Calculating AM directional ERP based on field strength?

[pianoplayer88key]

Hi all...

Does anyone know how to calculate effective radiated power (or whatever the correct term is) for directional AM stations?

For example, using 1170 KCBQ as an example, I know the actual power is 50kW, the RMS field (assuming 50kW non-directional using their antenna system) is 2499.5mV/m, and the actual field at 187.3° is 3750mV/m (mentally approximating/interpolating between the 185° 3559.01mV/m and 190° 3935.08mV/m). How would I calculate the power to produce that field?

I tried searching on google, but most of the results I found mention things about isotropic radiators, gain in dB, and that's not what I'm trying to calculate here.

 

[rfry]

Field intensity varies as the square root of radiated power. If the field at 1 km increases by a factor of 3750 / 2499.5 = 1.5, then radiated power has increased by 1.52 = 2.25 times, which value is 50 kW x 2.25 = 112.5 kW.

 

[pianoplayer88key]

Thanks.

Now is there a formula to calculate approximate field strength at a given distance, assuming constant known ground conductivity? Just eyeballing the charts doesn't quite cut it for me (and in some cases the numbers won't even be on the chart).
For example, taking that field of 3,750 mV/m at 1 km, how would I calculate the field at 15.05 km (my distance from KCBQ) assuming a conductivity of 8 (what the M3 map says for the path)? I want to guess it would be somewhere north of 200 mV/m, probably not exceeding 500 mV/m, but I'm not sure. (The indicated signal strength on my PL-606 is about 81 dBuV using its built-in 100mm ferrite loopstick, but you know how inaccurate that is, as you have a PL-310 that I've read in reviews is similar on AM.)

(I don't know if I've asked this before. I'd use search, but I have had problems searching for posts from specific people for some reason.)

 

[rfry]

There is no simple, accurate way of calculating such values, unfortunately.

Using the FCC groundwave propagation charts for that frequency and ground conductivity, that radiated power will produce a field intensity of about 124 mV/m at a distance of 15 km.

 

[pianoplayer88key]

Interesting... only 124 mV/m?
Based on how some of my radios behave with that signal (blocking/desense - the PL-606 and PL-380 show 45 dBuV with no signal audible on 1150 and 1190, or splatter - it's heard just as loud 30 kHz off channel on at least one or two radios I have), I would have expected it to be quite a bit stronger.    Basically it seems to overload my radios' front ends, which I had hoped should only happen with a much stronger signal, either starting with the blanketing contour or the maximum permissible exposure contour.  (But, then, chances are my definition of front-end overload may be inaccurate - I consider it to be any trace of impairment of reception of weak signals (that would be detectable if the local pest wasn't on the air) as close as the first-adjacent (including IBOC hash - if you can detect it, it's FE overloading), in a nutshell.)
As an example, reception of 1180 KERN is extremely difficult here (impossible using just the radio's built-in antenna except when KCBQ is off the air, suspected carrier barely detectable using the Select-A-Tenna).  I'm 237 miles, 144° from KERN (ERP 64.56kW toward me), but asking you to help calculate the received field strength would be beyond the scope of this post due to the distance and varying ground conductivity.  Estimating from the radio-locator coverage map, though, I'm probably about 1.85-2x past their estimated 150µV/m contour.
So the formula from which the FCC groundwave propagation charts were calculated is not known?

 

[rfry]

#1: I want to guess it would be somewhere north of 200 mV/m, probably not exceeding 500 mV/m, but I'm not sure. #2 Interesting... only 124 mV/m?

Probably your estimate in quote #1 above was based on a perfect ground plane. But the groundwave propagation loss over a path having a ground conductivity of 8 mS/m is greater than that for a perfect ground plane.

 

[pianoplayer88key]

#1:  I want to guess it would be somewhere north of 200 mV/m, probably not exceeding 500 mV/m, but I'm not sure.   #2  Interesting... only 124 mV/m?

Probably your estimate in quote #1 above was based on a perfect ground plane.  But the groundwave propagation loss over a path having a ground conductivity of 8 mS/m is greater than that for a perfect ground plane.

