Monday, February 11, 2013

Field Strength Calculator One: Update 1.0050

I tweaked the Field Strength Calculator One program a little and added some new calculations figures which may be helpful for mediumwave DXers. Added are expected distance coverage for Local (2.5 mV/m), Distant (0.5 mV/m), Fringe (0.15 mV/m), and Extreme Distance (0.05 mV/m) signal levels. Also added is a Custom mV/m level which you can enter and the program will calculate the expected distance.

For those new to Field Strength Calculator One, the program returns expected received field strength in millivolts per meter and dBu (also known as dBµV/m), based on ground conductivity, earth dielectric and several other input constants. It also displays the distance to the radio horizon and the signal path loss in dB, along with several more technical parameters. The resulting output of Field Strength Calculator One should be accurate in most cases to a couple of percent in the longwave and mediumwave bands. It compares favorably to ITU program GRWAVE and currently available FCC Ground Wave Conductivity graphs.

For further information on how field strength is calculated see the Field Strength Calculations Series previously published on RADIO-TIMETRAVELLER.


To download, see the link at the top of the right sidebar under LATEST PROGRAMS. The sidebar at the top right will have the most current link in case the program is updated. The link will change in the case of an update, so I would avoid copying and pasting it into a forum or other web page. Come to the main page of this blog instead.


Install is simple. Download the .zip file and unzip. Click on the FieldStrengthCalculatorOne.exe file to run. This program makes no registry changes and saves no data to your hard drive. It has been developed and tested in Windows 7. It should work fine in Windows Vista and XP environments, and Windows 8. It is written in the old standby Visual Basic 6.

Included in the .zip is a readme.txt file. Be sure to have a look.

Click image to enlarge.


Stephen said...

Hi Bill,

I see you were able to implement the field at specified distance feature. Nice touch to include the Radio-Locator-style "standard" fields, along with the "extreme" (0.05 mV/m) level. :)
Would it be possible to have it specify the distance in feet or meters when the distance is less than 1 mile or kilometer, respectively?

Also in case you want ideas for other fields to have it list the distance to (although what you have is good too), here's a few ideas:
0.005 mV/m (maximum overlapping signal of a Class A station's 0.1 mV/m)
0.025 mV/m (maximum overlapping signal of other stations' 0.5 mV/m)
0.1 mV/m (class A protected groundwave contour)
0.25 mV/m (maximum overlapping of a first-adjacent's 0.5 mV/m)
5 mV/m (maximum overlapping 2nd-adjacent contours)
10 mV/m (or 15 mV/m) - minimum signal that David Eduardo (David Gleason?) on the radiodiscussions boards says that general public (non-DXers) will listen to in a major city's metro area (LA, NYC, etc)
25 mV/m (maximum overlapping 3rd-adjacent contours)
1000 mV/m (blanketing contour)

So what's the progress toward being able to find the field strength over a path with multiple conductivities? How tough would it be (once you figure it out) to include the M3 map data & FCC database (except that it'd make for a huge download), and have the user specify the station and receive coordinates?
As an alternative to including the FCC database, maybe the user could specify the transmit coordinates and field at 1 km, and get the sought-after result. And/or, what about pulling the data (for M3 & FCC Data) from an online source and not requiring it in the download - would that be possible?

In the meantime, it'd be nice to at least have something like a custom path feature. You'd specify the conductivity and distance for each segment of the path, and be able to add and subtract segments as necessary.
For now, I'd be fine with doing it one at a time (if that'd be easier to get the next update out quicker ;) ) - specify mV/m @ 1 km @ conductivity to first distance, then specify mV/m @ x km @ y conductivity for 2nd, and so on. :)

Also any suggestions on how to do close-in measurements? I've read posts with Rich Fry (on various forums, including radiodiscussions) saying that when you're within something like 10 times the distance between the farthest towers apart in an array, the traditional ground conductivity and pattern plots do not apply.

A couple examples I'd like to be able to figure out are:

1170 KCBQ - &

590 KTIE - (that's the center tower of a 3-tower array - not sure from the FCC online query how I'd find the mV/m from a specific tower in an array)

73, Stephen

Oh and another thing -- in a recent Tower Oddities section (the one where you mentioned KFBK's and KSTP's franklins), you mention KFMB's 3545.89 mV/m where I think you meant KFBK? :)


Hi Stephen,

Thanks for the detailed reply.

I did include a "custom" mV/m level in the calculator for entering an extra level which one might want to test. There was no room to add other radio buttons on the small window for other fixed values. Extra levels could be added in a dropdown box I suppose, but I think it further complicates what I wanted to be a simple field strength program.

My Radio Database MW program (another separate program of mine which analyzes the FCC database) does a lot of what you are suggesting already. The exception is that I haven't incorporated multiple ground conductivities yet. I will try to tackle that project over this summer when I get back home to NY. The M3 data (available in the FCC's M3.seq file) would eventually have to be part of that download if that program ever gets to a distributable state.

