of closely-spaced carriers, each
> array having a carrier spacing equal in Hz to the p.r.f. and at 100
> kHz exactly, the two arrays of carriers line up.
> With this information and a few measurements, I was able to work
> out absolutely the values of the two p.r.f's, and hence the carrier
> spacings. They are 100000/14998 Hz and 100000/13462 Hz, or
> about 6.67Hz and 7.42 Hz. We can work out where the spurii come
> in the 135.7 to 137.8 kHz band by noting that one set of frequencies
> is the series N*100000/14998, where N goes from 18268 to 18551,
> and the other set of frequencies is M*1000/13462, where M goes
> from 20352 to 20667.
> What we want to do is find the gaps! The biggest gaps will be found
> either side of the places where the two sets line-up as closely as
> possible. They never line up exactly within the band. There are 7
> spots where they line up closer than 0.1Hz, so the best gaps are
> about 6.5Hz wide centred 3.3Hz either side of these spots. I give
> these spot frequencies below.
> Peter G3PLX