Actually it was more like a guess pulled out of thin air.  BTW V-Soft Zip Signal says for 92020 (I'm at the extreme southern tip) 1170 KCBQ is estimated at 235.64mV/m daytime, but it's probably sampling from a site a couple miles closer.

Until several years ago, KCBQ used to operate from a different site about 6 miles north of me with a slightly different 50kW pattern, and at that time had a powerful lobe aimed right at me (maybe 200-300kW but I'm not sure).  Is there any way to find historical station/pattern data so I could calculate what the signal would have been like then?  (Also being able to check historical V-soft or radio-locator sites, if possible, would be interesting, but I haven't had much success with that on the Internet Archive website.)

 

[rfry]

BTW V-Soft Zip Signal says for 92020 (I'm at the extreme southern tip) 1170 KCBQ is estimated at 235.64mV/m daytime, but it's probably sampling from a site a couple miles closer.

Using the 3750 mV/m field at 1 km (from your earlier post), that field at 15 km over a perfect ground plane would be 3750 / 15 = 250 mV/m.

But the FCC groundwave propagation chart for 1170 kHz shows that the field at 15 km for a conductivity of 8 mS/m is about 49.4% of the inverse distance field. Doing the math: 250 x 0.494 = 123.5 mV/m.

The V-Soft number probably is based on a location in that ZIP code that is closer to the tx site, and more in the main lobe of the radiation pattern.

Until several years ago, KCBQ used to operate from a different site about 6 miles north of me with a slightly different 50kW pattern, and at that time had a powerful lobe aimed right at me (maybe 200-300kW but I'm not sure). Is there any way to find historical station/pattern data so I could calculate what the signal would have been like then? (Also being able to check historical V-soft or radio-locator sites, if possible, would be interesting, but I haven't had much success with that on the Internet Archive website.)

The data for their 50 kW daytime pattern of Sept 2005 from a site at 32 50.367 N 116 59.517 W shows a tri-lobe radiation pattern with maximum ERP at 154 degrees. Another lobe was centered on 226 degrees towards San Diego, and there was a fairly deep null between them.

If you can post the distance and bearing from the old KCBQ site to your location then an estimate of their field could be made there.

 

[pianoplayer88key]

The V-Soft number probably is based on a location in that ZIP code that is closer to the tx site, and more in the main lobe of the radiation pattern.

Probably so.  Searching 92020 on Google Maps points to a location maybe 1.5-2 mi NNW of me.  KCBQ is 9.3mi N of me, and their main lobe is focused approximately southwest.

The data for their 50 kW daytime pattern of Sept 2005 from a site at 32 50.367 N 116 59.517 W shows a tri-lobe radiation pattern with maximum ERP at 154 degrees.  Another lobe was centered on 226 degrees towards San Diego, and there was a fairly deep null between them.

If you can post the distance and bearing from the old KCBQ site to your location then an estimate of their field could be made there.

Wow, no wonder it had such a huge blowtorch signal here back in the day!!  Distance and bearing from them to me was 9.818km (6.1mi), 153.62°.  So what would the day (and night - their drop was quite noticeable) field have been?

I suspect that even if I had a radio with good selectivity, midday reception of 1180 KERN (381.42km, 144.33° toward me) would have been impossible then... although if you know of an inexpensive portable (GE SuperRadio approx max size/price, maybe Sangean PR-D5(?) ok, prefer ultralight more like Sony SRF-59 but with digital tuner) that could do it without any splatter/blocking/desense from 1170 I wouldn't mind knowing which one(s).

Your mention of a tri-lobe pattern reminded me...  I remember I used to be able to hear them fairly well in the mountains north of San Bernardino near Crestline in the daytime when I'd go camping there, as well as at friends' houses near Riverside, Bloomington, etc.  Now it's a no-show, or at least very weak in Moreno Valley if there's any trace at all.
Is there any chance of some radio-locator style estimated coverage map existing for that previous pattern?

