## Saturday, June 25, 2011

### Field Strength Calculations: Calculating

As stated in the previous post, one more piece of information is required to complete the puzzle of calculating received field strength. That is the millivolt per meter level at 1 kilometer from the station transmitter, eminating in your direction. Notice I said "eminating in your direction". It is not good enough to simply calculate the mV/m level at 1 kilometer for the station's overall power output. Two things must be accounted for that change that result and would make our field strength calculation inaccurate. They are:

1. Antenna efficiency. A mediumwave tower or array of towers will be more or less efficient depending on their radiation length(s). The FCC provides us with a figure called RMS Theoretical for every station's antenna array whether it be one tower or several, measured in mV/m. Reflected in this figure is the efficiency.

2. Pattern gain. Multiple tower arrays inherently have broadcast patterns. Meaning, of course, they aim to broadcast a majority of their signal in a certain direction to cover their market audience and/or avoid co-channel interference with another station. Where you are in relation to that pattern is important. If you are in the major lobe of a 50KW station, it may be pumping upwards of 100KW towards you, or more. If you are in a sharp pattern null, it may only be beaming hundreds of watts towards you. The amount of millivolts per meter "facing you" is the important figure. The FCC provides that as well, in their pattern plots.

There is only one case where we will need to do a simple extra calculation to arrive at the full millivolt per meter level for a station. That is for stations with a single tower only. I will explain why in just a minute.

So let's put together what we need. First, pick a station within reasonable distance you think you'd like to log. Note its frequency. Next, gather the following four things:

1. The ground conductivity in mS/m between you and the station. Use the M3 conductivity map. If the station path crosses a couple of zones, estimate the average ground conductivity for the entire path. The resultant figure should fall between 0.1 mS/m and 30 mS/m, or possibly higher if part of the path is over salt seawater.

2. The Ground Wave Field Strength Versus Distance graph for the frequency of the station, one of the 20 graphs published by the FCC. Several frequencies are usually grouped into one graph. The graphs are in .PDF form. Have your .PDF viewer ready.

3. You will need to find your distance to the station in kilometers, and also the reverse bearing from the station back to you. Many calculators exist on the web which will compute this information. The FCC has a good one, be sure to check out their calculator. These calculators require you to know the latitude and longitude of both your location and the station's location.

www.wikimapia.org is a great way to determine your home latitude and longitude as it has a crosshair defining the center of the map, and thus the latitude and longitude. Move the map to your exact location and read the latitude-longitude in the web browser's address bar.

The station's latitude and longitude can be found in a couple of ways. The FCC's AM Query web page allows us to query the station by call sign. The search output will display basic information like latitude and longitude. Click on the call sign link and you will be taken to the FCC's web page for the facility (station). Example: WHAM-1180 page.

4. The last item. Get the millivolt per meter value at 1 kilometer from the station transmitter, headed your way. The method of locating this figure will depend on whether the station has one antenna tower or multiple towers in its array.

Determine if the station uses a single tower or multiple towers for the service you are interested in. This information can be found on the FCC's web page for the facility (station), as shown just above.

Note that stations may have more than one entry on the page, one for each service they operate under, i.e., UNLIMITED, DAYTIME, NIGHTTIME, CRITICAL HOURS. Be sure you are looking at the correct service. Sometimes stations use a different number of towers for day and night.

mV/m for multiple towers.

The mV/m figure is gotten from the pattern data. It's simple.

Multiple tower arrays will give you the option to display the pattern plot. The pattern plot link will be under a heading that looks like this:

Horizontal Pattern at 1 km radius (Sections 73.150 and 73.152):
Electric Field Strength pattern plot
Pattern Data for WXXI

Either link will give us the information we need, though the "Electric Field Strength pattern plot" gives a nice graphic pattern plot for the station. Click one of the links.

RMS Theoretical values, and in some cases RMS Standard or even RMS Augmented values will be displayed for each five degrees of compass, 0-360. Find the compass bearing that most closely matches the return bearing from the station to you. We need to record one value only. Preferably, record the RMS Augmented value, if given. If not available, record the RMS Standard value. If not given, record the RMS Theoretical value. These values are in millivolts per meter and are the value of signal level the station presents towards you.

