HOPE FOR US? MAYBE A WAY TO CALCULATE FIELD STRENGTH OF OUR TRANSMITTERS WITH A TECSON RADIO

Hope for sure!

Mark, I am very interested in this topic and will follow with great interest as comments are added to the post. I have for years considered my TECSUN PL-310 with its field strength indication a must-have part 15 tool to observe numbers at varying distance from the transmitter. In fact, I will use this stimulus as the right time to upgrade to a newer TECSUN model and renew my experimental activities in the art of micro-casting.

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@carl-blare I have thought long and hard to try to find a way to measure our transmitter's field strength without a $15000 machine that is big, heavy, needs a large 7ft dipole antenna, and is not practical. There's the Tenmars hand held meters that are quite expensive and measure down to 0.1mV/m(100uV/m) but are not frequency selective so they just measure everything coming from everywhere. I don't think it's so difficult to make a field strength meter that is frequency selective like the Potomac FIM-71 in a hand held device at a reasonable cost. Now, the cost prohibitive Potomac takes a field intensity in open space that induces a small voltage on the antenna just like radio reception and shows the reading that is a small voltage at the antenna input of the circuit board of the Potomac. Then some processing relates this to a meter that from what I saw is a volt meter. Then the readings are interpreted as a field strength in mV/m which you convert to get uV/m.
The Tecson/Grundig radios do the same thing. I will get a Tecson returnable at Amazon and see with someone here...well I can tell you who it is, Gerry from Procaster who I am sort of friends with here as he is where I am in Canada has a Potomac FIM 71 which has the frequency range that includes the FM band. This is the meter that the FCC uses for compliance. They also have portable ones they have in their cars, as does ISED(Canada).
I will take the Tecson radio to his place and with a transmitter see if with conversion from dBu to uV/m I get a similar result as the Potomac FIM. I'll try some experiment with adding or subtracting antenna length on the Tecson to see if there's a sweet spot to get close readings(as the FIM) at different distances. It may work..."may" is the key word.

I hope you can get it to work too.

As I recall, this has been discussed before, and the issue is obtaining correct absolute readings, i.e., proper calibration.  That's part of what makes a FIM so expensive to run, as you have to ensure that it is accurately measuring field strength.  Relative field strengths for the Tecsun, no problem.

Using a Potomac FIM, you might be able to get your receiver accurately calibrated with antenna length (it would be interesting to see if length would change across frequencies), but someone else's radio could be totally different.  Another experiment would be to attempt to calibrate one Tecsun radio, and see if the measurements stay the same on another under the same conditions.

Yes a lot of variables but if Tecson designed the chip and processor with the dBu readout to have some degree of accuracy you'd think there would be similar readings between radios.
Just like an FIM the readings from the Tecson will change with objects near it etc and that's why it is done in the open. It could also be done like this....if you get a dBu reading on the radio of say 12dBu and that corresponds to a reading of, lets say(just for discussion) 200uV/m on the FIM and then I go closer to the transmitter and now get a reading of 50dBu on the Tecson and see what the FIM now shows I could potentially devise some kind of table to interpret the dBu readings but with the conversion it would be great if I could calibrate the Tecson to approximate the FIM. The antenna position would have to be in the calibration also, vertical or horizontal. Trouble is, you need something to calibrate the Tecson radio with and that means an FIM!! The FIM-71 is the one used for FM frequencies and the FIM 41 is for the AM frequencies although it's not needed for AM as field strength is not a factor there.
FIM 71.... https://testequipment.center/Product_Documents/Potomac-Instruments-FIM-71-Specifications-32D80.pdf
All analog...frequency adjust, volt meter with range selection, just like an analog multimeter.
I'm sure the Chinese could easily make something like the Tenmars hand held with true frequency selection.
Here's dBu conversion to mV/m right from the FCC page... https://www.fcc.gov/media/radio/findvalues

I am guessing that using our best judgement to calibrate the TECSUN reading can put us in a fair and reasonable estimated ball-park of compliance that would go a long way to demonstrate to an FCC inspector that we were doing our sincere best to comply. These results should be kept with other documentation in our inspection file at the transmitter.

Very interesting idea, so I conducted a little experiment.  I have a tecsun 380 receiver, a C Crane fm transmitter, and a CZE-7C 1-7Watt (retekess 508) fm transmitter. 

