R. Fry said:
tfcwings said:
(I believe R Fry has a Tecsun PL-310, which one reviewer says is comparable to my PL-606), and I'm hoping I have something under my nose that would help me more accurately experiment with and research AM broadcast signal strengths.
The link below leads to a comparison of the display in "dBu" shown on a Tecsun PL-310 for the actual field intensity at the points of measurement. So if a PL-606 is comparable to the PL-310, and they both "meet spec," then this will give you three data points showing actual field intensity vs. the display shown for it on the Tecsun receiver.
Note that the dBu reading on the Tecsun receivers reports the conducted voltage across an internal resistance in the receiver. It is not a calibrated measure of the field intensity of the arriving radio wave
Others have measured the linearity of the Tecsun dBu display with varying r-f input voltages, and found it to be quite good. So as long as the radio is not over-driven, and the measured frequency is not much different from one shown in my comparison, then the
approximate field intensity at the location of measurement can be calculated from the data given in the link.
For example, on 1440 kHz my PL-310 displayed 58 dBu when the arriving field intensity was about 28.9 mV/m. So in another case if the Tecsun displayed 38 dBu for a 1440 kHz signal, then the arriving field intensity = 1/10^(dBu difference/20) * reference field, or 1/10^(20/20) * 28.9 mV/m = 2.89 mV/m.
If the measured field is greater than the reference field then the equation is 10^(dBu difference/20) * reference field.
For medium wave signals the receiver should be positioned so that the long dimension of the case is in the horizontal plane, and the meter reading should be maximum when the case is perpendicular to the compass bearing to the transmit antenna. If both of these do not exist together then the reading can have significant error. (The whip on the Tecsun receiver is not used for MW reception, and should be collapsed and stowed on top of the receiver.)
http://i62.photobucket.com/albums/h85/rfry-100/Tecsun_PL-310_Signal_Readings.gif
I understand that about positioning for MW signals. For me, to find the right compass bearing, I will first null the station (until it gives me the lowest dBu reading), then I will rotate it 90° to peak the signal.
I should mention that in some locations in the house I will get different compass bearings and/or readings. I'm suspecting that there may be some metal objects interacting with the radio. For example, 760 KFMB, which at night normally displays 82dBu at night, may display 85dBu on top of my upright Baldwin piano (I suspect the cast-iron plate), 87-88 dBu on the bathroom floor near the bathtub (I think a water pipe may be running under the floor - the particular spot is in line with the septic tanks), or 88-89dBu (after sometimes briefly saying 92-93dBu or so when I first tune to 760) in a hall coat closet below an electrical breaker panel.
Also, what do you do with less-than-good-quality signals? As a few examples, sometimes I will aim at a station, like 980 KFWB in my back yard, for example, and while I can hear the station, I get a 30/00 reading (30 dBu, 0dB SNR). If I wait several seconds, it may show a reading of something like 27/05. If I rotate-null then re-aim, I can sometimes get it to show a reading of something like 22/10 or so.
And what about weaker signals in the general vicinity of a stronger ones? For example, I've experimented with a crude signal generator on some signals, and have sometimes taken what would have been a 72/25 signal, and made it read 50/00 (and weak but audible) when generating a signal 20 kHz away that reads 98/25. (A 24/14 signal would be undetectable, reading 50/00 as well, in the same situation.) Also, at my grandma's house about 1/3 mile SE of 1300 KAZN and 1430 KMRB, while I haven't taken the PL-606 there yet, I have had the PL-380 and the Grundig G8 there, and in the daytime I get readings of 50/00 on 1230 KYPA, 1260 KGIL (is it KMZT now?), 1280 KFRN, 1330 KWKW, (50/15 on 1390 KLTX), 1460 KTYM, 1480 KVNR, (IIRC 1580 KBLA, or 50/something), and 1650 KFOX. Even though according to the radio-locator maps and v-soft website they all should have different signal strengths, they all indicate 50 dBu, most of them with 0 dB SNR.
I've made
several short video clips (unfortunately with no sound as the mic on the camera isn't working) of the display on the PL-606 in various situations. A few show what happens when I tune to one signal near a very strong one (in those cases, the strong one's signal is peaked with the Select-A-Tenna - there should be explanations in the descriptions). I notice that the reading might show one number when first tuning to the desired signal, then a second later has dropped by maybe 15 dBu or so.
In a few cases, the SAT is peaking the signal I'm tuned to. One thing I've noticed that to me is very peculiar. Normally the SAT is supposed to amplify the signals - for example 46 dBu on 640 KFI becomes 65 dBu, 43 dBu on 1000 KCEO becomes 64 dBu, and 39 dBu on 1550 XEBG becomes 55 dBu in one spot in the house.
However, in one spot shown in a video or two, the SAT was making the PL-606 read 85 dBu on 760 KFMB, whereas it normally reads 88 dBu, a
REDUCTION of 3 dBu! (This is after briefly showing a higher number like 94 dBu or so.) Also, speaking of compression at the upper end of the range, I get a reading of 82 dBu on 760 KFMB at home 7.3 miles southeast of their 50kW transmitter at night. To get to 90 dBu, though, I have to be on the road near one of their towers, maybe a little over 300 feet away.