A quick definition of RMS Standard and RMS Augmented values. RMS Standard is essentially the RMS Theoretical value plus 5%. It is a "guard" against interference to other co-channel stations by overstating the RMS Theoretical calculated value. If stations have pattern augmentations, and many do, the RMS Augmented field will be present. Augmentations are enhancements or detractions to the theoretical pattern.

I'll use WXXI-1370 for the example. At night it runs 4 towers and has a roughly figure-8 pattern north-south. WXXI's return bearing to me is 204.7 degrees. Checking the FCC pattern plot for the station, we see that the 205 degree return azimuth presents a facing RMS Theoretical of 350.59 mV/m and a facing RMS Standard of 368.88 mV/m. We will use the RMS Standard value in the final calculation.

mV/m for single tower.

A special case requiring a simple calculation. We will calculate the mV/m figure from the RMS Theoretical value.

Again go to the FCC's web page for the facility (station), as above. The RMS Theoretical value will be on this page.

Note again that stations may have more than one entry on the page, one for each service they operate under, i.e., UNLIMITED, DAYTIME, NIGHTTIME, CRITICAL HOURS. Be sure you are looking at the correct service. Sometimes stations use a different number of towers for day and night.

The FCC computes all RMS Theoretical values from a formula of course. The values are calculated for a distance of 1 kilometer. The FCC formula used generates accurate millivolt per meter values (as published) for multiple tower arrays. Single tower arrays are a special case, however, in that the published mV/m value is always based on a 1 kilowatt output power calculation. Hence, the only published single tower mV/m values we can use are those of 1 KW stations. For all others, we will do a simple calculation to arrive at the correct mV/m value. Proof of this is simple. For example, check the FCC's published figures for my local WHAM-1180 station out of Rochester, NY. This 50KW station shows a calculated RMS Theoretical value at 1 kilometer of only 376.59 mV/m. Now of course this cannot be correct for a 50KW station, as a 1 KW station running a quarter wave (.250 wavelength) monopole has an exact calculated figure of 305.768 mV/m at 1 kilometer.

376.59 mV/m would, however, be correct for a 1 KW station using the same single tower antenna that WHAM uses (a .492 wavelength antenna).

To accurately calculate the mV/m figure for WHAM (or any other single tower station, including those 1 KW stations), the following formula must be applied:
(Power in KW, distance in KM):

mV/m = RMSTheoretical x SQRoot(Power/Distance)

Thus in WHAM's case:

2662.89 = 376.59 * SQRoot(50/1)
WHAM's actual RMS Theoretical value is 2662.89 mV/m. And since it is a single tower antenna having an omnidirectional pattern, it presents this value of signal in all directions. Use the value you calculate for your single tower station of interest as the mV/m value that the station presents towards you.

Making The Calculation

Now we have all of our information. Let's get busy. We will use the FCC's Ground Wave Field Strength Versus Distance graph to arrive at the received mV/m signal level. Proceed with the following steps.

1. To make them universal, the FCC's Ground Wave Field Strength Versus Distance graphs are based on 100 mV/m levels at 1 kilometer. We simply need to calculate how many 100s our mV/m value is. Just move the decimal point left two places. In WHAM's case, 2662.89 mV/m, 26.6289 (26.6289 x 100 = 2662.89). The multiplier value we will use for WHAM is 26.6289. In WXXI's case, 368.88 mV/m, 3.6888 (3.6888 x 100 = 368.88). The multiplier value we will use for WXXI is 3.6888.

2. Find the station distance in kilometers on the graph, usually at the bottom. The bottom range is 10 to 1000 kilometers. The top range is 0.1 to 50 kilometers.

3. Draw a trace upwards (or downwards if using the top scale) until you hit the ground conductivity value curve that matches the average ground conductivity between you and the station.

