calculating antenna size or transmitter power based on field strength?

[pianoplayer88key]

So apparently using the dBu meter on my PL-380 isn't accurate enough. I'm not expecting perfection, though. I just want to be able to see that I'm indicating X dBu at Y meters, and know that I'm probably somewhere close to 250uV/m @ 3 meters (on FM), and be assured that it's not, say, 1uV/m @ 3cm, or 500mV/m @ 3km, for example.
Basically, I'm wanting to periodically transmit analog audio - a few hours at a time, once every few days or weeks or so. Is there any band that can be tuned on the Tecsun PL-380 (153-513kHz, 520-1710kHz, 2300-21950kHz, 64-108MHz) for which I can have a legal part 15 signal that will go over the horizon, without having to use an antenna longer than 3-4 inches, or a transmitter larger than 2 decks of cards? One of the reasons I picked 13.56MHz was due to the 15,848mV/m @ 30m field strength limit. (I do not intend to transmit via groundwave at that frequency - it'd either be skywave or direct wave.) I wish I could use the 160-190kHz band or the AM band in 15.219, but unfortunately there seems to be no provision in the rules for increasing transmitter power to compensate for a much smaller antenna than the legal limit. For example, if I was operating on 180kHz, the field strength limit would be 2400/180 = 13.33 uV/m @ 300 meters. (Not factoring in ground losses, I understand that field strength voltage is halved when doubling the distance.) Considering that, under that provision, the field strength over some distances I might want to cover may be something like 139.84547 nV/m (nanovolts) twice a week (for a few hours each time), 23.5538 nV/m twice a month (for several hours each time), or 9.0678 nV/m twice a year (for a few days each time). (That's assuming that the field strength voltage drops in half each time you double the distance, which obviously does not take ground losses into account.) I believe that... A - my Tecsun PL-380 is not sensitive enough to pick up signals that weak at 180kHz, and B - even if it was, those signals would be well below atmospheric noise levels.
Is there another solution?

 

[rfry]

I just want to be able to see that I'm indicating X dBu at Y meters, and know that I'm probably somewhere close to 250uV/m @ 3 meters (on FM), and be assured that it's not, say, 1uV/m @ 3cm, or 500mV/m @ 3km, for example.

You might get a very rough idea of the average dBu reading on your PL-380 for a 250 uV/m FM field by careful measurement of the field generated 3 meters in all directions from the antenna of a known legal Part 15 FM transmitter, in an open area (clear for at least 100 feet in every direction). You certainly could not trust that to prove Part 15 compliance, though.

Is there any band that can be tuned on the Tecsun PL-380 (153-513kHz, 520-1710kHz, 2300-21950kHz, 64-108MHz) for which I can have a legal part 15 signal that will go over the horizon, without having to use an antenna longer than 3-4 inches, or a transmitter larger than 2 decks of cards?

Unfortunately, no matter what size antenna and transmitter are used, the legal fields permitted from systems meeting Part 15 are too weak for useful reception over the horizon by any mode of propagation (ground, direct or skywave).

RF

 

[rfry]

There are some examples where people using specialized receiving equipment and quiet receive sites have received Part 15 LW signals over considerable distances.

Legal Part 15 LW permits the use of one watt of DC input power to the final stage of the transmitter, and limits the length of the antenna plus feedline plus ground lead to 15 meters. Of course this doesn't fit your requirement for a very small transmit system.

But below is a NEC analysis of a LW station using a transmitter with 0.5 watt of output power with a 15-m monopole. A total resistance of 50 ohms was assumed for the matching and r-f ground loss.

LOWFER Station 5/24/2010 7:52:43 AM

--------------- ANTENNA DESCRIPTION ---------------

Frequency = 0.19 MHz
Wire Loss: Zero

--------------- WIRES ---------------

No. End 1 Coord. (m) End 2 Coord. (m) Dia (mm) Segs Insulation
Conn. X Y Z Conn. X Y Z Diel C Thk(mm)
1 GND 0, 0, 0 0, 0, 14.986 75 9 1 0

Total Segments: 9

-------------- SOURCES --------------

No. Specified Pos. Actual Pos. Rel Amplitude Phase Type
Wire # % From E1 % From E1 Seg (V/A) (deg.)
1 1 0.00 5.56 1 1 0 I

-------------- LOADS (R + jX Type) --------------

No. Specified Pos. Actual Pos. R X
Wire # % From E1 % From E1 Seg (ohms) (ohms)
1 1 0.00 5.56 1 50 0

No transmission lines specified

No transformers specified

No L Networks specified

Ground type is Perfect

Field Intensity Calculation:

Power = 0.5 watts
Max field = 6.27518E-05 V/m RMS at X,Y,Z = 3000, 0, 0 m

Electric (E) Field (V/m RMS)

X (m) Y (m) Z (m) Ex Mag Ey Mag Ez Mag Etot
3000 0 0 0 0 6.2752E-5 6.2752E-5

Unfortunately a field of 63 microvolts/meter at 3 km would not provide very useful reception to most listeners, and 3 km is a long way from reaching beyond the horizon.

