So I went and bought one of these cute little Tecsun radios that have the signal strength dbu meters in them. Why? I guess because I felt like it at the time 🙂 I've always wanted to take one of these out and compare the dbu numbers to actual field strength readings in volts (well, mv anyway). Theoretically, if these radios are built with any decent quality control I should be able to determine what dbu = what mv, etc. and if this holds even slightly accurate from radio to radio ya'll could use the info for some non-scientific field strength readings. Seems to me there was an outfit selling Tecsun radios that came with a chart that did this, actually (at a rather steep markup if I recall). So, anyway, something to play around with.
At first I thought I had received a radio for a different country. Only tuned to 1620 (which worked for me since my station is at 1620) and the ad sad AM tuning 520-1710. but I discovered when I changed the AM tuning step from 9 to 10 KHz, it also changed the band for AM from 522-1620 to 520-1710, also changed FM from 64-108MHz to 87.5 - 108 and changed the temperature from C to F. Thsi wasn't mentioned in the instruction booklet (although I admit I had to read it with my reading glasses AND a magnifying glass).
Anyway, experiment results to come 🙂
Tim in Bovey.
... Theoretically, if these radios are built with any decent quality control I should be able to determine what dbu = what mv, etc. and if this holds even slightly accurate from radio to radio ya'll could use the info for some non-scientific field strength readings. ... experiment results to come _________________
Just to note that the "dBµ" number shown in the LCD displays of such receivers is NOT an accurate measure of the field intensity present at its receive antenna.
It is based on the r-f voltage present across the __input terminals__ of that receiver with reference to decibels above one microvolt across that input impedance.
That value can differ greatly from the field intensity in dBµV/m from the transmit system arriving at the receive antenna to produce that input terminal voltage in such a receiver.
Below is a graphic showing measured results, proving this.
I took the Tecsun PL-310et out with my FIM today just for a couple quick comparison readings. I realize the readings are not the same units, or the same methodology, but I thought it would be interesting to compare anyway.
At my usual test spot not far from my transmitter, I have a field strength of 1.2 mV. This is typical, never varying more than .1 up or down over the past 18 months. At this same point the Tecsun gave a reading of 46 dBu. Yes, of course I rotate the radio for strongest reading just like I do the FIM.
Then I went to what I consider the edge of my coverage -- the spot where day in and day out, I drive past and notice a clear drop in clarity in the car radio leaving town, and a noticeable improvement on the way into town -- it's amazing how defined this point is. This spot is about 7100 feet from the transmitter/antenna (if you've seen the photo of my coverage circle, it's the parking lot near the highway on the west side of the photo -- the coverage circle goes right through it). At this point I read a FS of 60 uV (note, this is now uV, not mV) and the Tecsun shows 22 dBu.
Interesting to note that at this "fringe" spot listening to the station on the Tecsun was nearly impossible because it picked up noise that no other radio I've tried at this spot picks up. I've tested at this same spot with 4 different car radios, a boom box, a GE "Super Radio" and various small portable transistor radios -- like the kind you used to buy for $3.98 to listen to rock and roll or the ball game on back in the 60's and early 70's. Each of which is very clear and listenable in this spot.
What does It all mean? Not too sure, but I'll say this, the little Tecsun radio is a really good sounding, small portable radio!
Tim in Bovey
Interesting, Tim.
The 20:1 voltage ratio measured by your calibrated FIM at those two locations is a difference of 26 dB. Your PL-310ET showed a 24 dB difference for those locations.
The PL-310 signal display reads out in decibels, rounded to the nearest unit, so that is fairly good accuracy/linearity for such an inexpensive receiver.
I think this Tecsun PL- series is designed to get quite "deaf" when the received S/N doesn't exceed some threshold value. That may account for the difference in its performance from the other radios you tried 7100 feet away from your tx antenna.
I posted the following clip in this thread earlier today ... The PL-310 signal display reads out in decibels, rounded to the nearest unit, so that is fairly good accuracy/linearity for such an inexpensive receiver. ...
My "fairly good" comment above applied only to the ratio of the fields measured by a PL-310ET compared to the ratio reportedly measured for those same two fields using a calibrated FIM.
Signal strength displays on a PL-310ET are not an accurate measure of the true values of the field intensity in units/sub-units of volts per meter arriving at its receive antenna.
Therefore such "stock" receivers cannot be used either to prove or disprove compliance with the maximum fields permitted by the FCC, or any other authority.