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Cool Tools

A

audioguy

Maybe you already know about this, but I just discovered a really neat tool for measuring the straight line distance between two points. This is useful when you're trying to figure out exactly how far your signal is going.

Check it out: http://www.daftlogic.com/projects-google-maps-distance-calculator.htm

The other thing is the Tecsun PL-310 portable radio. I learned about this over on the DX board. You know how we all struggle with trying to figure out how to measure field strength without having to spend $14,000 for a Potomac? Well, the PL-310 has a digital field strength meter. I plan to eventually measure it on the bench to see how accurate it is, but at least you can get repeatable readings out of it. How cool is that?!
 
I have a Tecsun PL-310, and can provide some data points about its signal meter. The display shows a value in "dBu," but they don't define their meaning of that term.

A distant station on 790 kHz has a field intensity of about 1.2 mV/m (61.6 dBµV/m) at my location, according to the FCC propagation charts for the radiated power, path conductivity and length. The Tecsun meter reads 30 dBu -- which if that was a field intensity would be 0.032 mV/m.

A local station on 930 kHz has a field intensity of about 134 mV/m (102.5 dBµV/m) at my location (FCC value). The Tecsun reads 80 dBu, which if a field intensity would be only 10 mV/m.

In both cases the loopstick antenna in the Tecsun was oriented for a maximum reading, which put the long dimension of the case perpendicular to the compass bearing to the station. When the radio was rotated 90 degrees the null was 20 dB or better. The radio was outside, and well away from power lines and other possible re-radiators.

For a check on FM field measurement I took the radio out in the middle of the street in front of my house, and with its whip antenna fully extended and in the vertical plane it read about 60 dBu (1 mV/m, if a field intensity) on a local station that uses a c-pol transmit antenna. It read about the same with the whip horizontal. The whip antenna was about 5 feet above the ground.

The expected field intensity from that station for the radiated power, transmit antenna height, path length and receive antenna height is about 12 mV/m, or 81.6 dBµV/m.

So based on the results above, the PL-310 does not provide even a roughly accurate measure of the field intensity that the radio is receiving (unfortunately).

RF
 
If the meter is even approximately related to the signal level by some constant of proportionality, a calibration factor can be added to convert the dBu reading of the receiver to field strength in dB(uV/m). If you have confidence in the FCC's contour for the local AM station at 930 kHz, the calibration factor would be 102.5 - 80 = 22.5 dB. This particular calibration factor would give an error of 9.1 dB for the distant station at 790 kHz. The error could be due to the meter circuit, or the FCC contours, or both. It would be nice to be able to compare the receiver reading to actual readings at different frequencies and different signal levels by a standard FSM.
 
In My experience The Meters on Radios are for show.
On Mine they only seem to do anything on AM on FM its always a Full 9
 
I just finished performing a series of bench tests on my PL-310. On the AM band I tested the unit at 1650 kHz, and on the FM band I tested at 90.7 MHz. All measurements were performed using a Panasonic VP-8179P analog signal generator.

AM measurements

Test conditions: 30% modulation; 1 kHz tone; E-field coupling

The minimum reading on my PL-310 is 15 dBu. A 15 dBu tone-modulated signal is audible. Readings below 16 dBu were indeterminate. At 16 dBu and above, the unit exhibits almost a perfect 1:1 correspondence between signal input and the meter reading. I could not produce a reading higher than 89 dBu with my particular test setup (signal generator limitation). This is a remarkable feat in of itself for such an inexpensive device-- over more than a 70 dB range, a 1 dB signal change produces a 1 dB meter change. Also, on my unit a reading of 30 dBu equals a S/N meter reading of 10 dB (I did not attempt to verify the S/N or SINAD measurement).

FM measurements

Test conditions: 22.5 kHz deviation, 1 kHz tone, E-field coupling.

