By 'Full Spectrum' is meant a kind of radio-activity managed throughout the HF, using very low power, improvised radios & antennas, on any or on a casual frequency.
If the web is surfed looking, for example, for 'shortwaves beacon', one can find a lot of sites on this topic. Many are experimenting with very low (and sometimes even not low at all) powers, in the range of tens or hundreds of milliwatts, by running beacons on almost any frequency on the Shortwaves.
A cathegory of these experimenters rely on those portions allocated to Short Range Device applications, industrial heating, medical applications, so to reduce the impact on the spectrum and therefore reducing possible disturbancies to some other radio stations.
This kind of activity is a new frontier. By suitable programs it is possible to filter the output from a receiver so that morse code sent by a few milliwatts can be perfectly decoded across the oceans. If the power is very low, the signal would not be noticed by a professional user of a given frequency, so everything would not be harmful. It is a frontier in the sense it is being experimented and investigated the real limit of the informaiton delivery by the ionosphere, with ultra-small power.
On top of this activity, a few more points should be added. Operating on non-conventional frequencies has always been a source of attraction by many. By the same many that, at a certain point, became licenced radio amateurs.
The bands today are often overcrowded and the users are just limiting themselves to push the buttons or to play with the menus of devices looking more domestic appliances than radios.
By operating improvised 'wireless sets', like, for example, a couple of transistors, crystal controlled, as TX, and further two or three transitor as Direct Conversion RX, with a power in the range of a few hundreds of milliwatt, anywhere in the HF away from the crowd, from the QRM and the infinitely boring guys using domestic appliances, is fun !
The fun is increased if these operations are run during outdoor trips, and agreed with the correspondent operating in the same conditions on a distant place, hundreds of miles away. Each time from a different location and on different frequencies. Low power, with the trusty piece of wire hanging between trees or hidden in a wooden roof.
Electronic components dealers offer plenty of crystals on frequencies in the good part of the Shortwaves, from 2 Mhz up to 12 MHz, where the limit of the experiments is only the imagination. These frequencies are often 'standard', so if more experimenters of this kind will be active, there would be chances to meet on the air other radio-fellows and establish contacts.
If receivers capable of demodulating a phone signal, typically AM, are needed, a NE602 mixer + a MC1350 IF amplifier would make a reasonable device, if the homemade way is preferred. Many 'world receivers' today available on Ebay, can be purchased by a few tens of $ and have more than sufficient performance to run a reliable radio-traffic.
Schematics with 3, 2 or even 1 solid state component transmitters can be found anywhere on the Web. The components are cheap, ready to get and perfect for cut-and-try optimizations, in which they might fail in search for the best performance. No worry, they are cheap and therefore the game will still run with no dramatic financial impact.
Sadly the Shortwaves Spectrum is day after day emptier. On the other side, those empty spaces might be used, occasionally, for unconventional radio experiments for very low profile radio traffic.
It can be often noted, besides, that on the same Forums where beacons-hunters and beacons-runners exchange their info ... there is apparently no discretion, and often people tell the time, frequency and power they will run with on a certain experiment. And they sign the posts by nicknames, but sometimes even not personal names (or ham calls !).
I have always thought that this is the prelude of a revolution regarding the HF Spectrum use, having its main origin in the frustration coming from the mess currently found on the 'official' bands. It is certainly generated by the curiosity of experimenting new things, and by the courage of operating out of band.
If this practice is not abused and managed with care (i.e. listening a lot, before sending a signal on a frequency, and, again, at extremely low power levels), for sure it will bring a breath of fresh air in the world of the radio enthusiasts!
Radioman, Jan 2018