- November 7, 2011 at 7:06 pm #7846mpmissParticipant
Total posts : 13
Had a great discussion with a guy today at work regarding AM vs. FM Part 15 stations. This guy’s dad was a retired FCC inspector who once worked out of the Nevada field office in the 70’s & 80’s before being transferred to another region of the country in ’92. He retired in in ’95, passed away in ’99.
He told me that his dad always thought more folks should’ve/should have sought to start Part 15 FM’s over the AM’s. In all of his years on the job, his dad was called to check on one Part 15 station due to a filed complaint. He said his father told him that his office was aware of several Part 15’s in the region operating on both the AM and FM Bands, but had never sought to check their operations in the 20 or so years he was on the job; They weren’t considered a big deal. The only one he was “forced” to take a look at was a station located in California. It was run, programmed and sounded like an FM giant and had a range of 3-4 miles. (Sorry I don’t know which TX it used).
It was located in a densely populated city in California. During his inspection the former FCC inspector observed nothing out of the ordinary at the station, which is now off the air due to its owner’s death. From what the father saw there, the station hadn’t run afoul of any of the rules the FCC has set aside for Part FM’ers. The complaint leveled against the operator was over his signal’s strength. The complaint was made to the FCC by a disgruntled dj who’d been fired for some reason or another. It was only after four such complaints over a two year period that the guy begrudgingly went to investigate.
The former inspector reviewed the guy’s set up and found a compliant 100mW TX connected to a mixing board, amp, processor and eq. He stayed there an hour drinking coffee and talking HAM radio with the owner. What WAS interesting to the inspector was how high the operator had installed dipole antenna over his office space which sat in a strip mall. It was elevated over 40 feet up! The son of the inspector said his dad always felt that there was/is a misinterpretation of the rules against the FM band Part 15’ers in that they/we mistakenly applied some of the rules for AM operation to FM; particularly as it relates to ground lead issues-which lead to antenna heighth problems and even field strength interpretations. His dad once told him, “It (misinterpretation) cripple people’s drive to building FM Part 15’s”. So I wondered…
What are your thoughts, Gents? I’ve posted a little FCC mumbo-jumbo here for you to peruse.
“The field strength of any FM emissions within the permitted 200 kHz band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector.”
My take: The measured power is not based on output power from the transmitter but rather on received energy at the test receiver antenna. Right? During a test of these units, I’ve seen transmitters placed on a table that rotates, 3 meters (10feet) from a certified antenna, with the antenna being raised and lowered and the table is rotated to find the strongest possible signal. After that factor is found then the test receiver attached to the test antenna measures the received power level (it must be less than 47dbm/v per meter using an average detector.
Last thoughts: The former inspector felt the elevation of the operators antenna was uniquely higher than he’d seen for an FM Part 15’er. But, he concluded that that had as much to do with the range the operator was achieving. That, according to him was the reason behind the station’s range and NOT anything else. It was a 100mW TX!!!! A question arises out of this for me… I wonder how an elevated whip would do for Part 15 FM’s? Say, 40, 50, 75 feet up in the air since they can go up as high as one would like and the grounding issues are so wide open for FM’s.
As for the inspector, I wish there were more around like him. He clearly wasn’t a ball-breaker. Oh well, back to work I go.November 7, 2011 at 8:49 pm #23166kc8gpdGuest
Total posts : 45366
probably a lot like him back then. but back then the government wasn’t in such a tizzy to shut up independent voices on the dial like they are today and their wasn’t a huge NAB fueled lobby looking to shut down independent voices on the dial like there is now. nab was around but part 15 did not seem to be a major concern to them or their members back then.November 7, 2011 at 11:08 pm #23167Carl BlareGuest
Total posts : 45366
I get the part about height. Think of a 7-watt light bulb. If you put it on the floor it lights a few feet in all directions. On the ceiling it lights a whole room. Outdoors at 100-feet high it would look like a star and be seen line-of-sight for several miles.November 8, 2011 at 12:21 am #23168RFBGuest
Total posts : 45366
Line of sight is the key to successful Part 15 FM coverage. The higher the antenna, the better.
There is no lead length limit or coax/feed line limit or antenna mounting height restriction whatsoever in the rules. There is not even any transmitter input/output power limit
either…and if there is..by all means anyone is free to show us all where.
There is only the field strength limit and bandwidth utilization specifications.
If I am wrong, I welcome the correction. And do not use the “too big o box for the job” excuse. That is not a rule, but an everyday irrelevant side note and opinion from some individuals.
RFBNovember 8, 2011 at 1:08 am #23173Carl BlareGuest
Total posts : 45366
Excited by this post I went outdoors and looked up.
There it is! The sky ! Imagine an FM antenna stuck up there sending “Look No Further” radio programs that will make people’s lives happier.
Actually, “Look No Further” would be a good TV slogan. On radio you’d say, “Hear No Further,” but that doesn’t come out right.
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