The minimum reading on my unit is 0 dBu. A 0 dBu tone-modulated signal is audible. From 1 dBu up to 70 dBu, the meter reading again exhibits almost a perfect 1:1 correspondence with signal input. Above 70 dBu, it saturates and there is no further increase in the reading. On my unit, a 13 dBu signal corresponds to 10 dB S/N, a 41 dBu signal corresponds to 42 dB S/N, and a 60 dBu signal corresponds to 60 dB S/N.

This confirms my earlier comment that this receiver should be very useful for Part 15 operators who want to make relative signal strength measurements. In other words, if you measure a 6 dB change in the reading, that should represent pretty close to a 6 dB change in the field strength. If you can compare the reading to a local station on a nearby frequency for which the field strength is known, you may be able to get an idea of the actual field strength that you are working with.

If anyone has any additional tests that they would like run, let me know and I will try to help. I will have the unit in the lab for a couple more days before I take it home.
 
audioguy,

If you have information about the measured field strength of a local broadcast or TIS station at a particular location, you could see what the meter reading is with the receiver. Then you can determine the calibration factor for the meter in the way I mentioned in my previous post in this thread.
 
audioguy said:
... In other words, if you measure a 6 dB change in the reading, that should represent pretty close to a 6 dB change in the field strength. ...
Click to expand...

That may be ~true for your/my PL-310, but such 6 for 6 (or 1:1) linearity in the indication of "signal strength" isn't necessary to peak the radiated field of a transmit system.

That is possible as long as whatever device that is used to indicate the relative strength of a signal on a given frequency at a given location shows different values for different radiated powers.

That functionality may be useful, but unfortunately, does not permit the accurate and provable measurement of field intensity needed to show compliance with the requirements of Part 15.
//
 
It is true that only a calibrated standard field strength meter can be used to demonstrate compliance with the rules. The roughly-calibrated receiver can be used for what is known as "pre-compliance" testing for just getting an approximate idea of what is going on.
 
The link below leads to the values displayed by the Tecsun PL-310 today for the signal strength of three different radio stations having known field intensities at the point of measurement.

The variations shown in the PL-310 readings can't be applied in general to other PL-310s at those frequencies, much less any other frequencies, of for how this or another PL-310 would follow changes in those incident fields.

But in a limited way it does give some idea of the "real world" performance of the PL-310.

http://i62.photobucket.com/albums/h85/rfry-100/Tecsun_PL-310_Signal_Readings.gif

//
 
I was asked about sensitivity measurements by one of the members here, so I ran a test on my PL-310 today. I could only test the FM sensitivity since there is no input connection for AM and I don't have any way of generating an accurate AM field.

On FM, I find that a -122 dBm signal at 90.7 MHz is just barely audible, meaning that you can hear when the generator is turned on and off. That is remarkable, but keep in mind that this would absolutely not be intelligible under any circumstances. However, an input of -104 dBm produces a quite respectable signal that is almost "noise free" although not "ultimate quieting" (in mono). The equates to a reading of 8 dBu. An input of -102 dBm (10 dBu) is very listenable in mono. That's quite good for a portable radio.

When I stated earlier that I believe the radio is useful for AM band comparison measurements, I did not intend to imply that anyone should be trying to use it to make absolute measurements to demonstrate Part 15 compliance (or even pre-compliance). All you need to demonstrate compliance (to 15.219) is an accurate DMM and a tape measure.

Things you could use it for: if you know, or can estimate the signal strength of a local station on a nearby frequency, you can use the radio to determine how much stronger or weaker your signal is compared to that station, at a given distance from your antenna. You can also use it to correlate reception quality with signal strength. Without such a meter, it's really very difficult to compare two signals because of the AGC operation in most radios. Finally, it would be very useful for seeing how much of a difference it makes when you add or remove ground radials, tune the transmitter, etc. I'm pretty convinced that at least my unit exhibits a good relative correlation between signal level and meter indication.
 
