I right now agree with PhilB that the language of what's posted as of this time regarding this Compliance Appliance leaves a lot unexplained.
For example, in large part Information Station Specialists (ISS) provides TIS AM installations, meaning - Traveler's Information Service AM Broadcast Stations as defined by FCC Part 90.242, a licensed service, with high towers and many Watts.
What is NOT made clear is whether the Appliance is claimed to be accurate for Part 15 use, on any band.
Yet, ISS does produce a Part 15 AM transmitter, so..... maybe....
Mark wrote: This (Eton-Grundig G8) radio could be a good indicator of compliance. 250mv/m = 48dBu approx.
Sorry Mark, but the Eton-Grundig G8, the Tecsun PL-310, and their equivalents do NOT display the field intensities arriving at their antennas.
Instead, they display conducted voltage values present across a certain (and unknown) impedance within those receivers in units of decibels with respect to 1 µV (i.e., dBµV).
Field intensity values are stated in units of the r-f voltage difference existing across a given, linear distance in space (customarily, one meter). That is not the same as the voltage produced at the output terminals of a one-meter-long receive antenna.
When the antenna factor of these receivers is known and accounted for, the AM/FM "signal strengths" these receivers display compared to the true field stengths in dBµV/m arriving at their receive antennas can be very inaccurate.
I have looked at both the G8 and PL-310 manuals and I am not convinced that you can say exactly what they are displaying.
The whole dB measurement system is a pet peeve of mine because there are so many suffixes and constant confusion surrounding them.
The text in both manuals say the signal strength is displayed in units of dBu. The Tecsun manual clearly shows dBµ (Greek letter mu) in the LCD display. In the various dB definitions there is a distinction between dBu and dBµ. There is a definition for dBµV, but no definition for dbµ. Even more confusingly, dBµV can also be written as dBuV.
So, it isn't clear at all what these radios are displaying. We know, the FCC standards state that 60 dBu = 1 mV/meter, so if they are really displaying dBu, AND they are accurate, these radios could be useful. But what they display is not clear.
If this is all very confusing to anyone, then join with me (at least in spirit) in the "Abolish the Decibel Measurement System" movement!
The following comments may be boring and/or too "picky" to many, but hopefully not everyone.
A big part of the confusion here is that the units for field intensities often are incompletely/incorrectly stated and used.
For example, the term "dBµ" really is meaningless, because it does not reference the base or derived SI unit* it applies to (volt, watt etc).
The "µ" in dBµ means 1/1,000,000 of the SI unit that follows. Many writers substitute u for µ when they write because it is faster than using the keyboard steps needed to produce the mu (µ) symbol.
But either way the expression has no meaning, because it doesn't include the basic unit of measure -- which in the case of the intensity of an electric field is volts/meter (V/m).
The correct form of expressing a field intensity of 1 mV/m in decibels with respect to 1 µV/m is 60 dBµV/m, not just "60 dBu" or "60 dBµ."
Units of field intensity always need to include the distance in space across which that value exists, usually one meter. That is the reason for "/m" part of dBµV/m. If the /m is missing, then that voltage does not exist as a field in space, but across two points in a physical electrical circuit of some sort.
The case of the letters used in these units also is important, and often written incorrectly. For example a lower case m must be used when writing the abbreviation for milliwatts (mW). An upper case M means megawatts, which might not be what is intended by the writer.
Usage, convention, context and experience are needed to understand what really is meant by a term such as dBu. But that could be eliminated if the correct form of these units was used.
Rather than supporting an "Abolish the Decibel Measurement System," perhaps we might better support a campaign to use the correct/complete units of measure when we write.
* see http://en.wikipedia.org/wiki/International_System_of_Units
As writers of technical language we can refine and perfect our skill so as to express accurately with correct language/symbols, but will that endow us with the editor's skill of being able to identitfy and correct the perfections/imperfections in technical descriptions written by others?
Perhaps abolition of the Decibel measurement system is the clean restart solution.
Perhaps abolition of the Decibel measurement system is the clean restart solution.
Decibels are not a measurement system, actually. They are just a logarithmic ratio between two values stated in the same SI units, and make it easier to do certain mathematical manipulations.
Decibels are calculated using these equations:
dB = 10 log10(P1/P2)
dB = 20 log10(E1/E2)
dB = 20 log10(I1/I2)
where P, E and I are values in watts, volts or amperes, respectively. When using E or I values they must be present across the same value of resistance/impedance.
For example, the decibel relationship between 100 watts and 50 watts is calculated this way.
10 log10(100/50) = 10 log10(2) = 10 x 0.30103... = 3.01 decibels, approx.
This board was not able to post the number 10 appearing after "log" as a subscript, which may lead to confusion.
That value of 10 refers to the use of common logarithms (based on the exponent of the number 10).
The transmitter side of legal Part 15 FM is based on field strength alone, and the resulting area coverage is usually very small. But not all FM radios are equal when it comes to low power signals, and it is the radios that can make it work!