Oh... and is there any chance you may be using the 4nec2 software to calculate those fields, or do the part 15 research you've posted in other threads? I have downloaded it in the last few months, and was recently trying to figure out a few things (one being the behavior (field at specified distance, for example) of an electrically very short antenna with a short (or no) ground plane), but so far don't know much about how to use it yet.

 

[pianoplayer88key]

I ran out of edit time, but if possible I'm curious... what possible signal levels do you think your PL-310 might have displayed with those former fields from KCBQ? (My PL-606, which I've heard is comparable to the 310, displays a level of about 81/25 +/- a dBu on KCBQ's current day pattern.)

 

[rfry]

Distance and bearing from them to me was 9.818km (6.1mi), 153.62°. So what would the day (and night - their drop was quite noticeable) field have been?

Day = 315mV/m, night = 33 mV/m

Your mention of a tri-lobe pattern reminded me... I remember I used to be able to hear them fairly well in the mountains north of San Bernardino (etc)

The 3rd lobe of their daytime pattern was centered at 355 degrees.

Is there any chance of some radio-locator style estimated coverage map existing for that previous pattern?

You might check with the station engineer in case they have something from those days.

is there any chance you may be using the 4nec2 software?

I used it for a few "Part 15" calculations, but in this thread I used the FCC charts.

what possible signal levels do you think your PL-310 might have displayed with those former fields from KCBQ? (My PL-606, which I've heard is comparable to the 310, displays a level of about 81/25 +/- a dBu on KCBQ's current day pattern.)

My PL-310 read 89 "dBu" in a 684 mV/m field on 930 kHz. Of course these meters are not calibrated to show field intensity, but the voltage across a known resistance inside the receiver. What your receiver(s) would show in that or any other set of conditions could not be guessed, with any kind of accuracy.

 

[fybush]

Is there any chance of some radio-locator style estimated coverage map existing for that previous pattern?

You can see a pattern plot, at least. Search in FCCInfo.com for KCBQ and be sure you've checked the "Include Archive Records" box. The very last set of license data that comes up (BL-19850314AC) is the data from the old Santee site where the Lowe's is now.

 

[rfry]

The link below also shows the shape of the pattern, minus the field values.

http://i62.photobucket.com/albums/h85/rfry-100/KCBQin2005.gif

 

[pianoplayer88key]

KCBQ's previous pattern plots are interesting, and do reflect what I remember seeing from them. Is there any way I could see the fields at 1 km? I can't seem to find it on FCCinfo.com's links.

is there any chance you may be using the 4nec2 software?

I used it for a few "Part 15" calculations, but in this thread I used the FCC charts.

Ah. I've been wanting to experiment a little with the software, but am still trying to learn some things. You wouldn't by any chance have any .nec files available from your part 15 experiments that I could download? (One of the types of things I want to experiment with is electrically short antennas and short or no ground systems, which would be off the scale for the FCC's figure 8 calculator. One example would be portable operation under 15.225 (15,848µV/m @ 30m, 13.553-13.567 MHz). Fitting the transmitter, antenna and power supply all in a case the size of an iPod Shuffle (29 x 31.6 x 8.7 mm) would probably be asking just a bit much of miniaturization, though although it'd be interesting to figure out what power would be needed with an antenna+ground that small. I suspect it probably wouldn't be legal to calculate the necessary power assuming a ground conductivity like on Long Island, then use it near salt water at the same power level? )

what possible signal levels do you think your PL-310 might have displayed with those former fields from KCBQ? (My PL-606, which I've heard is comparable to the 310, displays a level of about 81/25 +/- a dBu on KCBQ's current day pattern.)

My PL-310 read 89 "dBu" in a 684 mV/m field on 930 kHz. Of course these meters are not calibrated to show field intensity, but the voltage across a known resistance inside the receiver. What your receiver(s) would show in that or any other set of conditions could not be guessed, with any kind of accuracy.