I have the PL-380. I've noticed it has the 15dBu minimum reading on AM like the 310, but for some reason my PL-380's maximum reading is capped at 63dBu RSSI, 25dB SNR for AM and 36dB SNR for FM.
Also, while the minimum reading is 15dBu on the PL-380 (and, in a place with a not-so-strong RF field, like between Escondido and Temecula, or inside a car at home, I can get readings of 16dBu and 18dB SNR), it does seem to desense when faced with very strong signals. For example, my grandparents in San Gabriel live 1/3 mile away from a 23kW on 1300 and a 50kW on 1430, and it reads 50dBu even in "empty" channels across a significant portion of the band. At my house here east of San Diego, the "noise floor" is typically 30dBu across most of the band, except in the 1100s where it goes up to about 45dBu on 1150 or so. I'm 6 miles from a 10kW on 1130 and 9 miles from a 50kW on 1170. If I use a Select-A-Tenna, tune it to 1170, and put it and my PL-380 up to a power pole with a ground wire (hidden inside a wooden piece) running down it, it seems to couple the overhead powerlines to my radio as if it were an antenna.
http://cid-6bdd1917662288cb.skydrive.live.com/browse.aspx/AM radio files/1170 KCBQ - 2010-04-16
At that link are a few pictures and audio files. There's a pic of me with the SAT and PL-380 at the power pole demonstrating what I do to amplify the signal. Also there are a few recordings of 1170 on the PL-380, and also on my Panasonic RQ-SW20 which has very poor selectivity - comparable to the Sony SRF-M37W. With the PL-380 and SAT at the pole, I was actually able to literally get 1170 to overload the audio section of my radio - this is demonstrated in one of the audio files at the above link. Also, the "blank" channels were reading 50dBu across pretty much the entire AM band, and if I remember correctly, were about 45dBu at 153kHz in LW, and didn't start dipping much below 48-50dBu until around 3 MHz in SW. Also, KCBQ's harmonics were quite strong with that setup - reading 63dBu on 2340 (2nd harmonic) and I think around 48dBu +/- 6dB on 3510 (3rd harmonic).
For those of you that have the PL-310... how well does it do when you're within a fraction of a km of a 50kW stick, of not being overloaded by it?
 
I'd be really surprised if any new AM radio had a proper tuned RF amplifier ahead of the converter.
It's too much work to try to get varactor tuning to track properly in multiple stages for manufacturers to bother.
So they just mix it all and try to use the IF selectivity and the DSP to make up the difference.
The result is what you've encountered, an apparently "raised" noise floor.
It would be fun to decouple the loop on one of these and add a proper tuned RF amp with variable gain.
The select-a-tenna is doing its job of tuning, but there's still no shielding to ensure that the ONLY signal which gets to the radio's
internal loop would be from the select-a-tenna.

Is anyone aware of a digitally tuned radio they know for SURE has a tuned RF amp on AM?
 
