Minimum Power: 12mW : 61dBu/25-sn : SA= 33dBm
Mid Power: 101mW : 71dBu/25-sn : SA= 44dBm
Maximum Power: 261mW : 76dBu/25-sn : SA= 49dBm
An interesting post, Carl.
Other things equal, a transmitter input power change from 12 mW to 101 mW (unmodulated) would result in a field intensity change of +9.25 dB. For this reported power change your Tecsun showed a difference of +10 dB in its "dBµ" display.
Again for other things equal, a power change from 101 mW to 261 mW would result in a field intensity change of +4.1 dB. For this reported power change your Tecsun showed a difference of +5 dB in its "dBµ" display.
So, if these (unmodulated) d-c power input values to the output element of the final, active r-f stage of your AMT5000 transmitter are accurate, and the r-f output power of the AMT5000 is linear with respect to that d-c input power for such conditions, and the location and physical orientation of your Tecsun PL-310 was the same in all cases, and the readings were all taken under the same propagation/noise conditions -- then the __relative__ (not absolute) "dBµ" readings on your Tecsun PL-310 could be rather accurate for those conditions.
The formula for determining the input (I mistakenly said "output" in the earlier post) to AMT5000 is provided as a formula in the Manual and using test points provided on-board the transmitter.
All circumstances and conditions for the reported test were stationary, nothing was moved, the tests followed in succession with no time delay.
Appreciate the encouraging report.
Appreciate the encouraging report.
Thanks for the thought, but just to reinforce that my comments earlier in this thread do NOT encourage/support a belief that the Tecsun PL-310 and similar DSP-based receivers with signal strength readouts expressed in units of "dBµ" are accurate as to the real values of the field intensities arriving at the loopstick antennas of such receivers.
Reported data shows that such readouts are low by 20 dB or more.
Would it be reasonable to add 20dB to every reading and go around believing that the resulting number was a bit more accurate compared to nothing?
Adding such a number could still leave a lot of room for error. From the few data points we have, the correct value appears to be somewhere between 20 and 30 dB.
If, for example, 20 dB was added when the correct value was 25 dB, the measurement of field intensity would still be low by about 180% of the true field.
As it is now, the correct value to add for the frequency being tested is only very coarsely estimated, and that is based on sparse data from only two Tecsun PL-310 receivers on a handful of frequencies in the AM broadcast band.
Such accuracy is far short of what is needed for a "compliance appliance."
It will be interesting to learn if the SMR receiver from I.S.S. has significantly different performance than shown by the Tecsun PL-310. The webpage linked below reports (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.
That quoted line from I.S.S. assigns a much higher field measurement accuracy to its SMR receiver than is indicated by the Tecsun PL-310.
Also note that the field intensity limit that needs to be observed for a TIS station using a vertical monopole antenna is 2 mV/m (66 dBµV/m) when measured with a standard field strength meter at a distance of 1.50 km (0.93 miles) from the transmitting antenna system.
This 2 mV/m field intensity also can be produced by an unlicensed AM station fully compliant with FCC §15.219 -- just closer to the transmit antenna. So the thought that the SMR receiver is useful only to TIS stations is incorrect. It would measure both types of systems with the same degree of accuracy.
What that degree of accuracy really is has yet to be made public.
http://www.theradiosource.com/products-smr-receiver.htm
Carl's readings with the 3 different power settings show good linearity for the Tecsun meter. His readings show essentially the same linearity as his spectrum analyzer results.
As I said before, the Tecsun readings could be converted to fairly accurate real field strength values with a conversion chart created under controlled conditions by recording the Tecsun indication for each of its 99 increments to actual calibrated FIM readings. For the additional price of the Grundig/Eton radio from I.S.S. (same receiver chip) this is what I would expert. The linearity of the receiver chip doesn't seem to be in question thanks to Carl's measurements. The remaining question is whether the readings would vary from unit to unit in the long term production run. There is no specification provided by the manufacturer defining the possible variation.
Carl's measurements of real broadcast stations vs. Radio-Locator maps are too widely out of agreement to say that these maps could be a viable way to create a meaningful calibration chart.
Phil, I am convinced that like the S-meter, the Radio-Locator maps are a conversation piece and nothing more.
Back in Post # 9 PhilB put forth a measurement suggestion for the TECSON PL-310. Here is what he said:
Reduced Coverage of Your Station at Night:
Take a Tecsun reading during daytime at a fixed location a moderate to fringe distance from your transmitter. Go back there within a few hours after sunset when skip is significant and your signal probably sounds weaker on the car radio. Most likely the reading will be the same or even higher at night because you are actually reading the composite of your signal, several distant stations, and increased atmospheric noise. On the car radio, with its excellent AGC action, it just sounds like your signal gets weaker. It doesn't. The background signals get stronger and begin to swamp out your signal. You can use this evidence to bust the myth that Part 15 AM transmitter signals are weaker at night.
All right, Mr. B. Here is Part 1 of this Day vs. Night Measurement.
At 1:20 PM right after the noon hour I went to the NE edge of the driveway right at the street, where my 1680kHz signal tends to be weaker due to the lack of ground radials in this direction. The reading on the TECSON was 50dBu/25-SN.
Transmitter & Wintenna are way on the rear of the house, about 150-feet from the spot, with AMT5000 set for 79mW input to the final.
Later, after the sun goes down and the critical hour gives way to nighttime skywave, I'll take the second reading for comparison. You might want to stay home for this.
Three times I have been down to the NE corner by the street, but I am not satisfied with the reading. It is something like 45/23, not what expected.