4. From the previous point, draw a trace leftward to the scale on the left side of the graph. This is the base millivolt per meter level based on 100 mV/m at 1 kilometer. Multiply this value by your multiplier value. In WHAM's case, multiply times 26.6289. In WXXI's case, multiply times 3.6888. This resultant value is the received field strength in millivolts per meter at your location.

There you have it. You have ballparked the approximate field strength of your station of interest. If done correctly, you should find this in fairly good agreement with V-Soft's figure if you are near the zipcode point they based their calculation on. With a list of expected receive field strengths for various stations, you can judge the approximate sensitivity of your receiver. After a few times trying this, you will find the calculation to be rather simple to do.

Hope you have enjoyed this series.

If you are interested in the history of field strength calculations, be sure to see Field Strength Calculations: A History on RADIO-TIMETRAVELLER.

Stephen said...

Hi Bill...

I enjoyed reading the series, as always. :)
There's another antenna that's not featured in the list of antenna sizes you mention that's more efficient from any of those. I'll let the FCC query results for KSTP and KFBK do the explaining. ;) Anyway... would it be reasonably possible to calculate fields from antennas like those?
Also at the other end of the spectrum, with the "electrically short antenna", would that by any chance be useful for part 15 radiation? Also how much of an effect would a ground radial system have, especially if you eliminate it entirely? For example, I've wanted to calculate the power needed to be legal in the unlicensed band around 13.56 MHz, with the entire transmitter + antenna + ground being contained within a battery-powered device approximately the volume of the Sangean DT-400W due to space considerations. (Sometime I'm wanting to experiment with that frequency range.)

As for ground conductivity, what do you do when there are wildly different conductivities along the path?
On 1290, I get much better reception of 500-watt KZSB Santa Barbara, about 195 miles away, than I do of 5kW (slight lobe toward me at ~ 32°45'38.6"N 116°56'44.8"W but I don't remember the exact field in my direction) KKDD San Bernardino, CA, only about 95 miles away. KKDD can be faintly heard on the barefoot PL-380, and is probably slightly better on the PL-606 and the SRF-59. Earlier this afternoon, once I tilt-nulled some strong locals to reduce the desense then repositioned the radio, I was seeing a reading on 1290 KZSB of about 22/10 or so using the PL-606's stock loopstick. (The strongest daytime local is 1170 KCBQ, which checks in at about 81/25. KFMB hits 82/25 at night using only the built-in loopstick.)

Within the past few weeks I was in Cameron Corners at approximately 32°37'47.7"N 116°28'22.6"W. Among other things, I noticed that even that far inland, KZSB's signal was still slightly better than KKDD's signal, although they were closer in strength than at home. That saltwater really has an effect on signals. :)
I've heard the conductivity in some places back east is quite poor. Is it possible that a 250-watt station on 1600 kHz with a 1/4-wave antenna over saltwater could beat a theoretical 50kW on 540kHz with a 1/2-wave on Long Island or northern New England?

Also another thing that would help me with the calculations, knowing what to expect, etc.... you wouldn't by any chance know of some resource that gives typical sensitivies of various types of radios & antennas (for example maybe a Drake with a tuned longwire beverage, a PL-600 with a 2-foot loop, a barefoot SRF-59, etc), as well as typical noise levels in various areas ranging from inside a factory near downtown LA/NY to thousands of miles from any electricity grid, lighting storm & aurora?

So far it seems my PL-606 is more sensitive than the V-Soft figures will allow me to calculate. At home I can get 960 KIXW weak (27/00 after nulling locals (including 910 KECR 67/25) to reduce desense, but it's there) even though I'm about 33% farther than Radio-Locator's 0.15mV/m contour. At Cameron Corners I'm able to get a fair signal on 1160 XEQIN in spite of being over 2x past their 0.15mV/m. I suspect, though, that Radio-Locator may be severely underestimating Baja California's ground conductivity - look at the day pattern for KBLU (which can be received at both locations I mentioned, although very faint at home - about 17/01). Also at CC I can get a fair signal from 1090 XEPRS in spite of being at least 3x past their 0.5mV/m. (I should mention that XEPRS uses their nighttime directional antenna 24/7.)