Yeah.  I have an old analog Zenith Royal 705 AM radio (probably 60s or 70s vintage) that I believe has a tuned RF front end... but obviously it's not digitally tuned. :p
One of my main reasons for getting my PL-380 was to get some stations that my previous portable had absolutely no chance of getting even WITH a large loop antenna, because of proximity to strong locals.  Unfortunately, it doesn't seem to be quite up to what I was expecting.  It's almost sensitive enough when there isn't too much strong RF, but it can't handle strong signals very well.  Sure, the audio splatter doesn't extend much past 13 or 14 kHz, but the raised noise floor can extend halfway across the band or more.
At my house, I have strong locals on 1130 (KSDO) and 1170 (KCBQ).  I can hear a station on 1110 barefoot, but it's buried quite far under a ~35dBu noise floor. KNWQ on 1140 requires the SAT and careful nulling of 1130 to hear, and KERN 1180 requires similar nulling of KCBQ to hear them at all.  KSDO's 10kW transmitter is 6 miles north of me, and KCBQ's 50kW transmitter is 9 miles to the north, by the way.  As for KTLK 1150, I probably would be able to hear it using only the built-in antenna, if it wasn't for a 45dBu noise floor on that frequency, AND the fact that I'm just outside the "fringe" contour line on KTLK's Radio-Locator daytime map.  When the noise level isn't so high, I can hear stations outside that area, though - for example 960 KIXW Apple Valley's signal is supposed to have their "fringe", according to Radio-Locator, just north of Escondido, CA... but I'm about 30% past that or so and can hear them from here without any external aid.
I was just wanting a portable radio that could go in my pocket that would be able to hear some weak signals in the presence of strong ones.  I had read some reviews of the PL-380 that claimed it had excellent overload resistance, selectivity, etc... but apparently the reviewers didn't test it from my grandparents' house in San Gabriel, CA.  They live 1/3 mile away from a 50kW station on 1430 (KMRB) and a 23kW station on 1300 (KAZN), both of which diplex off each other's sticks.  I've wanted to be able to hear 1290 KKDD San Bernardino from their house (Radio Disney - doesn't restrict analog audio bandwidth to 5kHz like KDIS does, but they're near the edge of the "fringe" coverage), but with my PL-380 there's absolutely no chance in heck of hearing KKDD next to KAZN.  On my previous radio, a Panasonic RQ-SW20, KAZN (1300) and KMRB (1430) would continuously splatter strong enough to light the tuning indicator from about 1220 or so up to almost 1710.
I wonder if something with a tuned RF front end AND a DSP chip might have a better chance?  (It would have to fit in my pocket, though.)  And, even if I can't hear KKDD from my grandparents' house with 23kW KAZN being only 1/3 mile away... I'd still like to be able to get solid signals of stations about 25% outside the "fringe" coverage, even when I'm within a couple miles or so of a 50kW station 10kHz away.

Oh.... and since R. Fry posted before I got the chance to post this... that looks interesting.... but it still apparently doesn't help at my grandparents place.  (Is there NO way to hear an 0.1mV/m signal 10kHz away from a 1/2-mile distant 50kW stick, without resorting to external antennas or larger than pocket size radios?)
Also, I just took another look at that... and it only looks like a tuner in front of the LNA, not an RF amp. I could be wrong, though.
 
tfcwings said:
...I had read some reviews of the PL-380 that claimed it had excellent overload resistance, selectivity, etc... but apparently the reviewers didn't test it from my grandparents' house in San Gabriel, CA. They live 1/3 mile away from a 50kW station on 1430 (KMRB) and a 23kW station on 1300 (KAZN), both of which diplex off each other's sticks.
Click to expand...

The field from KMRB at your grandparent's house will depend on the direction and distance of their house from the center of the KMRB array.

KMRB's directional pattern is shown at the link below. The power they radiate in various directions varies from about 213 kW to about 178 watts.

The inverse distance field intensity values shown on the graph are for a distance of 1 km. At 0.5 km (about 1/3 of a mile) they would be about twice as high.

In the maximum of their pattern you could have a field intensity close to 9 volts/meter, so trying to receive a signal of 0.1 millivolt/meter there would need a receiver with 99 dB selectivity just to make the KMRB signal equal to the signal you want to receive.

That is some rather stellar performance, and probably can't be done with practical, affordable receive systems. Using your external loop may help if its null can be aimed at KMRB without reducing the signal you want to receive too much.

http://www.fcc.gov/ftp/Bureaus/MB/Databases/AM_DA_patterns/1261620-104249.pdf

RF
 
They are about 0.6km away, basically southeast of the transmitter. According to FCC data, KMRB's augmented field strength at 1 km is 153.32mV/m, and the standard field strength of KAZN (1300), which transmits from the same antenna array, is 962.5mV/m.

