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C. Crane FM Xmitter Mod
- Thread starter Bongwater
- Start date
I had recently reviewed the C. Crane transmitter for my web site and it's a nice little performer, which seemed to conflict with some reports I had read on the web about poor coverage. The transmitter is aligned to be within the power range it was certified for (FCC Part 15). If the "mod" makes the equipment violate the section of the regs it was certified for (i.e., field strength) it would be in violation.
While I'm sure this comment will ruffle some feathers, if you were to perform this "mod" and the effective coverage area improved by a somewhat insignificant range (let's say 20 or 30 feet) would anybody really care? I highly doubt that little transmitter is going to cover a 5 to 10 mile area as I've heard other harp and cry about.
While I'm sure this comment will ruffle some feathers, if you were to perform this "mod" and the effective coverage area improved by a somewhat insignificant range (let's say 20 or 30 feet) would anybody really care? I highly doubt that little transmitter is going to cover a 5 to 10 mile area as I've heard other harp and cry about.
Bongwater said:
Yes it does violate the rules since no modifications to a certified device are permitted.
Bill DeFelice said:
Just a reminder that the FCC part 15 rules for FM BCB operation are not based on power. They are based on field strength at a distance. Obviously, this is affected by power but the rules are not written in terms of power. Bill's comment conveys the idea correctly that the FCC wants to limit the range but it is not based on power. I just wanted to clarify. Minor point but one of constant confusion regarding FM "power limits"..
Neil
Neil E. said:
Thanks Neil. I should have referenced the field strength part of the rule instead of mentioning power. I was thinking of the testing that was done in the lab and how the "mod" in question actually works.
OK Bill. That makes sense and I missed the connection at first reading. At the factory, they are probably adjusting this to yield a certain power or voltage somewhere in the circuit which duplicates the setting for a certified unit in terms of the field strength. At least that's how I would do it.
Neil
Neil
The mod does work! I have a series of Belkins and am already looking to a swap out of one that I feel doesn't get the range like the others. Perhaps, the one I use for my Sirius XACT.
Bongwater said:
I don't know but that should not be the question. Here's why:
The maximum field strength for part 15 FM set by the rules is 250 uV/m at 3 meters for most of the FM band. When trying to relate this to ERP (effective radiated power) or transmitter power output to the rules one meets several obstacles, primarily the gain of the antenna. The FCC intends to limit the range and interference potential of these transmitters and therefore chose the most all inclusive measurement affecting this, namely maximum field strength at a distance in any direction. This removes from consideration the antenna gain and grounding and the power of the transmitter.
But, given an antenna situation, such as a dipole, we can calculate back and state the ERP required to produce the legal field. Doing so yields an ERP from a dipole of about 11.8 nanowatts. (A nanowatt is one millionth of a milliwatt so the much cited 25 milliwatt output from a transmitter produces about 1414 times the legal field even without accounting for the gain of the dipole which makes this number greater.)
So, though I cannot answer your specific question, I can state that probably no one else can either since power measurements at these levels is beyond the capability of most laboratories. When the device was certified, the field strength at a distance was measured, not the power. It is most reasonable to conclude that this modification increases the power and thereby the field strength over the legal limit assuming that the manufacturer set this to obtain the maximum legal field strength to optimize the performance when the device was manufactured. (Note the disclaimer by the author at the linked posting.)
Perhaps someone has made the field strength measurement before and after the modification and could comment on the results.
Neil
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