Re: [Discuss-gnuradio] Spectrum from 1 to 10 Mhz

[Dave Emery]

On Wed, Sep 01, 2004 at 07:22:49PM +0100, M Taylor wrote:
> On Wed, Sep 01, 2004 at 11:22:49AM -0600, Jung Ko wrote:
> > 
> > Thank you very much for all the answers. From what you guys said it
> > seems to me (please correct me if I'm wrong), that the only consistent
> > (aka commercial broadcast) in this frequency range would be either AM
> > broadcast or SW broadcast that also uses AM modulation. There is no
> > way to guarantee that a certain ham mode will *always* be at a
> > specific frequency since they are operated by amateur radios, right?
>  
> Most broadcast (one-way) is fairly consisent while two-way communications 
> tend to be bursty unless automated (i.e. digital).

        This is accurate - broadcasters appear and disappear on
schedules they set - illuminating different frequencies at different
times of day (and seasons and levels of sunspot activity).  But for the
most part they reliably use particular frequencies every day at the same
times of day.   HF Radio propagation, however, is not all that reliable
and the signal on a particular frequency may be strong some days and
completely inaudible on others.   AM MF broadcasters of course almost
always operate during most of the day and evening (though some are
required to reduce power at sunset).

        Military and aviation SSB comms are found on specific channels
(accurate within under 1 hz typically) but are by their very nature
intermittent push to talk traffic.   Almost all of these networks have
families of specific HF frequencies up and down the spectrum with ground
stations active on several at once and the aircraft choosing a frequency
that propagates well to the ground station (either with ALE or
manually).

        For the most part at least some audible traffic can be found on
the military and aviation channels 24 hours a day 7 days a week.
Channels in the 6 through 13 mhz range are likely to be in use 24/7 with
traffic every few seconds to every few minutes depending on time of day
and propagation.    Use of particular frequencies changes rather slowly
so these are among st the stablest allocations on HF over time.   

        Most are on 3 khz steps (carrier frequency) 3 khz spaced apart -
it is standard to allocate 3 khz bandwidth to such signals. In recent
years some military networks (notably the Coast Guard) use odd 100 hz
steps like .5 or .8 - virtually all the transceivers currently used are
tunable in 100 hz steps (few are 10 hz or 1 hz settable) and all of
them have accurate reference oscillators and usually 1 hz or so 
accuracy of carrier frequency.

        Some of the encrypted digital signals in the USA are radiated
24/7 by various government and military entities - they can be found
present all the time if one is within the appropriate distance from the
transmitter for energy to propagate.

        Hams are NOT allowed to own a frequency (save for VHF/UHF
repeaters specially licensed and coordinated) so NO ham traffic is
either going to be certain to be on a particular frequency or for that
matter reliably on any particular frequency at all.  Ham HF rigs are
universally tunable and usually not very accurately calibrated so one
can reasonably state that it is unlikely that one will find much of any
ham traffic precisely on a specific frequency.   Exceptions to this do
exist, however, as there are digital and a few SSB nets that attempt to
stick to a precise frequency. But this is the exception, not the rule
unlike the entire rest of the HF spectrum where EVERYTHING is precisely
on frequency on an assigned channel.


-- 
   Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass 02493