Yeah, I know its meter isn't accurate. In fact, I think once it gets into the 80s dBu range, it starts compressing (i.e. reading lower than it should). Then, once it gets to 90 dBu (if it doesn't snap down, for example briefly displaying 94 when first tuning in the station then 88 a second later), it seems to be more linear, until it pegs the meter at 98 dBu.
Also at the location I suspect V-Soft sampled KCBQ for 92020, it was reading 86 dBu. It was 32°47'34"N 116°57'59"W, about 12km (198.68°) from them - 200° field @ 1km is 4502.99mV/m (RMS 2499.5mV/m).

I recently took sample "readings" from a few places closer to some stations...

Near 590 KTIE (34°4'20"N 117°17'52"W) mid morning:
34°4'18.98"N 117°17'58.17"W - 160.868 meters, 258.714° - 90 dBu
34°4'18.96"N 117°17'56.87"W - 128.437 meters, 255.81° - 95 dBu
34°4'19.01°N 117°17'54.6"W - 73.092 meters, 245.265° - 98 dBu
On that last one, I was actually between two of the towers. Distance and bearing from the nearest tower was 34.968 meters, 153.2488°. I don't recall the exact signal reading on the 2nd harmonic, but I guess it was somewhere in the upper 50s dBu or so.
I also got 98 dBu (on the fundamental) at 34°4'20.03°N 117°17'55.07"W - 3.6 meters, 84.694° from under the center of the tower base. (I estimate the tower to be about 4.658 meters wide at the base.) Second harmonic was reading 81 dBu, fourth (the PL-606 wouldn't tune the 3rd) was 91 dBu.

Near 760 KFMB (32°50'33"N 117°1'30"W) right after sunset:
32°50'34.19"N 117°1'25.29"W - 127.55 meters, 73.26° - 89 or 90 dBu
I was basically east of the site, and closer to the east tower (32°50'35.5"N 117°1'28.89"W) than the distance between the east and west towers (there are 3 in line).

Near 1290 KKDD (34°7'20"N 117°14'14"W), mid afternoon:
34°7'16.31"N 117°14'32.89"W - 584.7 meters, 235.645° - 85 dBu
34°7'19.93"N 117°14'36.68"W - 619.268 meters, 249.37° - 83 dBu
34°7'25.38"N 117°14'36.73"W - 582.96 meters, 265.081° - 84 dBu
34°7'25.56"N 117°14'31.51"W - 449.63 meters, 264.328° - 86 dBu
34°7'25.40"N 117°14'28.03"W - 361.89 meters, 262.1574° - 87 dBu (also saw it display 88 as well as 85)
34°7'28.39"N 117°14'26.62"W - 325.316 meters, 277.58° - 89 dBu
34°7'28.36"N 117°14'25.06"W - 382.7 meters, 312.4° - 87 dBu
34°7'28.38"N 117°14'24.54"W - 373.4252 meters, 313.844° - 88 dBu
34°7'28.38"N 117°14'20.82"W - 311.896 meters, 326.03° - 92 dBu
34°7'28.30"N 117°14'19.90"W - 297.2656 meters, 329.5254° - 94 dBu
34°7'26.7"N 117°14'20.28"W - 261.767 meters, 322.19° - 97 dBu
On the last one, I was actually more like 31.9634 meters, 304.117° from the nearest (southwest) tower.

I don't expect estimated field readings on all (or even most) of those. If possible, though, I wouldn't mind an estimate of KCBQ's field at the distance given above (12km, 200°), KTIE 3.6 meters from the center tower, KFMB in the example given above, and KTDD 32 meters from the southwest tower. For the KTIE and KKDD examples, M3 conductivity looks to be on/near a border of 8 and 4.
When I'm checking fields as close as in the above examples, is there a distance within which I should no longer take ground conductivity or directional pattern into account, and just calculate inverse distance field - for example between the towers? And in very close cases when you're closer to the tower's edge than the width of the tower (for a self-supporting (non-guyed) tower), do you measure from under the center of the tower, or the nearest part of the structure?
Also speaking of KFMB, what do you estimate their night field at 11.73 km, 140.81° may be? (At 140°, it's 1813.92 mV/m @ 1 km, path along the conductivity is 8 according to M3.) I'm guessing something like 121.5 mV/m - how close would you say my guess is?