Hi Stephen,

Checked out KSTP-1500 and KFBK-1530. Calculations should follow the same procedure as for other stations. KSTP runs a single tower, effectively .498 wavelength tall. It is top loaded and sectionalized (type=3). KFBK is a two tower array, each tower being .500 wavelength tall. RMSTheoretical can be used in each calculation per the guidelines in the article. Neither station has augmentations.

I usually estimate the ground conductivity where it varies between transmitter location and receiver location. You must interpolate between the graph lines if the value is between those on the graphs. There is a numerical method to calculate the value, but it is complicated and I don't have it at hand. It is somewhere in the FCC documentation (Part 73), as I have seen it.

Both KZSB and KKDD should be fairly weak to you at your location. KKDD, miles inland, also seems to cross the 15/8 mS/m ground contour. KZSB, though much greater distance, is across a pure salt water path. The ocean is making the difference here.

Though I am a Ham operator for many years, I've not been active in many years and I don't recall much about Part 15 operation. I do recall that there are limits on antenna length (50 ft.?). Electrically short antennas can be effective if matched correctly. Build yourself a transmatch tuner and use it with an SWR meter. I have worked people across the world on CW using short lengths of wire. I even loaded up a 6 foot section of corner drywall bead (in place) and worked a guy in Russia once (from Denver), with no ground system at all. Of course this is on the Ham bands on CW and not mediumwave AM voice-modulated.

One could plot the signal strength of a 250 watt 1600 KHz station on a salt water path and a 50KW 540 KHz station over a lossy ground path. Don't know what the results would be. It's easily done with the FCC ground conductivity charts though. Found a link for these, by the way:

ftp://ftp.fcc.gov/pub/Bureaus/MB/Databases/AM_groundwave_graphs/

Get the .PDF version. They are also available in .JPG and other formats.

Sorry, I don't have a resource for typical sensitivities for known receivers. Noise levels are always relative to the location. I can walk ten feet from one location to another in my house and go from almost zero noise to a horrendous buzz that disrupts all signals. The house does have a general noise factor though. I find if I get 100 feet or greater away, this general noise factor basically disappears. Lightning storms produce intermittent static of course. Dust storms and dry wind storms can produce a constant noise (frying sound) level. They are dangerous to front ends of receivers as they are basically static electricity.

Ground conductivities are for sure relative and may vary more from place to place than the maps show.

Good DX,

Bill

Stephen said...

Actually, KFBK's antennas are each a full wavelength tall (two half waves stacked one on top of the other), and KSTP is slightly shorter. Scott Fybush has written a good article about KFBK, and has also mentioned KSTP in another article.
KSTP: http://www.fybush.com/sites/2009/site-091127.html (Look near the bottom of the page.)
KFBK: http://www.fybush.com/sites/2005/site-051028.html

As for calculating fields based on conductivity, is it reasonably possible to calculate the FS in stages while taking into account changes in conductivity? One way I just thought of is after finding the initial conversion factor, trace the first conductivity curve to the appropriate distance. Then, calculate another conversion factor for the next conductivity (estimated actual field calculated for that spot based on previous measurements, divided by field indicated on the new curve), then trace that to the next change or your destination, whichever is applicable, recalculating conversion factors as necessary. Would that work?

Regarding noise levels & sensitivities, I was mostly wondering about them in general terms. For example, a cheap pocket radio (like a Coby) might be approximately "A" uV/m at the noise floor, a better pocket radio (like a Sangean or well-designed Sony) might be "B", a larger portable (like one with an 8" loopstick) might be "C", some home stereos / higher end receivers may be around "D", hooking up / inductively coupling a 12 inch loop would make your radio "E", a 9-foot tuned loop maybe "F", a full wavelength beverage upwards of "G". As for noise levels, an urban area like L.A. or NYC might be "A" mV/m downtown, a suburban area 10-20 miles outside a place like San Diego might be "B" in summer and "C" in winter (not taking into account RFI generated inside a house, for example), a somewhat rural area might be "D" in summer and "E" in winter, a very rural area may be "F" in summer and "G" in winter, and a place a several thousand mile radius from any electrical grid or storm activity may be something like "H" nV/m. That's basically along the lines I was thinking. You wouldn't by any chance know of an online resource (preferably not on IEEE's site, as I don't have nor can I afford a subscription) that discusses/details that? Also doesn't latitude affect noise levels as well?