It seems that all FM radios work well in receiving the high-power licensed stations, but in these times of more LPFM and the weakest of all, Part 15 FM, the differences in radio performance are astounding!
This became known to me when I started testing the TECSUN PL-310 FM band reception, and it actually receives my flea signals from two certified FM transmitters everywhere indoors and all over the yard on the outside!
My other high quality FM portables DO NOT share this sensitivity on low power FM signals, and can only hear my stations within a few feet of the transmitters.
As the new week begins, we should be hearing more information about the ISS Compliance Appliance, and one question to have is "How sensitive is it with low power FM signals."
As the new week begins, we should be hearing more information about the ISS Compliance Appliance, and one question to have is "How sensitive is it with low power FM signals."
Carl -- regardless of the answer to that question, it should be recognized that receivers uncalibrated as to the field intensities arriving at their antennas are not useful as a "Compliance Appliance," unfortunately.
After a pleasant conversation with a spokesman for ISS (Information Specialists) we can share the clarity that's not exactly been known thus far in speculations and high hopes.
The ISS name itself is the key, "Information Station Specialists." As we have mentioned, so-called "Information Stations," also T.I.S. (Traveler Information Station) is an F.C.C. licensed type of AM station with a field intensity limit based on up to 50-foot towers with up to 10-Watts. Getting the field strength right determines actual height and power.
The "Compliance Appliance" is presented for use in TIS station compliance, with no intended application for Part 15 radio stations. I was told that AM signals in Part 15 are below the lowest sensitivity range of this radio.
The engineering realities outlined by PhilB and Rich stand up and we have not, too bad for us, finally been granted the wish for an affordable field-intensity measurement device.
Carl Blare posted (reportedly based on his recent telcon with someone at ISS):
The "Compliance Appliance" is presented for use in TIS station compliance, with no intended application for Part 15 radio stations. I was told that AM signals in Part 15 are below the lowest sensitivity range of this radio.
Just to note that per FCC §90.24(b)(4), a TIS station is permitted to generate up to a 2 mV/m groundwave field at a horizontal radius of 1.5 km (0.93 miles) from its transmit antenna.
Such fields also are possible from an unlicensed and functionally compliant system operating under FCC §15.209 and §15.219. However they would occur closer than 1.5 km away from those transmit antennas.
To whatever extent the ISS "Compliance Appliance" is able to accurately measure a 2 mV/m field from a TIS station, it would be able to measure a 2 mV/m field with that same accuracy from any other class of transmit system in the AM broadcast band -- whether that transmit system was licensed, or not.
However per an ISS reply posted earlier in this thread, and other information posted in this thread -- the ISS "SMR" does not measure the true value of the field intensity arriving at its receive antenna.
Therefore it would be incapable of proving compliance to any FCC Rule based on a limit for field intensity such as §90.24(b)(4), §15.209, and §15.239 for licensed TIS, unlicensed AM, and unlicensed FM operations, respectively.
im glad this isnt viable for part 15 honestly. splitting hairs syndrome has ruined this country enough
Trying to determine the accuracy of the on-line description of the SMR receiver (link below), I emailed Bill Baker at I.S.S. and provided him with data measured by a Tecsun PL-310 showing about -25 dB errors with respect to the arriving field intensity in dBµV/m. The PL-310 uses the same Silicon Labs si4734 microchip as the Eton-Grundig radio apparently marketed by I.S.S. as the SMR receiver.
I pointed out that a -25 dB error when measuring a 2 mV/m field from a TIS station would mean that the arriving field was only 0.1125 mV/m, and that this is a substantial error that would not be useful in proving FCC compliance.
His last emails to me on the subject (today, 9/9/13) included the text below (shown in bold font).
We make no claims that a SMR is equal to a calibrated Field Intensity Meter. But we find the unit to be a good relative reference that a TIS licensee can use to gauge compliance.
I then emailed him: Bill, would I have your permission to post only your text above, attributed to I.S.S. and you, on the Web? It might help clarify this issue for many people.
Sure.
But note that the link below states (first paragraph): "The Signal Measurement Radio (SMR) Receiver allows an Information Radio Station operator to measure accurately the level of his/her AM signal to comply with FCC rules."
I politely pointed out this discrepancy between the I.S.S. website description and his emails about the SMR before his last emails to me, but he was unconcerned.
http://www.theradiosource.com/products-smr-receiver.htm
I guess the term "relative" could imply that soon after a properly engineered TIS site was place into operation, measurements made at a proper distance from the antenna, could be recorded and assuming no degradation in receiver performance, along "exact" duplication of measurment techniques, this receiver could be usful in detecting significant changes that could signal (no pun) out of compliance operation but nothing more. For sure not "proof of performance."
Has anyone purchased one of these G8 radios? Although it will not receive SSB or CW as there is no BFO, I'm interested in the relative signal strength meter, even if it isn't calibrated.
They are available on Ebay for $35 with free shipping.