KKDD (1290) is about 77 km away, at a heading of about 90° from my grandparents' house toward the transmitter. Their standard field strength toward the receiving location would be 454.27mV/m @ 1 km. (I don't know how to convert that field strength at that distance to the field strength at the receiving distance, while taking into account ground conductivity/losses/etc. Ground conductivity in that area appears to be 15 close to the coast, 8 a little more inland, and 4 farther inland (but probably beyond where KKDD's transmitter is, but I'm not sure.))

So, they are well within their null, but even so, on my Panasonic RQ-SW20 (which has absolutely abysmal selectivity anyway - a 50kW station 9 miles away can be clearly heard +/30kHz, -20kHz (there's a 10kW station 40kHz below the 50kW station, 6 miles away, that's clearly heard +/-20kHz), KMRB is heard quite loudly from 1350 up to the 1600s or so (it does a little better job of selectivity on the low side than on the high side), and the IF image at 530kHz can be heard strong enough so its as if that too was a local-grade signal, completely wiping out reception of the LAX TIS on 530 at that location. On the PL-380, it shows a 50dBu noise floor across most of the band.

Interestingly, even though KAZN's signal has a MUCH higher field strength toward their house, KMRB splatters MUCH wider on the RQ-SW20. Am I close enough to the transmitter there that the directional pattern has little if any effect?
I had recorded a bandscan on the radio with poor selectivity, the RQ-SW20, and have uploaded it.
http://cid-6bdd1917662288cb.skydrive.live.com/self.aspx/AM%20radio%20files/KMRB%20^0%20KAZN%20QRM%20-%202-20-2010%20-%20san%20gabriel^J%20CA/RQ-SW20%20quick%20bandscan%202010-02-20%20end%200530pm^J%201430%20to%201710^J%20520%20to%201430%20^5KMRB^6.mp3
It starts off on 1430kHz, tunes up in 10kHz steps (you can hear a beep each time the radio tunes) to 1710, then loops around to 520kHz, and continues up until it meets 1430 again. That recording was made at the west end of Ardendale Ave, across a culvert from the transmitter property. Even though KAZN's field strength is supposed to be much higher than KMRB in that direction, I'm thinking... it's probably at least one of two things - I'm close enough to the array to make directional antennas moot, or, at the time I recorded that (2-20-2010), KAZN wasn't broadcasting from that location, but from another site, for example their nighttime site 2 miles away. What would be most likely?
I remember doing a radio-locator search at the time from my grandparents' coordinates, and KAZN was listed as being I think 1.8 miles away. However, doing a search now lists KAZN and KMRB as I think 0.6 miles away.

One thing I find interesting is that power lines can greatly amplify signals (and static) across the band. I have experimented by inductively coupling my radio to a power pole (it needs to be one like in this picture: http://cid-6bdd1917662288cb.skydriv...io files/1170 KCBQ - 2010-04-16/PowerPole.jpg , and a pic of me, select-a-tenna, and PL-380 on-location: http://cid-6bdd1917662288cb.skydriv...2010-04-16/Me^J PL-380^J SAT @ Power Pole.jpg ), and have found it GREATLY increases the received signal. (Of course it doesn't help selectivity, and also increases the noise, so it obviously won't work for trying to pull in a signal that needs that much amplification JUST to break even with the galactic (yes, I meant that, not atmospheric) noise level at, say, 549kHz, when you have a 50kW local IBOC buzzsaw on 550kHz and you're right outside their antenna property.) For example, at that pictured location, from NINE MILES AWAY (!!!) from (1170) KCBQ's transmitter, I actually got my PL-380 to OVERLOAD - have distorted audio ON the frequency! Also, it was showing a 45dBu noise floor as low as 153kHz, and up to about 2.5 or 3 MHz shortwave, not to mention the 2nd harmonic was 63dBu and the 3rd was 49dBu or so. (Related files for that experiment are located at: http://cid-6bdd1917662288cb.skydrive.live.com/browse.aspx/AM radio files/1170 KCBQ - 2010-04-16 ) Also, I experimented (and recorded) how much it splattered on the aforementioned RQ-SW20. Last time I remember going by the transmitter on a highway about 1/4 mile west of the site, it didn't splatter anywhere CLOSE to as much. (BTW, since I don't have a good field strength meter (translation: none at all), my way of determining relative strength of strong stations is by how far they splatter off frequency.)
 