Also.... if you're in a very low noise area with an extremely sensitive receiver + antenna combination, how would you calculate an estimated received field strength if, for example, you're receiving a signal from a 2 megawatt station on 540 transmitting into a Franklin antenna across a saltwater path, and it's off the charts?

Hi Stephen,

Yes, I am familiar with the Scott Fybush site. A great site which I look at now and then. Unfortunately, the FCC's antenna engineering records for KFBK-1530, which I referred to when checking out their tower array, only refer to a single section "A" for each tower being .500 wavelength tall. They do not indicate a stacked configuration for some reason. That was the info I was relying on, supposedly "official". I guess not in this case.

Yes, I believe it is reasonable to calculate the FS in stages while taking into account changes in conductivity as you stated. That may be the way the FCC does it in their formula. Unfortunately, I can't remember where I saw it.

Sorry, I don't know of a source for general noise levels around the country or radio baseline noise levels. Cheap portables are not usually measured like the expensive tabletops are.

The calculation for a signal from a 2 megawatt station on 540 transmitting into a Franklin antenna across a saltwater path is done just like the others. You need the RMSTheoretical at 1km pointed at you. The level is calculated from the charts using this figure. The formula to calculate the actual RMSTheoretical value in mV/m is very complicated, and the math well beyond my ability. It is buried in the FCC's code library.

Bill

Stephen said...

Hi Bill...

I found it interesting that KSTP's antenna is actually slightly more efficient than KFBK, as configured. I took the field that KFBK puts out, converted it to 1 kW "3545.89/50^(1/2)" and got 501.464573 mV/m @ 1 km for 1 kW. (KSTP is 511.77 mV/m, so 50kW for them puts out 3.618.766037 mV/m @ 1 km.

The FCC has a figure 8 antenna efficiency calculator:
http://transition.fcc.gov/mb/audio/bickel/figure8.html
It does have limitations, one of which it won't calculate sectionalized antennas directly like ones with capacitance top hats or Franklins, for example. I have found a partial way around it, though. Look up the figure in the FCC database for a known Franklin, then with the help of the calculator you can come up with a conversion factor. For me, I find it easier (although it required trial & error) to come up with a field of half the value I want, then convert accordingly. For KSTP's 511.77 mV/m for 1 kW, for example, I put in an antenna length of 30° with a ground system of 120 90° radials, returning 255.886 mV/m. (That should be close enough, I think, to 255.885, half of 511.77.)

Now, about that 2-megawatt station.... I now remember two things that previously slipped my mind. One, I think I remember seeing a 2.5 MW transmitter mentioned somewhere. Two, would it be possible to feed each section of the Franklin with its own transmitter (2.5 MW), so it'd be like you're transmitting with 5 MW?
If I just put 5,000 kW into the calculator with that antenna config, though, it'll peg the calculator's 4-digit meter. Quartering the power halves the field, though, so I can get it in range by quartering the power to 1,250 kW, which gives a field of 9,046.926 mV/m @ 1 km. I double that field number twice (once because I quartered the power to get the result within display range, once because my 1 kw field was set up to be half the value I actually wanted) to get a field of 36,187.704 mV/m @ 1 km.
Applying the conversion ratio to the chart, it bottoms out at approximately 36 uV/m. I've found a few sources that lead me to believe noise levels in extremely quiet & rural areas may be down in the single digits or even fractional. Also another site mentioned that often skilled operators can dig out a signal 20dB below the noise.
So, is it possible to calculate a distance to the minimum discernable signal in such cases when they exceed the FCC's charts & calculators?
BTW that figure 8 calculator won't let me calculate extremely short antennas/grounds, like 0.0005 wavelength, nor does it work outside the AM band, so I still don't really have a way to calculate those. I do have a free version of 4NEC2 software but really have no idea how to properly use it.

