While pondering Tim in Bovey's curious findings regarding "Certified" Part 15 FM transmitters, in which every transmitter measured was either over or under the specific field strength seen in 15.239.
Since then we have assumed that the whole system of certification is not being seriously adherred to by either the manufacturers, the FCC, or both.
Preachers have issued once-an-hour reminders that the operator of these transmitters is the one responsible for actual compliance with 15.239, despite having no means of ascertaining that compliance and no appeal to the FCC nor manufacturers for their slapdash results.
Something doesn't make sense with this picture. A lot of somethings don't make sense.
That's why an evolutionary thinker, good morning, tends to believe there's an untold part of the story. He thinks the certifying labs have a different set of parameters, not simply the 15.239 rule, giving a certain margin of error in recognition of the loose tolerances of consumer level electronics manufacturing.
Makes sense, but this is human business which isn't bound to making sense.
The reproducibility of a measurement becomes important if a transmitter is going to be run right at the 15.239 limit. If lab X’s field strength meter reads 250 but lab Y’s reads 251, then the device may be noncompliant. It is only a difference of 0.4%, but without knowing how the FCC’s field strength meters compare to the ones used at lab X or lab Y, that could be enough for a NOUO.
In a lab a test chamber can be designed so reflections do not influence the field strength meter reading. In a real world install reflections that add in phase at the field strength meter could lead to a field strength being measured over the limit. In a lab measuring the field strength they will be careful to not have random bits of metal that can act as parasitic elements directing energy from one direction and sending in another. In the real world the interactions between a transmitter and stuff around it could cause a directional lobe to form that would have field strength higher than the limit.
15.239 is an upper limit. Being over it is bad. It is safer to use a transmitter that measures under the limit so reflections and interactions with metal objects do not cause the field strength in some location to be over the limit.
There are a couple of NYC NOUO’s that were double or less over the limit. For the FCC to issue them I would think they must be confident in the accuracy and reproducibility of their field strength measurements.
Who is "an evolutionary thinker, good morning," ?
Neil
Neil had a question: "Who is "an evolutionary thinker, good morning," ?"
Our polite thinker was in the process of being upwardly mutated during morning rush.
stvcmty u-turned away from progressive thought so as to keep the status quo the way it was.
1. The FCC requires a site attenuation test before they will accept data from any test lab. A properly designed Open Area Test Site or chamber will not depart more than +/- 2 dB from the theoretical plot.
2. Because of this the FCC builds in a fudge factor in favor of the suspect operation when determining compliance.
To wdcx: Below is the text of an inquiry made to the FCC OET on this topic. Their reply to it didn't much help to answer the important issue stated in the last paragraph of the inquiry, but still might be interesting.
____________
Inquiry on 10/22/2015 :
Could the FCC please consider the text below along with the attached graphic, and offer their evaluation/comment?
Compliance lab testing for FCC certification under §15.239 produces one set
of measurements for one physical configuration of the transmit system for
one set of propagation conditions. If those measurements are accepted by
the FCC then a compliance certificate is granted to the equipment
manufacturer.
However the fields that such certified equipment will produce after its
installation by the end user in a somewhat different, or even the exact same
configuration as in the test lab likely will not be the same as measured
there, because the propagation environment and the distance and height of
the receive antenna can vary from that used in the test lab.
The attached graphic illustrates this. Note the wide variation in the
fields at 3 meters with respect to their height above the earth.
Even if the field intensity meter used was perfectly calibrated, the field
it would measure at an h-distance of 3 meters and an elevation of, say, 2.5
meters is quite a bit less than 250 µV/m, due to cancellation by reflections
from the ground plane (the earth). This occurs even though that transmit
system would produce 250 µV/m at a distance of 3 meters in a free space
environment (no obstructions, no reflections).
This might lead the user to increase the transmitter output power until the
FIM read 250 µV/m including the antenna factor, or slightly less.
But by doing so, the fields at higher elevations above the earth will be
considerably greater than 250 µV/m.
The distance and height of the FCC's FIM receive antenna, the height of the
transmit antenna, reflections along and near the propagation path, and earth
conductivity all will have a significant affect on the field intensity the
FCC will measure, wherever they measure it.
