Dear John, LF Group,
First, many thanks for your continued efforts in getting us access to these
out-of-the-way, but interesting, bits of spectrum, and I certainly look
forward to an extension to my NoV.
Re - possible limits on transmission bandwidth. Most of the modes that have
been used on 500kHz will fit within a 200Hz bandwidth, so having this
restriction would rarely be a great hardship . However, I think it is fair
to say that none of the current modes have proved ideal for the particular
features of this band, in particular the deep QSB, which often mean that DX
contacts get interrupted several times during the duration of the contact.
The HF-type modes struggle with the poor SNR, while the very low-speed LF
modes get chopped up by the QSB. In my opinion, this might be handled well
by some kind of specialised ARQ mode designed for weak signal conditions and
long fading periods. There has been some good work done on the effectiveness
of existing data modes on 500kHz, but these tend to be designed for HF and
above, with the expectation of a TX/RX bandwidth of around 3kHz, and so are
mostly wider than one would like to use at LF/MF. I guess for software
writers, it is difficult to justify all the effort in creating new modes
specially for 500k when there is only a very small number of potential
users. In the long term, when hopefully 500kHz will become a fully fledged
amateur band like 136k, it will probably be neccessary to have an upper
limit on the bandwidth just so that a reasonable number of stations can use
the band simultaneously. But in the meantime, with only a small pool of band
users, quite wide signal bandwidths do not really cause a problem to other
amateur stations. Provided the signals stay within the allocated frequency
range, it seems unlikely that it would affect other spectrum users, none of
whom seem to use frequencies close to the current UK amateur allocation.
Perhaps what is needed is a specification on the maximum out-of-band levels
permitted?
Re: TX ERP levels - Some extra power would always be welcome, although it
might not make a massive difference to the actual coverage achieved,
especially since the band is currently largely confined to the UK. My
antenna experiments suggest that with reasonable"back garden" antennas, 10W
ERP at 500k would require TX power levels of the order of 1kW, somewhat less
than needed to achieve 1W ERP on 136kHz, and a rather lower antenna voltage
than required to achieve 1W ERP on 136k. So actually achieving this ERP
level ought to be practical for stations in average locations.
Was it stated which services are of particular concern to MCA regarding
interference? I guess the activity in mainland Europe and the USA should be
a good guide to possible interference problems - I have not yet heard of any
serious QRM problems caused by these higher ERP stations.
Cheers, Jim Moritz
73 de M0BMU
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