From the ALPB website about using a Tecsun receiver to determine compliance with FCC §15.239: "... Can't remember where I read this or heard this but it has been talked about in an ALPB meeting: for a distance of 10 feet at a height of table level approx 4 Ft from ground level your signal strength to a Tecsun 880 Radio should be 48 DB's. Again can't remember the source. ..."
The above-quoted belief that such receivers are capable of showing compliance with FCC §15.239 probably is related to the fact that 48 dbµV/m converts to an e-m field intensity of about 250 µV/m -- which is the maximum limit permitted by §15.239.
But unfortunately, using the signal strength value shown as "dBµ" on the display of a Tecsun PL-310, PL-880 and similar receivers does not show the field intensity arriving at the antenna of that receiver.
Below is a re-post comparing the readings on the dBµ display of a Tecsun PL-310 when receiving a known field intensity.
The errors shown on the receiver display are quite large.
The main point advanced by the preceding post: "Using the signal strength value shown as "dBµ" on the display of a Tecsun PL-310, PL-880 and similar receivers does not show the field intensity arriving at the antenna of that receiver."
That being the case, we in part 15 work are hindered by the FCC's failure to provide a practical and reasonable means to achieve precise compliance of the FM rule.
Therefore a dBu measurement tool provided by TECSUN and the like is "good enough" and no doubt "close" to compliance. This point has been argued hundreds of times yet continues to be raised.
It's probably just as good as the FCC 200 foot 'rule' for an effective (whatever that is) signal (to a receiver of unknown sensitivity).
...Therefore a dBu measurement tool provided by TECSUN and the like is "good enough" and no doubt "close" to compliance. This point has been argued hundreds of times yet continues to be raised.
All of the errors shown in the graphic in the original post of this thread understated the actual field intensity at the receive antenna by more than 20 decibels.
This means that the Tecsun signal strength meter display was less than 10% of the true value of the field intensity in dBµV/m arriving at the antenna of that receiver.
That level of accuracy may be good enough for some, but hopefully not without understanding the true accuracy level that this technique/hardware really provides.
I am guessing that the indicator methodology emeployed by Tecsun is simply a tuning indicator much in the same fashion as an S-Meter. It would be interesting to place 2 Tecsun's side by side and see what the indications would be.
Also, it should be noted that for compliance testing below 30,000 kHz a magnetic loop antenna is employed. So once again I am guessing that any signal strength observation is relative only.
What about for FM frequencies?
Would the accuracy be any different?
Mark
FM Measurement
Mark. the subject here is about FM, since AM is not subject to field strength compliance with 15.219.
The margin of error: "The Tecsun signal strength meter display was less than 10% of the true value of the field intensity in dBµV/m arriving at the antenna of that receiver."
We are talking very small numbers here, so 10% in the small number arena isn't the magnitude that would be encountered at higher power levels.
Setting that aside, what method would be better for the hobby radio operator wishing to comply with 15.239?
Telling the problem is only half of a presentment.
One needs to conclude by offering a solution.
What about for FM frequencies? Would the accuracy be any different?
I just took my Tecsun out in my front yard and "measured" the signal strength when tuned to a local FM station 4 miles from me that radiates 28 kW both in horizontal and vertical polarization from an antenna height of about 400 feet above ground level. The FCC F(50,50) propagation curves for this setup show the field intensity at a 30 foot height above level ground at my house to be about 95 dBµV/m. It would be several dB less than 95 dBµV/m closer to the Earth, where I sampled it.
The Tecsun displayed the following values in dBµ when moving the radio along a 20-foot path with the plane of its fully extended whip antenna perpendicular to the FM station propagation path, and centered at an elevation of about 5 feet above the Earth:
- Horizontal polarization ranged from about 51 to 59; vertical polarization ranged from about 53 to 62.
So the values shown on the Tecsun display also are well below the actual FM field intensities arriving at its receive antenna.
...what method would be better for the hobby radio operator wishing to comply with 15.239?
If that method is to involve field intensity measurements, that would require a calibrated field intensity meter operated by someone with a background in making such measurements, accurately. Such meters (with their calibrated antennas) are very expensive.
Tim in Bovey posted some interesting results for his measurements of the fields of "Part 15" FM transmitters, which appear in the archives here on Part15us.
Therefore, in conclusion, we can say that the dBu read-out from a TECSUN-like radio is the best we can do short of an exceedingly expensive field test measurement.