It is not generally known how, or even if the FCC accommodates this set of
realities before deciding to issue an NOUO.
//
* * * * *
| FCC Response on 10/22/2015: |
| The Commission considers many factors, including those in your inquiry, before determining the rules for equipment authorization and operation of an RF device. The rules specify limits that the device must comply with to obtain equipment authorization of the device. The purpose of the limits for unlicensed devices is primarily to assure that the device does not cause interference to licensed radio services. The Commission also provides or refers to a measurement procedure for testing of the device to show compliance with these limits. For the subject device the measurement procedure is ANSI C63.10. The rules and the measurement procedure cannot account for all possible conditions under which a device may operate. This is impractical; so there must be a standard procedure for determining compliance with the rules. The rules and the measurement procedure provide for operation of the device for its intended purpose while protecting licensed services from interference. |
Attachment Details: Part 15 FM Fields
To Rich: I agree with your assement as presented to the FCC. I think the concern with regard to protecting "licensed services" is that in the case of a residential radio or television is that the likelyhood of being withing the 3 meter range is very high. Also in the case of Class A (Industial) and Class B (residental) digital devices, would expain why the measurment distance is 10 meters and 3 meters respectivly.
Included is a link for Site Attenuation procedures: https://www.vcci.jp/vcci_e/activity/regulation/y09/kiyaku12_0804.pdf
I'm under the impression if you can't make sure you are legal, you shouldn't operate.
It is pretty obvious when you are going over the limit.
A typical part 15 fm transmitter is typically set to be safely within limits from the factory so the layman without technical knowledge can use it. The internal adjustment some transmitters have is for the advanced users that know what they are doing. Keep in mind the nature of VHF means a set power level is going to give different field strengths with different setups.
The FCC response in post #6 did not answer the question asked. The reply addressed the rationale and means of compliance testing for certification but did not address how or if the unknown factors are accounted when measurements of the field of the device in use may result in NOUO.
Those who use FM transmitters with the intent of being within the limits of the rules remain in the dark about their exposure to a citation.
Neil
IF, as Mighty said: "I'm under the impression if you can't make sure you are legal, you shouldn't operate"...
Then WHAT is 15.239 intended to enable?
The problem is that you're not always sure that you are legal.
Case in point. Elevating an FM install to 10 meters above the Earth. Your field strength increases. But by how much - if you're under at 1 meter (due to the manufacturer being careful), are you significantly over at 10 meters? Is the increased range due to line of sight to the receiving antenna, or that increased field strength?
We already know that the sensitivity of radios varies considerably, so range checking isn't accurate either (unless you've got an accurate measurement of the sensitivity of the radio - and if you have that, you can probably measure field strength).
Thats the unfortunate risk you take on part 15 FM. Naturally a slight overage in field strength is unlikely to cause any issue. I'd say for the most part so long as you are using a certified FM transmitter you should be okay with most any installation. If you want full legal range on FM you need the right equipment to do so.
Granted, if you stuck that transmitter on top of a chimmney or water tower that would be obvious intent.
Again I hate to point this out, the FCC did not ever intend part 15 to be used for broadcasting. It was for short distance experimentation and those phono oscillators. Likewise .239 is not meant to have much usable range. Would the FCC REALLY want completely uncontrollable content to actually go far?
I'd venture to guess .239 was based on the original usable distance of .209 and .219 equivalents from back in the day of highly inefficient electronics.
Conclusion: The FCC Rules for low power broadcasting are inconsistent.
IIRC, weren't the h-pol fields of 2 of 3 FM transmitters certified under FCC §15.239 found to be non-compliant with §15.239 -- as measured by Tim in Bovey when using a calibrated field intensity meter at an h-distance of 3 meters, and tx/rx antenna heights of 7 feet above the earth?
Reply to post #13,
If this is viewed with an interpretation that the rules are not intended to facilitate broadcasting then it is easier to accept that they are to minimize the chance for interference while allowing for personal use with very limited range. For example, a person who wants to listen around their home or in their car doesn't care about nor need more than a few tens of feet of range.
It seems that when we pursue the idea of broadcasting with listeners around the neighborhood that we need to push the limits and really feel the constraints.
The perceptions of inconsistency and severe restrictions may be psychological and result from what we are trying to do.
Neil