Also should I look at some of the ITU documents to find out about noise levels, among other things?
http://www.itu.int/pub/R-REP/en

I don't remember if you've posted it somewhere else, but what do you think the field of your strongest daytime local, WYSL (IIRC) would be at your QTH? I've calculated my strongest, 1170-KCBQ, to be about 130 mV/m at my location. At night, 760-KFMB is about 135 mV/m. For one station I'd like to be able to listen to in the daytime (preferably armchair copy with near-NRSC audio and no adjacent splatter), 1180-KERN, I'm approximately twice as far as Radio-Locator's indicated 0.15 mV/m contour. (I haven't yet used the graphs & M3 maps to calculate their field.) I have heard them one time when KCBQ was off the air at around 1pm several months ago, but 1130-KSDO's ~63 mV/m was still fairly severely desensing the radio.
On my PL-606, KCBQ indicates about 81 dBu and KSDO about 75 dBu daytime, while KFMB at night checks in around 82 dBu. All three are pegged at 63 on the PL-380.

Hi Stephen,

Re: efficiencies of KSTP versus KFBK - you have to be careful when comparing efficiencies (really mV/m levels) of stations with different tower configurations. KSTP has one tower, KFBK has two. Because of the differences in phasing and field ratios sent to each tower, the number you have computed for the 2 tower KFBK array based on its RMSTheoretical is not entirely correct. Too, I'm not sure the method of calculation being used even applies to these two stations, as they use Franklins which are essentially full-wave dipoles turned vertically. KSTP and KFBK are the only two AM broadcast stations which use these antenna types as far as I know. The FCC Figure 8 calculator is also suspect for this antenna type. It is for monopole antennas with ground radials, not Franklins.

The 2.5 megawatt station. That amount of power fed into a quarter wave monopole, perfectly matched with 120 radials, should result in 15,288.4 mV/m at 1 kilometer, by the formula (mV/m = 305.768 * SQR(2500/1).

You wouldn't normally feed each leg of the Franklin (vertical dipole) with a separate transmitter. You are getting into phased arrays here, something usually done horizontally. Power does combine (and subtract) in phased arrays, however.

You asked is it possible to calculate a distance to the minimum discernible signal in such cases when they exceed the FCC's charts & calculators... Yes it is, the math is high-level and complicated. That's why the FCC produced tables. I wish I had the source code to do it, as I would put it to use. In my Radio Data MW program I have reduced the FCC charts to table data for a selected ground conductivity. The program then just looks up the figure for the distance and calculates it.

I took a look at the ITU documents you mentioned. I didn't see anything in there pertaining generally to noise levels. Per a previous question, higher noise is usually found in the tropics, lesser noise as you move towards the poles. Excepting, of course, aurora at the geomagnetic poles. Think thunderstorms for the tropics. When I was in the service in the 1960s, we ran equipment that had the capability of detecting thunderstorm static out to approximately 5,000 miles. It does get out.

Yes, my strongest local station here is WYSL-1040, 7.9 kilometers south of me. It runs 20KW daytime. Signal level here at the farm is about 176 mV/m. Blanketing for 30 KHz in both directions on many receivers.

Plugging your latitude-longitude in, KCBQ-1170 (15 km) should be near 122 mV/m at your location. The next strongest is KSDO-1130 (10.1 km) at 62 mV/m. Followed by KFMB-760 (11.7 km) at 44 mV/m. These are all daytime levels, based on a ground conductivity of 8 mS/m.

Hope this helps. Good DX!

Bill

Hi Stephen,

You had mentioned an interest in Part 15, or low power broadcasting.

Kyle Drake wrote a comprehensive user's guide to low power broadcasting in the MW AM band that is available to all through Creative Commons.

http://part15.us/files.p15/lpamhandbook.pdf

Bill

Jim Potter said...

