Those with the patience to read through the thread linked below may find something of interest.
These subjects are raised fairly often here on Part15.us, and just recently on a hobby board.
http://www.eham.net/ehamforum/smf/index.php/topic,69856.0.html
Below is a chart showing the fields near a dipole transmit antenna for the conditions shown.
Understanding them in relation to compliance with FCC §15.239 is not "straightforward."
Others have commented on this subject on other boards, but with no technical documentation or analysis supporting their statements.
In the presentation by Rich several things are brought to this viewers attention...
Keeping in mind that the described antenna is 5-meters above earth, we consider that human field strength measurements would be taken 3-meters from the antenna's support structure, at ordinary human altitudes from the earth, the measuring equipment located +/- 1-meter above the earth.
From the measuring position I've just described, the measuring equipment would be physically farther than 3-meters from the antenna itself, and only +/- 1-meter from the earth surface, therefore receiving a combination of direct radiation and reflected earth radiation, subjecting the measurement to nulls and/or signal enhancement, depending on very small spacial positioning of the measuring equipment.
Even knowing the imprecision involved with such measurement, I'm not sure what an inspector might do to improve accuracy of his field strength readings.
I happened to notice the snide snickering on the other forum. Except for poor grammer, they find themselves quite superior.
"... Keeping in mind that the described antenna is 5-meters above earth, we consider that human field strength measurements would be taken 3-meters from the antenna's support structure, at ordinary human altitudes from the earth, the measuring equipment located +/- 1-meter above the earth." etc.
However FCC §15.239 defines the maximum permissible field existing at a distance of 3 meters from the transmit antenna itself, not the field 3 meters from its support structure.
A field measurement made 5 meters above the (flat) earth, and 3 meters in the horizontal plane from the transmit antenna described in this NEC evaluation would result in a path length of exactly 3 meters from the radiation center of that transmit antenna.
Measurements made from this system at a horizontal distance of 3 meters from the vertical centerline of the transmit antenna, and closer to the earth than 5 meters would have path lengths somewhat longer than 3 meters.
Compliance measurements for certification require the transmitter to be placed 0.8 meters above the ground-plane and the antenna used for receiving is placed 3 meters from the EUT. That same antenna is raised and lowered 4 meters in both the vertical and horizontal polarization to determine the maximum emission point.
Now in the case of other Part 15 devices such as door openers etc., the EUT is rotated along with antenna manipulation do find the maximum emission point(s).
Quoting Rich:
"However FCC §15.239 defines the maximum permissible field existing at a distance of 3 meters from the transmit antenna itself, not the field 3 meters from its support structure."
Our question, therefore, is...
What means does the inspector use to elevate himself or his measuring equipment as far above his own height as necessary to be 3-meters from center point of transmitting antenna?
If the antenna is mounted to a chimney surrounded by other buildings, all on private property, how does the inspector avoid trespassage.
Think I responded in another post. The equipment used extrapolates the distance and includes a fudge factor in favor of the operator. Bottom line: If the signal measured is way over the Part 15 limit at 30 feet away, it's not going to get any better getting closer.
"What means does the inspector use to elevate himself or his measuring equipment as far above his own height as necessary to be 3-meters from center point of transmitting antenna?"
An FCC inspector might use any means needed, including a sufficiently long, non-conducting supporting pole with their sampling antenna mounted at its top.
"If the antenna is mounted to a chimney surrounded by other buildings, all on private property, how does the inspector avoid trespassage."
The FCC has successfully defended their right under U.S. law to inspect radio transmit systems subject to their jurisdiction even when such systems are located on private property.
But they can also measure those systems from a distance several hundred meters away if they need/want to, and come to their conclusion about the field that would exist from such an unlicensed FM transmit system if it was measured from a distance of 3 meters.
From talking about comparative VHF measurements across a wide swath of frequencies (opening post) we narrowed the focus in Post #2 to systems subject to FCC 15.239, FM band, and on Chart Comment # 3 says:
"Relating such accurately measured fields to compliance of an unlicensed system with 15.239 will depend on those making/evaluating the measurement."
This seems to say that "interpretations may vary" depending on differences among inspectors.
Yet subsequent posts describe athletic machinations that might be undergone as an inspector seeks to examine the frequency field of interest, suggesting a quest for minute precision.
Now would be a good spot to summarize what's actually been said in this thread with Special English for the benifit of less endowed readers.
"Compliance measurements for certification require the transmitter to be placed 0.8 meters above the ground-plane and the antenna used for receiving is placed 3 meters from the EUT. That same antenna is raised and lowered 4 meters in both the vertical and horizontal polarization to determine the maximum emission point."
Two questions...
1) Where is this particular test procedure specified in the FCC Rules?
2) If for Part 15 FM compliance testing, the transmitter is placed 0.8 meters above the ground-plane, what is the physical location/orientation of its antenna with respect to the ground plane?
The FCC normally does not conduct tests of systems for certification under Part 15, rather the tests are done by outside agencies who submit their findings to the FCC on behalf of the manufacturer. Don't those external agencies design their own test protocols, which may vary with different agencies?
But ultimately the FCC must evaluate and accept/reject the data and test procedures of those agencies as appropriate to show Part 15 compliance.
1) Where is this particular test procedure specified in the FCC Rules?
ANSI C63.4
2) If for Part 15 FM compliance testing, the transmitter is placed 0.8 meters above the ground-plane, what is the physical location/orientation of its antenna with respect to the ground plane?
Based on the user provided manual. Typically vertical but if the testing agency want to be thorough, boht horizontal and vertical if the antenna can be manipulated.
The FCC normally does not conduct tests of systems for certification under Part 15, rather the tests are done by outside agencies who submit their findings to the FCC on behalf of the manufacturer. Don't those external agencies design their own test protocols, which may vary with different agencies?
No. Also, the FCC can and often does requests samples and the laboratory will employ the same test procedure as out lined above.
These discussions are usually pointless and only serve to increase paranoia.
If you are obviously doing something wrong you'll most likely get nailed if someone complains or your operation is in the limelight.
If you are 100% legal you'll most likely get nailed if someone complains or your operation is in the limelight.
The horror stories which end with NAL's and huge fines are usually due to poor or no communication between entities.
Where subjective interpretation of rules is excepted practice based on hearsay, burden of proof is a moot point as who can afford to fight for it?
Be Happy! Don't Worry! Join The ALPB and Live for YOUR RADIO DREAMS...
The purpose of my posts was to educate those on the certification process. This is based on my own experience in compliance testing for years. I also have experience with several visits to the FCC Laboratory in Columbia, MD. The audit testing they perform is consistant with the ANSI spec quoted earler. No fear mongering here.
Post #8 contains the anecdotal statement: "The FCC has successfully defended their right under U.S. law to inspect radio transmit systems subject to their jurisdiction even when such systems are located on private property."
It would be useful for stations that maintain verified documentation to have access to source information demonstrating the claim that FCC authority supercedes the privacy boundaries of property owners. It would be inadvisable to surrender home security based on an unreferenced claim informally made on a hobby website.
Joining the ALPB impresses others.
This is the orginal test procedure that was unitilzed for tesing personal computers back in the day. The newer ANSI and IEEE documents were crafted from this document. The basic test set-up remains the same including the 0.8 meter table. Why 0.8 you might ask? It's the hight of most work surfaces: 30".
http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/measurement/mp4/mp4-1987.pdf