Also, if you're off frequency from a strong signal trying to read a weak station, I've noticed it will also read weird numbers, as well. I tested this using a small oscillator some weeks back.
For one test, I tuned to a station that was easy armchair copy (but with some noise) and reading about 30 dBu or so. I set up the oscillator to read 98 dBu about 20 or 30 kHz down, then upon returning to the weak station, its signal was completely undetectable and the radio was reading 50 dBu.
I even tested it with a somewhat stronger station - this one reading somewhere around 70 dBu, a noise-free local-level signal. Set the oscillator for 98 dBu 20 or 30 kHz down, returned to the station... I could tell something was there, but it was near threshold level and, again, was reading 50 dBu.

 

[rfry]

KCBQ's previous pattern plots are interesting, and do reflect what I remember seeing from them. Is there any way I could see the fields at 1 km?

The peak field at 1 km in the 154-degree lobe is about 5000 mV/m. The field at other bearings can be directly scaled from that, i.e., if the pattern value is 50% of the peak of the lobe at 154 degrees then the 1 km field at that bearing is 2500 mV/m.

You wouldn't by any chance have any .nec files available from your part 15 experiments that I could download?

Suggest you start by using one of the models included in the program, such as HFVertical > Gwave.nec, and edit that. There is a learning curve to this, and it would be better if you went through that yourself. It does take an understanding of antenna theory and practice, and “The Rules of NEC” to develop reliable NEC models.

I recently took sample "readings" from a few places closer to some stations...

Some general comments: The “dBu” readings made on the receivers you are using have little value as far as determining the true field intensity – especially in the very high fields existing close to the transmit antenna/array where you are trying to measure them.

Secondly, even with accurately calibrated, professional equipment, the measurement must be taken in the “far field,” which even for a single MW broadcast tower does not form until several wavelengths away from it. For directional arrays that distance is much greater, depending on the number of towers and their physical spacing.

As far as estimating the other (far) fields for KCBQ and KFMB, suggest you re-read the earlier posts in this thread showing how that may be done, and make those calculations following those examples.

 

[pianoplayer88key]

The peak field at 1 km in the 154-degree lobe is about 5000 mV/m. The field at other bearings can be directly scaled from that, i.e., if the pattern value is 50% of the peak of the lobe at 154 degrees then the 1 km field at that bearing is 2500 mV/m.

Ah ok so it's a linear (not logarithmic) scale. I figured that anyway, although if there's a place that might have the field measurements you get when you click "pattern data" from an FCC detailed AM query link, that would be nice.

Suggest you start by using one of the models included in the program, such as HFVertical > Gwave.nec, and edit that. There is a learning curve to this, and it would be better if you went through that yourself. It does take an understanding of antenna theory and practice, and “The Rules of NEC” to develop reliable NEC models.

I've been experimenting with a few of them - so that Gwave.nec is a good one? Any others you suggest experimenting with? Also are the "Rules of NEC" available in the program, or online somewhere?

Some general comments: The “dBu” readings made on the receivers you are using have little value as far as determining the true field intensity – especially in the very high fields existing close to the transmit antenna/array where you are trying to measure them.

I figured they weren't very accurate/linear in the far field. When I get 81 dBu 9 miles from a station, then only 89 dBu 1/4 mile from a station, something's not accurate there, as I would have expected the actual field to go up by a LOT more than 8 dB.

Secondly, even with accurately calibrated, professional equipment, the measurement must be taken in the “far field,” which even for a single MW broadcast tower does not form until several wavelengths away from it. For directional arrays that distance is much greater, depending on the number of towers and their physical spacing.