RF

 

[Ermi Roos]

The environment for which the Potomac instrument accuracy is specified is a standard calibration test site, not the real world in which actual field strength measurements are made.

 

[druidhillsradio]

You might get a very rough idea of the average dBu reading on your PL-380 for a 250 uV/m FM field by careful measurement of the field generated 3 meters in all directions from the antenna of a known legal Part 15 FM transmitter, in an open area (clear for at least 100 feet in every direction). You certainly could not trust that to prove Part 15 compliance, though.

Also, as Rich correctly points out, while not accurate enough to determing compliance, the antenna should be extended if possible to 1 meter, and those careful measurments should be made in the vertical and horizontal plane.

 

[rfry]

The environment for which the Potomac instrument accuracy is specified is a standard calibration test site, not the real world in which actual field strength measurements are made.

The intrinsic accuracy of a Potomac Instruments FIM-41 is independent of the site at which it is used.

Whatever is the net field incident on the loop antenna of an FIM-41 meeting its spec, that field will be measured to the accuracy of the FIM-41 -- which is about 0.52 dB error, max, including the uncertainty in the NIST standard field used to calibrate the FIM-41.

Therefore it is incorrect for anyone, no matter how learned, to state as a generalization that the accuracy of a MW field intensity meter such as the FIM-41 is not better than +/- 2 dB.

What may vary by 2 dB or more is the value of that measured field compared to the observer's expectation for it.

However that 2+ dB variation is the result of the observer's analytic approach, and not that of the FIM-41, itself.

RF

 

[pianoplayer88key]

I wonder if it's possible I'm incorrectly extrapolating field strength as a function of distance. I thought that when not taking ground conductivity into account, the field strength voltage would drop in half when the distance doubled (for example, 250µV/m @ 3m would be 125µV/m @ 6m).

Also, do I (in?-)correctly understand that when halving the field strength voltage, the level is reduced by 6dB? My Tecsun PL-380 specifies a sensitivity of 20µV/m with a SNR of 26dB. Extrapolating down (weaker signals) from that, I would think 20dB would be 10µV/m, 14dB = 5µV/m, 8dB = 2.5µV/m, 2dB = 1.25µV/m, -4dB = 625nV/m, -10dB = 312.5nV/m, -16dB = 156.25nV/m, -22dB = 78.125nV/m, and so on.
If I remember correctly, my ability to hear a 3kHz sine wave under white noise fizzles out at somewhere around -24 to -30 dB or so. I also just tested my ability to hear a musical selection under noise, by importing a few pieces of music into Audacity and normalizing it to peak at 0dB, and generating white noise, scaled at -0dB. I was able to hear the music at -10dB with moderate difficulty, but at -16dB it was almost gone. At -4dB it was easily copyable, though, so that would probably be good enough for me, although +2dB would be nice if possible

So... my calculation for 13.56MHz, assuming direct wave (although skywave coverage would be less because of the distance the waves have to travel to get to the ionosphere and come back, and any ionospheric loss during refraction), is as follows (I don't expect to use groundwave at all, and I didn't factor in any possibility of atmospheric absorption (although I hear that's not an issue till 10+ or 100+ GHz))... (I also included some steps related to the specified sensitivity and signal/noise figures of my PL-380. Also I truncated (and rounded) some of the decimals.)

15848µV/m @ 30m = 98.425 ft
7924µV/m @ 60m = 196.85 ft
3962µV/m @ 120m = 393.7 ft
...
30.953µV/m @ 15.36km = 9.544 mi
20µV/m @ 23.772km = 14.77 mi (26dB SNR)
15.5µV/m @ 30.72km = 19.09 mi
10µV/m @ 47.544km = 29.54 mi (20dB SNR) (Most of my intended transmissions would be well within this range.)
7.74µV/m @ 61.44km = 38.1770461 mi
5µV/m @ 95.088km = 59.085 mi (14dB SNR)
3.87µV/m @ 122.88km = 76.354 mi
2.5µV/m @ 190.176km = 118.17 mi (8dB SNR) (A few of my intended transmissions would be within this range.)
1.9346µV/m @ 245.76km = 152.7 mi
1.25µV/m @ 380.352km = 236.34 mi (2dB SNR)
967.285nV/m @ 491.52km = 305.4 mi
625nV/m @ 760.704km = 472.68 mi (-4dB SNR) (I would prefer not to go below this signal level.)
483.64nV/m @ 983.04km = 610.833 mi
312.5nV/m @ 1521.408km = 945.36 mi (-10dB SNR) (I could possibly still use this signal level if I absolutely had to.)
241.82nV/m @ 1966.08km = 1,221.7 mi
156.25nV/m @ 3042.816km = 1,890.72 mi (-16dB SNR) (This is probably too weak, though, unless the music has 0dB dynamic range and is always at full scale, which will NOT be the case - there actually will be NO dynamic range compression being used, and it's definitely possible the peaks may be at -6dB to -12dB or so.)