Bill: Excellent worked examples, thank you. If you please, I need some help with the next step. I want to calculate losses in segments of different ground conductivities along a path. In Edmund A. Laport's Radio Antenna Engineering textbook (1952) he gives an example on page 84 of sequential conductivities, but I can't get his next numbers using the included Ground Wave Field Strength chart. Question: Once you arrive at the first field strength reading from the chart for a given conductivity, what is the next step? Do you divide this first result by 100? What distance do you use for the next conductivity? Start from zero again? Your help would be much appreciated.
Thanks/Regards/Jim Potter K3NSW
jpotter@jpotter.com

Hi Jim,

Am not familiar with LaPort's book, but somewhere in the FCC's Part 73 documents I do believe I have seen an additional example of calculating field strength over paths having different ground conductivities. I just can't remember exactly where I saw it.

What I have usually done, and I find it works out in a general sense when reviewing many stations to generate a receivable database list, is to average the ground conductivity over the entire path and use that figure for the overall conductivity for the path.

It is simple if exactly half the path is at one conductivity level and the other at another, as they can be directly averaged, however becomes complicated when the distances are not equal, in that different weights must be assigned to each loss factor.

If I can get some time I'll have a look and see if I can find a reference to that FCC example.

All the best. Will be looking forward to your comments. Will continue with e-mail if you desire.

73s,

Stephen said...

You may be looking for Part 73.183 - this seems to have something about calculating groundwave over varying conductivities.

What about calculating very close-in fields?

For example:

https://picasaweb.google.com/118228966367965758611/TecsunPL606#5597872520572769682 (near the center tower of 590-KTIE in San Bernardino, CA)

https://picasaweb.google.com/118228966367965758611/TecsunPL606#5597873169625893938 (near a tower (forget which now) of 590-KKDD in San Bernardino, CA)

(ok, those last two were edited ;) )

(last 2 near 1170-KCBQ in Lakeside, CA (licensed to San Diego, also same transmitter site for 910-KECR El Cajon, CA), also would like to figure out distance I would need to be to receive the same strength as at signal peak using only the built-in antenna, and equivalent field)

http://www.youtube.com/watch?v=-w49RVGQzFc (recorded 9.2 miles S of KCBQ, boosted signal here is stronger than the barefoot signal in the above videos recorded about 300 feet from the nearest tower in the center of the main lobe)

http://www.youtube.com/watch?v=5ay5k3txG_Q (recorded in my front yard of a weaker station to demonstrate SAT+Utility gain - 1550-XEBG is taken from about 30 dBuV to about 86 dBuV or so)

Last but not least ... I guess with strong signals like that one would wanta a radio that nulls better than this? ;) http://www.youtube.com/watch?v=f7fU5hIIuOY

Jim Potter said...

Bill & Steve: Thanks! Very useful. Yes, 73.183(d) describes the 'equivalent distance method' for calculating fields where they traverse differing ground conductivities. I'll work with this immediately.

And... Bill and Steve -- I have the pdf of Radio Antenna Engineering by Edmond A. Laport, 1952. This incredible book covers everything from longwave to UHF in theory and practice. It's an amazing textbook. it's 38.5 meg, too big for email attachment. I'll be happy to furnish a CD to you through US mail. Contact me by email jpotter@jpotter.com with your mailing addresses, and I'll burn you a copy. I printed it and it takes a 3" ringbinder, but -- man -- it's the Holy Bible of antenna design and engineering. Thanks again for your help and insights, fellas. Regards/Jim/K3NSW

Stephan,

Thank you for looking up the Part 73 reference. Sorry for the delayed reply. Be busy and also away on a trip.

Calculating field strength gets a little hairy at closer than one kilometer. The FCC's field strength formulas are based on "far field" interference patterns and will not work or be accurate at distances of 300 feet. A one kilometer distance is minimum.

Figuring distance for a required signal level is just a matter of reversing the way you look at the FCC graphs. Cross the desired signal level in mV/m with the ground conductivity and read the distance off the other scale.

Converting mV/m values to dBm or dBuV can be done with various calculators found on the web. Two good ones are:

http://www.giangrandi.ch/electronics/anttool/field.html

http://www.giangrandi.ch/electronics/anttool/decibel.html

Bill