The C crane works as advertised, with the pot mod and 30 inches of wire on the antenna, I get about 300 feet.  Measuring 3 meters from the antenna the tecsun gives me a reading of 50 dBu, which converts 316 mV/m.  About what you would expect for the modded C crane.

With CZE-7C set at low power I can get 1/2 mile of clear reception and a mile of spotty(same circuit as the retekess 508). At 3 meters it gives the tecsun a reading of 65 dBu, which converts to 1778 mV/m...way over the limit of 250 mV/m! 

When you look at the part 15 test report for the CZE-C7 pulled from the FCC site, the numbers they report of 1225 mV/m are in the ballpark sort of at least.  

I guess I would conclude that using a tecsun receiver to test anntenna output gets you in the ballpark.  And although the CZE-7E is part 15 certified, it's at least over the limit 5 fold.  

Welcome.
Those readings in mV/m have to be converted to uV/m so 316 mV/m you got from the Ccrane is equal to 316,000 uV/m which seems you didn't get the conversion right from dBu which the Tecson displays to mV/m. That is WAY too high a reading from the Ccrane! Or even the Retekess! Did you mean mV/m as uV/m? Micro-volts per meter is written uV/m, not mV/m. mV/m is millivolts per meter.
Here's a conversion from dBu to mV/m. Then to get uV/m you move the decimal point 3 places to the right.> https://www.rfwireless-world.com/calculators/dBu-to-mV-per-m-converter.html

The only way to calibrate the Tecson is to check it with a Potomac FIM and get some co relation with the dBu reading and the FIM. I will be seeing someone here soon with one and I will buy a Tecson then from Amazon(returnable) and see if indeed the Tecson can be able to give a reliable idea of actual field strength. But the legal limit in the USA is in uV/m @ 3 meters(250) which translates to .25mV/m which translates to approx 48dBu
Also, if you have a Retekess and at 1 watt power you only get 65 dBu on the Tecson at 3 meters away that would translate to 1.778 mV/m which is 1778 rounded off uV/m which should be WAY more than that. The reading on the Tecson should be much higher! It could be that the dBu readings on the Tecson have a max of 65 or 70 dBu and after that won't register higher. I am not encouraging you to do this but for testing purposes if you put the tecson with 1 watt power right against the antenna see what you get in dBu, Try at 7 watts the same thing. Again I can't condone this but just for a minute and see what the tecson says. I have a feeling that the Tecson radio will max out at 65 or 70 dBu no matter what which wouldn't be reliable readings. I read in the Tecson specs on one of the more expensive ones that the dBu reading goes from 0-99 dBu.
I think you have made some discoveries here that the Tecson may not work!
Note that 48dBu is equal to 250 uV/m so I'd be interested in seeing just how high a dBu reading you can get on the Tecson.

Thanks for the reply, welcome, and math check.  Yes, it looks like I messed up my decimal point and labels, but I think I ended up with the correct numbers.  

With the C crane the Tecsun gives me a reading in dBu of 50 at 3 meters, which converts to 0.316 mV/m.  Moving the decimal to the right 3 places gives you 316 uV/m.  

Moving the Tecsun to an inch from the retekess set at 1 Watt gives me a dV/m of 68 (2.511mV/m, or 2511uV/m 10xs the legal limit!).  When I first got it out of the box I set it to high/ 7 watts.  I was able to pick it up at two miles which clearly was asking for trouble so I haven't used that setting since or measured it.  Perhaps you are right and there is a limit to what the tecsun will read.  Still, the numbers are ballpark what the Part 15 test report for the unit states, so I think you're on to something using the tecsun to measure output.  I had the same thought myself but didn't know how to do the math conversions.

I've only recently become interested in micro broadcasting and Part 15.  It fascinating and frustrating to find information that is accurate and not contradictory.  From what I gather the chinese FM transmitters can be part 15 certified, but still not street legal?  That assumed, I haven't gotten an understanding on what the FCC does about it in the real world.   I see they have cracked down on some "pirate radio stations" with warning letters and heavy fines, but I assume (hope) those are not just people tinkering with their Retekess's or C cranes in the den for a couple of hours on the weekends.   

I have the Schlockwood SW-300 field strength modulation meter. From the same company the makes the SW200 processor. It seem pretty accurate, but it only measures AM.