[Lowfer] Lowfer history

[JD]

>>> I wonder what was the purpose of the definitions given in Part 15? Was 
>>> it to maintain a radiated field strength below a certain value? Why not 
>>> just define that?

In one sense, that was the purpose...and many sections of Part 15, covering 
different bands and different uses, indeed ARE all about radiated field 
intensity and how it should be measured.  For mass produced devices, 
especially, verification and certification are best based on actual 
emissions because the devices will all nominally be just the same anyway. 
In addition, for one-way gadgets like remote controls, it is assumed most 
users of those devices won't have access to or won't regularly be monitoring 
receivers at the frequency of operation.  Thus, strict definition of the 
interference potential is generally more critical in such cases.

However, the AM broadcast band and the 160-190 kHz region are somewhat 
different animals.  There's enough leeway in what signal levels might be 
harmful that it is sufficient to state basic power and antenna criteria and 
let it go at that.  Also, given the relatively confined areas within which 
these signals are expected to be heard on normal radios with average 
antennas, you'd have a difficult time getting a valid field intensity 
reading for compliance.  Of course, even complying with the letter of the 
alternative Rules provisions does not mean you're allowed to interfere 
anyway--Part 15 is very clear on that point--but the third difference at 
lower frequencies (especially within the broadcast band) is that it was 
expected most users would have suitable radios and the incentive to monitor 
their own signals.  After all, who's going to want to play DJ with his 
personal station buried right under the local 50 kW powerhouse? :) 
Non-interference is something of a two-way street in the AM band, after all.

Those are some of the planning factors that clearly went into the thinking, 
and there may be others.

I find Australia's approach to low power RF devices interesting.  Here, we 
say "if you're under a certain signal level or power/antenna size, no 
license is required."  Down there, the fundamental communication law 
apparently requires that every intentional radiator must be licensed.  But 
then what do you do about gazillions of cordless phones, garage door 
openers, wireless mics, etc.?  You don't want to issue licenses for every 
single transmitter and/or to every individual user...the paperwork would be 
a killer!  So, their equivalent to Part 15 is the LIPD Class Licence.  If a 
device operates in certain bands within certain parameters, it is deemed a 
Low Interference Potential Device, and is covered by the Class Licence. 
Voila!  No unlicensed transmitters, but no gazillions of separate licenses 
issued either.  Same end result as the FCC's approach, and the same 
underlying goal of managing interference risk, but from a subtly different 
philosophical starting point.  (I seem to recall the FCC implemented or at 
least considered the class license approach for cell phones in the early 
days, but I haven't kept track of that field.)

The takeaway message about purpose of the Part 15 Rules is twofold.  They 
were not written to facilitate what we do; and they were not written to 
hinder it.  They are, as the late Jim Erickson once pointed out, a framework 
of measures that are deemed sufficient to prevent harmful interference in 
most cases.  It's just that simple.  If you're observed causing unintended 
interference while otherwise complying with the letter of the Rules and stop 
when notified, you have met the requirement of the law and are safe...but if 
you are caught causing interference or otherwise draw attention to yourself, 
and are NOT in compliance with a justifiable or understanding of the Rules, 
then you can rightly expect to sweat.

>>> Also, I think harmonic content would've been a concern of the FCC.

It certainly was, and it still is.

All subparts of Part 15 impose limits on radiation outside the relevant 
band.  In addition, it is stated that tighter limits will apply if actual 
interference is caused by harmonics or spurs.

I wouldn't worry too much about the 180 kHz square wave example.  A good 
tuning coil (high Q and high self-resonant frequency) very easily suppresses 
harmonics by more than the required 20 dBc minimum.  But since the Rules 
also require no harmful interference, period, a smart operator will also 
provide additional filtering or suppression of upper HF and VHF frequencies 
in his design, and will check out possible RFI and TVI in his vicinity 
anyway.  I think everyone in this group has been pretty conscientious about 
that over the years.

73
John