Attempting to calculate field strength in mV/m at distances less than about one kilometer is not advisable. Unfortunately, theoretical radiation pattern mV/m calculations (the FCC formula in Part 73.150) are not accurate at distances much less than one kilometer. And the Norton calculation for field strength requires this value. Accurate results at distances less than one kilometer would not be possible with these formulas. These types of formulas for field strength are based on "far-field" wave interference, i.e., distances greater than several wavelengths (call it one kilometer or greater). Ideally, one should use some sort of field strength measuring device for closer distances.

The effect of ground conductivity at distances much less than 1 kilometer is negligible. At these close distances, the well-known inverse-square law is supposedly fairly accurate - at least for predicting field strength for a point source or perhaps a single tower. Multiple towers are a different and much more complicated story.

Thanks for the tip on the typo in that article. I have corrected the text.

Good DXing,


Stephen said...

Hi Bill,

Yes, I see the "custom" level, and often make use of it. :) Having only one does mean I have to do multiple calculations for various levels, but at least it can be done. (How tough would it be to have a spot to enter a few custom levels?)

Don't you mean "WHEN" the Radio Database MW program gets to a distributable state? ;) Any idea when I might be able to download it and try it out? Possibly this summer, perhaps?

As for multiple conductivities, without having to include & calculate the M3 data, would it be possible (with the Field Strength Calculator program) to either...
A: have a dropdown with which the user can specify multiple lengths and conductivities (for example 0.8 km at 4 mS/m, then 288 km at 5000 mS/m, then 18 km at 15 mS/m, then 7 km at 8 mS/m), OR...
B: have the user be able to specify the "reference" X mV/m @ Y km (or mi) and the conductivity - for example 480 µV/m at 280 km at 15 mS/m?

Ok so attempting to calculate those near-fields might not work in the current version of the program ... but is there either a way a future program could handle this, OR some other way to calculate it? I've been wanting to calculate the fields my radio is sometimes subject to when they encounter overload-level signal conditions. :)

One example would be what field would my radio be subject to if I took it to a spot where, using only the built-in ferrite antenna (or better yet no antenna at all), it was overloading on 1170-KCBQ as severely as the maximum overload in this video near KCBQ's site ;) (I was right smack in KCBQ's 200 kW main lobe, using the 4 towers that I was lined up with on the utility pole. The nearest tower is just a little over 300 feet from the pole.)

To give some idea of how much gain the Select-A-Tenna + utility ground wire (on the pole) gives ...

This video of 1550-XEBG in front of my house goes from about 29-30 dBµ barefoot on 1550-XEBG to about 86-87 dBµ with the combination SAT+Util. Recently, using the Tecsun PL-398mp, I've seen it once or twice briefly flash 98 dBµ when doing that on XEBG. (This happens if, after nulling the station, I quickly turn the radio to peak signal, and the display happens to update before the radio's front end has time to desense with the strong signal. This requires extremely precise timing and has only happened once or twice in several hundred tries so far, and have been unable to record a satisfactory video as of yet.)

This is 1170-KCBQ 0.1 mi N of my house, barefoot then using SAT+Util. ERP toward me is about 112 kW, conductivity 8 mS/m along the entire path per the M3 map, resulting in an approximate received field of around 120-130 or so mV/m. You may note that the signal from KCBQ using the SAT+Util in this video is STRONGER than the barefoot signal on the road right by the transmitter site in the previous video! :)
Also comparing 2nd harmonics to a few feet from 590 KTIE's center tower (98 dBµ on 2*1170 vs 81 dBµ on 2*590) and sound (no recording of KTIE, but 590 has a bit of modulation blips trying to break through similar to 760 KFMB's 50kW after pattern change 7.3 mi NW of my house, whereas 1170 is totally blocking / silent / unmodulated carrier effect / whatever you want to call it in the video recorded up the street from my house.)

73, Stephen


Hi Stephen,

Three of the mV/m levels you mentioned I think would be a good addition to the program so I increased the height of the window some and added them (the 10 KHz, 20 KHz, and 30 KHz adjacent channel interference contour levels). I am basically out of window height at this point as I don't want to go much past 700 pixels.

For the time being anyway, multiple ground conductivity options belong in my other program, Radio Data MW, as it has mapping abilities and the ability to drag a cursor over the map and directly read out signal levels, and a few other options which make this a more natural place to implement multiple ground conductivity.

Wow, if you are standing only 300 feet from a broadcast tower with a Select-A-Tenna hooked to a ground wire for extra signal pickup, no wonder your radio is overloaded. Not even a military grade $5000 radio could stand up to those conditions. You are suffering overload and desense, and all kinds of intermodulation products making your dBu readings inaccurate and irrelevant.

At 300 feet distance you are not only in the extreme near-field, but more in OSHA human exposure territory. OSHA has an interesting web page with a very good explanation of near field:

Check the Near Field paragraphs and you will see an explanation of why measuring (let alone calculating) accurate mV/m levels in the near field is next to impossible due to all the factors involved (magnetic levels enter into it, as well as multiple types of polarization). I have never seen a comprehensive formula for it. In any event, I'm sure the math would be horrendous.

Thanks for your input. Will post an update to Field Strength Calculator One soon.