R. Fry said:
Tom Wells said:
Is anyone aware of a digitally tuned radio they know for SURE has a tuned RF amp on AM?
Click to expand...

The block diagram of the PL-310 (link below) shows a tuned circuit ahead of its MW/LW LNA...

http://i62.photobucket.com/albums/h85/rfry-100/PL-310_Block_Diagram.gif

//
Click to expand...

Okay, I see a loopstick antenna, in parrallel with two series connected capacitances, one fixed, one variable.
I suppose one is lumped capacity to put reasonance in the MW range, the other is the varactor diode for "fine" tuning.
Perhaps varactor diodes behave a little differently in the presence of very strong fields.
What kind of shielding does this radio have?
When I was doing an alignment on a 1973? Motorola car radio last week, I was amazed at just how much the metal case and using every single screw to
fasten the case was necessary to ensure that ONLY signals from the coax made it in on AM.
Even leaving one of two scews out was a noticable difference.

It is likley that an "active yet passive" junction which uses DC to create a virtual capacitor is going to respond somewhat differently than a truly passive
physical capacitor, the junction could be responding "actively" to such a strong field, when the design never intended it to be
"used" in the near field of transmitters.

My next step would be a metal box for that radio, with a separate loop and physical cap, then coupled into the receiver in the box.
Or just put up with it, as that's about how all modern AM radios with varactor tuning seem to work for me.

For a cheap comparison of something with real selectivity, find a decent 1962-1966 AM only car radio on ebay for 25 bucks or so.
I think you'll be amazed at the difference.
 
tfcwings said:
... Am I close enough to the transmitter there that the directional pattern has little if any effect?
Click to expand...

A rule of thumb is that the pattern isn't well formed closer to the array center than ten times the widest spacing of the active towers in the array.

Below are the monitoring points for the directional patterns of KMRB. The closest monitoring points to the antenna site are about 1.5 km away from it.

Callsign: KMRB
Special operating conditions or restrictions:
Monitor Point Descriptions:

109.5° - Point located 0.24 km south of intersection of Longden and
Kauffman at mailbox of 6145 Kauffman, 2.3 km from site, max 45.6 mV/m
daytime.

139° - Point located at southeast corner of Loma and Hermosa at walk to
9144 Hermosa, 1.86 km from site, max 67.2 mV/m daytime.

346.5° - Point located 0.08 km west of intersection of La Presa Drive
and Orlando at walkway to 3174 Orlando, 1.48 from site, max 929.0 mV/m
daytime.

93.5° - Point located 50 feet south of intersection of Baldwin and Val,
on curb at 1918 address marker just south of 1918 driveway, 2.63 km
from site, max 11.7 mV/m nighttime.

342.5° - Point located west of intersection of La Presa Drive and
Orlando at end of Orlando in cul-de-sac at fire hydrant, 1.56 km from
site, max 16.9 mV/m nighttime.

//
 
Tom Wells said:
What kind of shielding does this radio have?
Click to expand...

The case is just plastic-looking from the outside. Don't know if any conductive surface(s) surround the inside components, though. I haven't taken it apart, and probably won't unless it dies.

//
 
I've dismantled my PL-380 a couple times, and have some pictures up on picasa.

http://picasaweb.google.com/PianoPlayer88Key/TecsunPL380#

There are a few pics on the yahoo ultralightdx photos section of the PL-310 and a couple other DSP chip radios, but you have to be a member of the group to access them, so posting the links won't work here.

Also, Rich, where'd you get the info on the monitoring points? Is it available somewhere at http://www.fcc.gov/fcc-bin/amq?list=0&facid=52913 (FCC link to KMRB station info)?
 
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