I fired up the $26 SDR receiver and have determined that it is sensitive enough to measure FM Part 15.239 field strength out to 30 meters. The included antenna is suitable and can be used as a calculable dipole. One more thing is needed to perform the measurement and that is a calibrated signal source. If anyone is interested we can discuss this.
Another way to "calibrate" the unit is to use an off-the-air signal as another member posted. The field strength reported by FMFOOL.com can be defined by height above ground.
BTW, the 250 uV/m at 3 meters is, I believe, for a linear polarization; either vertical or horizontal polarization. Using circular polarization we should be able to run twice the RF output power. This allows both vertical and horizontally polarized receiver antennas to be used with similar (good) results.
I spent some time designing and researching circularly polarized antennas for this use. These can be bought for a few hundred dollars or homebrewed for much less.
... One more thing is needed to perform the measurement and that is a calibrated signal source. If anyone is interested we can discuss this. ...
But unfortunately, even when a radiating VHF (and above) transmit antenna produces a known field in free space, that field is still subject to large variations when its value is measured at various horizontal distances and elevations above a reflecting surface such as the earth.
It can be shown by calculation that a center-fed, 1/2-wave, linear dipole radiating about 11.4 nanowatts will generate the 250 µV/m maximum field limit permitted by FCC §15.239 at a distance of 3 meters, when operating in a free space environment.
Below is a graphic showing the fields that exist when the dipole setup described above is operated and located close to the earth. The vertical axis shows the elevation above the earth in meters, and the horizontal axis shows the field intensity in µV/m, for those elevations.
Even with a perfectly calibrated FI meter, it would be very difficult for the person taking those FI measurements near the earth to know with certainty the true value of the radiated field itself, versus the radiated field including the earth reflection.
The FCC defines the §15.239 maximum field at a distance of 3 meters from the transmit antenna, which distance tends to include some significant effects of reflections arriving at the receive antenna of the FI meter. However many of the FCC FM NOUOs show the fields they measured much further away than 3 meters.
It is generally unknown how, or even if, the FCC accounts for the effects of reflections on the fields they measure on an inspection visit.
I think the aim of part 15.219 stations measuring field strength is not to avoid a NOUO, but to comply with 15.239 to the best of one's ability. To that end it seems to me we can measure field strength at 1-2 meters above ground.
FCC NOUOs suggest that only FM stations grossly over the limit receive NOUOs. A May 8, 2013 part15.us post lists the seven part 15.239 FM NOUOs for the month of April. The measured field strengths vary from 113 to 19773 times the limit. These stations were radiating 41 to 86 dB over the limit, making me think we don't have to worry about the FCC if our measurement is off by a bit. My worry would be cheating my station out of legal range.
Up above in #10 davec said: "I fired up the $26 SDR receiver and have determined that it is sensitive enough to measure FM Part 15.239 field strength out to 30 meters. The included antenna is suitable and can be used as a calculable dipole. One more thing is needed to perform the measurement and that is a calibrated signal source. If anyone is interested we can discuss this."
I very much encourage pursuit of this avenue of exploration, davec!
Using radio dBu meters, S-meters, spectrum analyzlers, and now SDR's to investigate the field strength of our mini transmitters is all welcome by FM radio hobbyists.
In a more recent post davec said, "FCC NOUOs suggest that only FM stations grossly over the limit receive NOUOs."
And that has been consistently true in the entire history of NOUOs.
Member Rich has made it clear that the FCC's methods in regard to hobby FM are nebulous, so we need to invent our own ways of going about it.
An indirect measurement can be simpler yet effective. We know how much RF power into a known antenna will produce 250 uV/m at 3 meters. The user can add coaxial attenuators to adjust their FM transmitter RF output power and feed an antenna such as a dipole or ground plane.
Is there any interest in an FM RF power meter?
Why not just add 30 dbu to the actual reading you get from your Tecsun Radio @ 3 meters?
So for 3 meters your actual reading should be around 18 dbu on your Tecsun Radio. Duing this will get you closer to legal and this again should be at table level. Now we know as you raise your transmitter your range will improve but you'll be legal.
Another way is to use a C. Crane to calibrate your Tecsun as the C. Crane is under the legal limit. Doing this and camparing your transmitters you should get a good idea if your even close to legal for FM.