So, listen, let's do it again.
Tomorrow I will grab another daytime reading, and I think I should get the power exactly at 100mW to that RF input.
The evening reading will be final. This has got to end.
Hi, this is Carl
Hi Carl, thanks for all your measurement efforts. I think the day/night measurements may be of more interest to Part15.us members than the contour map stuff that proved to be pretty a bust, thanks to your great measurement efforts. So, I hope you don't give up on this too soon.
There are a few things that should be established for this to be a valid experiment.
1. Are the daytime measurements consistent from day to day.
2. Since your wintenna is essentially an indoor antenna, are there any variations due to indoor electrical variations (which lights and appliances are on).
3. Does the Tecsun PL-310 reading respond to various modulation levels? For example, if your transmitter is sending a 100% modulated signal with highly compressed and high density modulation, the reading may be higher than an unmodulated signal.
4. Is there a lot of vegetation (trees and bushes) around your measurement spot.
5. Has it rained between noon and post-sunset readings?
6. Is there heavy dew on the vegetation at the post-sunset reading time?
Given the variables above, I think it would be best to take the readings over several consecutive days while trying to duplicate the variables. Unless you can convince yourself that modulation level doesn't affect he Tecsun reading, set the modulation to zero for all readings.
If you can't prove or disprove the myth, you could always resort to what the Mythbusters always seem to do -- blow something up! That might make you feel better about the time you spend on this.
Guided by the list of steps provided by PhilB we are starting fresh.
The main objective of this test is to compare day readings to night readings using the dBu scale on a TECSUN PL-310 radio.
TEST LOCATION is the NE corner of the driveway at the street, about 150' from the transmitter/antenna, which is located in the rear of the building.
The NE corner happens to be the weakest signal point on the lot, owing to a lack of ground radials toward the north, however the signal is much stronger at the NW corner at the street which is right under the power line. We deliberately chose to observe the weakest signal location for this test.
The transmitter is AMT5000 adjusted this morning for 123mW output to RF final stage. The power adjustment will be left unchanged, and will be measured at each step of the test. No audio will be present during these silent tests.
The spectrum analyzer is located at a stationary position indoors 35' from the transmitter and displays a peak strength of 44dBm. Changes to this display will be noted over time.
We have turned electric lamps and other appliances off/on watching for changes on the spectrum reading, but the RF signal has so far remained constant.
15-minutes was spent trying to find the TECSUN radio, which is small and easy to misplace.
At 12:45 Noon-Hour we took our position at the NE corner of the drive at the street and measured 56dBu oriented toward the transmitter, s/n=25, with a wide generous angle of good field.
The null was very limited in its arc and required careful attention to find, and read 47/23.
This test location is open space, street, driveway, lawn strip, sidewalk, privet bushes 3.5' high about 10' away to the SW.
Weather mostly sunny, temperature 67-degrees, dry conditions, no moisture felt on grass or dirt, some cracking in the ground from long dryness. The last brief rain was 30-hours ago.
Part 2 when the sun goes down.
We weren't planning to do another measurement until after dark, but it was noticed that the reading on the spectrum analyzer had gone higher, so we did a new set of readings. The audio was off.
Spectrum Analyzer = 47dBm at 3:15 PM CDT (was 44dBm at 12:45 Noon)
TECSUN PL-310 field reading at NE corner driveway/street = 56/25 (was 56/25 at 12:45). It has stayed the same.
Null reading is 46/23 (was 47/23).
AMT5000 Input to Final RF = 121mW (was 123mW).
Weather sunny 69-degrees.
7:55 PM CDT me down at the NE corner of driveway and street for field readings after dark on testing radio TECSUN PL-310. No modulation.
At 12:45 Noon in Sunlight reading was 56/25 with a null of 47/23;
At 7:55 PM just now the darkness reading was 55/25 with null 48/20.
Spectrum Analyzer shows 45dBm (earlier 44dBm), and I've found why readings change on the screen every once in awhile. There is a crane-type desk lamp that reacts with the antenna on the front of the analyzer. I am always moving the lamp into different positions.
Temperature tonight is 65-degrees, it's very dry.
Input to final RF of transmitter = 123mW, same as during the noon hour.
I watched the radio's field strength reading during normal program modulation and the numbers did not change. I will try again later with 100% sine wave modulation.
This is encouraging in that you are getting repeatable results which will be useful baselines to monitor for changes. They will also be useful as changes and improvements are made.
There is a question about the spectrum analyzer reading though. dBm is an absolute power reading conventionally referenced to 1 milliwatt at the input. 44 dBm represents 25 watts at the input being dissipated by the input R of the analyzer. Are you sure this is correct?
For reference, the power can be calculated as P(Watts) = .001*(10^(dBm/10)).
Neil
The graticules on the spectrum analyzer show peaks of RF signals intercepted by the 13" supplied antenna, but adequate description of what is being read is something I'm not sure how to express.
I have been saying "dBm" because the manual states that the signals seen are dBm, for example the manual says every graticule line represents 10dBm, with the top line being 80dBm, but of course without the precise calibration you have asked about, they are "dBm" only in the way a stopped-clock is right two times a day.
So, given the realization that the readings are "wild, undefined" indications of "some amount of signal," how can I best express what it is?
The peaks seen on the screen of course show "relative differences" between a handful of nearby signals, but there are many stations across the local dial whose spectrum readings are down in the noise because of the small antenna.
From now on I'll say "dB?" unless there's another suggestion.