So the far field estimations (taking into account ground conductivity, pattern, etc) are done several wavelengths or more out. Some of the values I would like to calculate/estimate are in the near field --- maybe the VERY near field. Would I stop using ground conductivity (and begin just using the inverse distance field calculations) when I'm within several wavelengths, or where? Also would I stop taking directional pattern into account once I'm closer to the nearest tower than the spacing between the farthest spaced towers? And, for very near field calculations (obviously I can't trust a radio whose signal strength meter is being pegged as in that photo), when estimating the distance to the tower (for the IDF calculation) do you measure from the nearest edge of the structure, or from under the center? (KTIE uses self-supporting towers, and I was actually closer than the width of the tower.)

I noticed my previous post has been playing hide and seek (I'll come refresh the page and it's gone, come back a few hours later and it's back, now it's gone again.) Also yours seems to have gotten duplicated with the same timestamp as the earlier one. The duplicate wasn't there when I started composing this reply, though - I got the notification of a new reply when I hit the preview button. Maybe something's up with the board software? (Upon posting this, my post returned and your duplicate is gone.)

 

[rfry]

Also are the "Rules of NEC" available in the program, or online somewhere?

The Help screens available through the program are a good start. The best source probably is this: http://www.nec2.org/other/nec2prt3.pdf .

So the far field estimations (taking into account ground conductivity, pattern, etc) are done several wavelengths or more out. Some of the values I would like to calculate/estimate are in the near field --- maybe the VERY near field.

Again, there is little point in using the receivers you have to try to measure near-field intensities, or far-field intensities for that matter. Little sense can be made of those meter readings as far as determining the true intensity of an electromagnetic field.

 

[pianoplayer88key]

Thanks for the PDF link.

While I may note readings just for fun in the near field, I don't consider them nearly as accurate as I consider things in the Bible.
More often, I'm checking the radio's selectivity (and sometimes directivity) near the station. My grandma (who I haven't visited in several months) lives about 0.6 km SE of 23kW 1300-KAZN and 50kW 1430-KMRB. My radios have difficulty hearing several locals from 1230 to 1650. (there's even a 50kW 1580 whose reception is noticeably degraded!) So far I have yet to be able to get a clean signal (or any, for that matter) from 1290 KKDD San Bernardino, CA (looking at radio-locator's map, she's probably near their 0.25mV/m field or so). (Using SSB wouldn't be an option as there's a semi-local KFRN (Family Radio) on 1280 in Long Beach, which itself is massacred by 1300.) Sure the program is available locally on 1110 KDIS, but the local restricts their analog audio to 5 kHz (whereas I've heard (and recorded & looked at the data near their local area) KKDD up to at least 8kHz.

I was just wondering how to estimate/calculate the inverse distance field - not from the reading on the radio, but from the FCC data and my distance to the station's towers. (Sometimes I might want to measure from between towers, and in a couple cases I've been closer to a self-supporting tower than its width at the base.) I'm assuming for the very near fields I would just calculate based on the RMS IDF from the nearest tower? Also do I measure from the nearest part of the tower structure (using a ruler tool on Google Maps overhead view) or under the center?

 

[rfry]

Sometimes I might want to measure from between towers, and in a couple cases I've been closer to a self-supporting tower than its width at the base.

You might want to read FCC Bulletin 65 and its supplements available at http://transition.fcc.gov/oet/info/documents/bulletins/ .

The radiation close to AM broadcast towers can be a health hazard.

With this post I will finish my comments in this thread.

 

[Nostalgia]

While I may note readings just for fun in the near field, ....
More often, I'm checking the radio's selectivity (and sometimes directivity) near the station... My radios have difficulty hearing several locals from 1230 to 1650....
I'm assuming for the very near fields I would just calculate based on the RMS IDF from the nearest tower? Also do I measure from the nearest part of the tower structure (using a ruler tool on Google Maps overhead view) or under the center?

My God man, time to get another hobby! I'm with Mr Fry on this one, read Bulletin 65 and stay away from the bases of hot towers!

Perhaps the reason your radios are having difficulty hearing some stations is that your have taken them right up to the base of one of these towers while it was on the air and fried the front end (and a couple of strands of DNA) in the process. Just a guess/observation from far away. 8)