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Here is part of the info found on the Part 15 Radio Stations of North America website:
Shortwave or HF Part 15 Stations (13,553 – 13,567 kHz)
(HIFER) = Acronym for “High Frequency Experimental Radio”, basically the same thing as a LOWFER or MEDFER but at HF (see LF and MF Keys for more detailed explanations). “New” term coined by the LOWFER/MEDFER community to accommodate this “newly discovered” and rapidly-burgeoning field of experimentation.
HF Part 15 stations occupy a narrow slice of the 22 meter shortwave band in accordance with the provisions of 15.225, which specifies a field strength limit of 10,000 uV (10 mV) per meter at 30 meters from the antenna. In practice this works out to 1.8 mW transmitter output power into a 1/2 wave dipole, or 3 mW into an isotropic (0 dBi) radiator. While this power level may sound minuscule remember that by comparison, only a fraction (< 1%) of the power a 100 mW Part 15.219 AM transmitter feeds to its antenna is actually radiated by even the most 'efficient' 3 meter antenna; in fact, the amount radiated is roughly comparable what is permitted in this HF band, yet some impressive DX as well as local coverage has been achieved at MF. On this HF band, unlike Part 15 mediumwave, there is no limitation on the size or type of antenna, feedline or ground nor on power output of the transmitter, except with respect to whichever combination of antenna and power chosen is required to reach the legal limit of field strength without exceeding it (and recalling that any gain effect even if incidental, such as might result from proximity to a reflecting suface or object, would cause an increase in field strength in the direction of maximum reflection which must be taken into account). In any case while the power output allowed here is not tremendous, it is indeed enough to "work the world" based on Amateur Radio observations on similar frequencies and power levels, via the excellent ionospheric skip characteristics of the band. Local range depends of course on the receiver used, its antenna, the prevailing noise and interference levels, etc. but should be at least comparable to and probably significantly better than that of a mediumwave Part 15 station - unlike MF, at HF especially this high in frequency, there is essentially no significant groundwave component and thus a major source of path losses is removed - local propagation is via direct wave - and also the baseline atmospheric noise level is far lower here than at mediumwave. The other main requirement under this Rule subpart is a frequency stability of +/- 0.01% under supply voltage variations of 85 to 115% of normal at 20 C (68 F) and under temperature variations of from -20 to + 50 C (+4 to +122 F). This might tend to scare off some potential builders but it is actually only 100 ppm, which is easily achievable with any reasonably decent oscillator and power supply circuit design and good quality crystals which are readily available from a supplier such as JAN or Bomar. Another possible design example: a CB crystal for 27.120 MHz and a simple 74HCTxxx series frequency divider would give 13.56 MHz and whatever the stability of the crystal originally was, it would be doubled (drift halved) using this scheme. It is also probable that this rule really applies to commercial manufacturers seeking Type Acceptance and for the homebrewer the FCC would tolerate a lesser degree of accuracy. Canadian RSS-210 rules, Section 6.2.2 (e) permit operation in the same band but at slightly higher field strength, 15.5 mV/m at 30 m. Their frequency stability requirements are similar. Canada also has another license-free HF band not available in the USA; 6,765 - 6,795 kHz. Field strength and frequency limits are the same as those for 13,553 -13,567 kHz. For AM broadcasting use, assuming a standard 5 kHz max. audio frequency (10 kHz occupied bandwidth), there really is only one frequency which can be used in such a narrow band: 13,560 kHz (this leaves a 2 kHz guard band on top and likewise, on the bottom of the band as well). It is recommended, in the interest of allowing as many as possible to use the band, that those considering broadcasting here either (a) reach an equitable time-sharing or scheduling agreement with other stations to avoid mutual interference, if using AM, or (b) operate SSB (Single Sideband), which is used by many full-power HF broadcasters around the world and occupies much less bandwidth than AM does. Inasmuch as most shortwave receivers can "hear" SSB (have a BFO), this should not be a problem. Using less audio bandwidth on AM, when possible (for instance, 3.5 or 4 kHz instead of 5) can help reduce interference potential as well. Narrowband FM can also be used but unfortunately, many shortwave receivers likely to be owned by the public cannot "hear" this mode. NBFM might be better than AM though, due to less noise, for wireless studio-to-transmitter link (STL) use of this frequency range in feeding other Part 15 units on AM (MF) or FM bands (an idea originated by the author). To the author's knowledge, no commercial manufacturer currently sells Certified transmitters for this band; or not, at least, for hobbyists/broadcasters - so it's "homebrew heaven" for those so inclined. Here is a page with some schematics to get the experimenter started. One company, MaxNet, does offer lease of Certified NBFM transmitters for this range, at a HIGH cost, for STL/TTL use as part of a complete system of Part 15 FM's to be used along highways as sort of a commercial "TIS" setup. Contrary to what some folks have been (erroneously) told by FCC personnel not intimately familiar with Part 15 Rules, this band is not strictly for ISM (industrial, scientific, and medical) use only; the author, in consulting with contacts in the FCC who write these Rules has learned that broadcasting here is indeed permissible (there are no written provisions to the contrary). However, United Parcel Service is planning to use this spectrum for specific applications and being a Part 15 operation as well, sharing the band and acceptance of occasional interference will necessarily be the order of the day (let's hope UPS sees it that way!) Also, those electronic anti-theft tags (RFID) found on clothes, CDs and just about everything else in stores today operate here too; you may want to avoid transmitting on this band too close to your local Mall! The lack of experimental broadcast stations operating here is truly amazing, considering the relatively generous field strength allowance and great (world-wide!) propagation in this frequency range. This Part 15 band is available in the USA and Canada for use by people like us: hobbyists, low-power radio and broadcast enthusiasts, and experimenters; not many people in other countries are so lucky, and in the face of commercial pressures there increasingly is less and less spectrum available anywhere to the average RF experimenter - why not make good use of the band?