Also the low end of the readings bottom out at 15 dBu, unless you have strong signals nearby, in which case it progress up in a series of 17, 19, 20, 22, 24, 25, 27, 29... and so on with the same pattern up to 50 dBu for a "minimum" reading. Is it possible to guess strengths with readings near the between-channel threshold like that? (BTW I've been able to get a
15 dBu and 25 dB SNR reading on at least a few occasions, one being when I partially peak 1170 KCBQ with the Select-A-Tenna and tune to 2340 kHz in the daytime. That pegs the meters both directions - minimum dBu and maximum SNR - simultaneously. In cases like that is there any way to calculate field strengths, or should I be looking at other methods?
Also, have you figured out how to measure LW, FM and SW signals? (The Stoddart I have only goes up to 25 MHz so is useless for FM.) I do understand, though, that the other bands, based on the review, are slightly different in performance, but I'm wondering if there's a way I could use meter readings to see an approximate 250 µV/m field in 88-108 MHz, or 15,848 µV/m from 13,553 to 13,567 kHz, for example. (I should mention that the whip antenna on my PL-606
is broken right now.)
Have you ever noticed "skewed" readings like what I've described?
Also would you by any chance know of a resource where I could find out how to get this FIM mentioned in the thread title fully functional? Or do you think, considering it is a tube-based design from no newer than the 1960s, the Tecsun radio would be better for reading field strengths?
Another reason I'd want to get the Stoddart NM-20A working is so, if possible, I could measure how much gain various inductively coupled antennas have. I have attempted to guess that on a couple occasions, but I seriously doubt my methods would even be within +/- 30dB of accuracy.
For example, if I hold my Tecsun PL-606 and Select-A-Tenna up to a utility pole ground wire up the street from my house, I can get some massive overload on 760 KFMB from 7.3 miles away at night, with, for example,
an 83 dBu reading on the 2nd harmonic, 1520 kHz, and a
85 dBu reading on the 4th harmonic, 3040 kHz (the Tecsun won't tune the 3rd harmonic, 2280 kHz). When I was about 160 feet or so from one of their towers,
1520 was only reading 59 dBu. On another day, I was able to visit the site of 2.5 kW 590 KTIE in San Bernardino (I was in the area on business), and when I was somewhat close to their tower, I was getting a reading of
81 dBuV on 1180, the 2nd harmonic, and
91 dBuV on 2360, the 4th harmonic, using only the built-in ferrite bar antenna.
I have also come across a daytime photo of 1170 KCBQ's 2nd harmonic,
2340 kHz, showing 98 dBu in the same spot where I peaked 760, but I haven't had the chance to visit KCBQ's site recently to find out how close I have to get to have the same readings. I have, however been there with the PL-380 in the past, and while it won't indicate above 63 dBu, I've gotten it to indicate
21 dBu on the 60th(!!) harmonic, 70.2 MHz in the FM BAND(!!) outside the property, peaking it the same way I had peaked 760 near home. Also at that level the radio gets overloaded so severely that somehow the signal bypasses the RF sections or something. When I turn the radio on, the distorted audio is
immediately audible, as well as when I use functions that normally temporarily mute the radio. Any idea what's happening with a signal level that high, and what field strength would produce that effect using only the built-in ferrite antenna?
Also, one evening a couple weeks or so ago, I got a
97 dBuV reading on 1580 KMIK 300 miles away in Tempe, AZ, peaking it the same way I had peaked 760 and 1170. A few days after that, I visited 5kW 1290 KKDD, and needed to be about 90 feet away from a tower to get the same reading of
97 dBuV using the built-in ferrite bar antenna.
I had attempted to calculate the gain with a fairly complicated method. It involves noting the distance I was with the bigger antennas, looking up the FCC field at 1 km for the station, figuring the ground loss with the ground conductivity curves (the M3 map says "8" where I am), then comparing that with the calculated IDF reading (based on the 1 km field) at the close distance to the tower. For the skywave from KMIK, I found a skywave loss graph on the FCC's site (don't have the link right now, though) to try to calculate the received field. I seriously doubt I've been making the calculations correctly. Also how do you measure distance from a tower when you're somewhat close (like in my photos taken at 590 KTIE's tower), for example so you're closer to one leg of the tower than the distance between the tower's legs? Do you measure from the nearest leg to you, or from the center under the tower?
I think it would be better if I could get the NM-20A FIM working so I could make more accurate measurements (although I'm not sure if I could inductively couple the Select-A-Tenna and utility ground wire to it) of antenna gain. Also, I think it would be safer for the stronger signals - I'm guessing to duplicate the same behavior I got outside KCBQ's transmitter site using the SAT + groundwire, while using just the ferrite antenna, I'd probably have to be
touching the 50kW station's hot tower.