As I said above, I was making the assumption that doubling the distance means halving the field strength voltage, and halving the field strength voltage means a reduction of 6dB. Am I making the calculations all wrong?

Does anyone know of any inexpensive (the Rangemaster for AM is several times too expensive, but the entry-level Ramsey AM kits are well within my budget) part 15 compliant 13.56MHz transmitters that will produce the legal field at the specified distance with an electrically-short antenna?

 

[Ermi Roos]

It is not clear what Fry means by "intrinsic accuracy." Perhaps he means "calibration accuracy."

 

[rfry]

My Tecsun PL-380 specifies a sensitivity of 20µV/m with a SNR of 26dB.

Are you sure about that? My PL-310 manual states "Less than 20 µV" for an "S/N=26dB" as its SW (HF) sensitivity.

What is not stated there is the reference value for that specification. If it is a field strength, it should be stated in terms of some value of voltage per meter. Otherwise it probably applies to an unknown impedance value at some circuit point within the radio -- which was not given in that spec.

But regardless, your calculations of the field intensity for a "direct wave" on a 13.56 MHz path are not in agreement with this clip from the applicable Rules for Part 15 at http://frwebgate.access.gpo.gov/cgi...E=47&PART=15&SECTION=225&YEAR=2001&TYPE=TEXT:

Sec. 15.225 Operation within the band 13.553-13.567 MHz.

(a) The field strength of any emissions within this band shall not exceed 10,000 microvolts/meter at 30 meters.

Also, your calculations do not account for earth curvature, which means that the values of the "direct' fields that you predict will be greatly reduced from your calculated values once they pass beyond the radio horizon.

A further concern is that few HF receive systems would produce a useful output even from interference-free incident signals with field strengths having much less than a few microvolts/meter.

RF

 

[Ermi Roos]

I am on the alert when Mr. Fry uses the word, "intrinsic." A couple of years ago, Fry made the incorect statement on another website that a half-wave dipole in free space has the same gain as a quarter-wave vertical monopole over a ground plane. I corrected him, saying that the quarter-wave monopole has 3 dB more gain than the half-wave dipole. His response was, "The intrinsic gain is the same." It still surprises me that he made such a meaningless statement using the word, "intrinsic." The best I can guess is that he just didn't want to admit that he was wrong.

 

[pianoplayer88key]

The link you gave was a rule from 2001.

Here's what the 2009 rules say:
http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=47&PART=15&SECTION=225&YEAR=2009&TYPE=TEXT

You're right that they don't account for earth curvature. Also, they don't account for skywave bounce. If I knew approximately how far up the ionosphere was, and the estimated loss during refraction, then knowing the over-the-earth distance, I could possibly calculate the angle, actual distance (going up and coming back down) and received field. Also, I was assuming that microvolts per meter was implied. :-\

Also, what approximate level would you consider "useful" output? I could make do with this if I had to...
http://cid-6bdd1917662288cb.skydriv...L-380 - 2010-05-24/WNSB415 - faint signal.mp3
http://cid-6bdd1917662288cb.skydriv...iles/PL-380 - 2010-05-24/WWV - very faint.mp3
Although, I'd prefer it to be a bit stronger, like this, and I'd be willing to use a better receive antenna to pull it in...
http://cid-6bdd1917662288cb.skydriv...- 2010-05-24/WNSB415 - fairly weak signal.mp3
Sure, I'd love to have a nice signal like...
http://cid-6bdd1917662288cb.skydrive.live.com/self.aspx/AM%20radio%20files/PL-380%20-%202010-05-24/KCBQ%20-%20strong%20signal^J%20good%20SNR.mp3
but I wouldn't expect that at the distances I'm trying to cover, but maybe within 2-3 miles or so. (I would hope to hit the 25dB S/N cap on the PL-380 at about 15 miles, or is that too much?)

As for the 20µV vs. 20µV/m argument... one of the designers of that Si4734 chip (Scott Willingham) is a participant on the ultralightdx yahoo group. I may see about asking him there what that means.

 

[rfry]

...A couple of years ago, Fry made the incorect statement on another website that a half-wave dipole in free space has the same gain as a quarter-wave vertical monopole over a ground plane. I corrected him, saying that the quarter-wave monopole has 3 dB more gain than the half-wave dipole.

intrinsic adj. Of, or relating to the essential nature of a thing; inherent.

A center-fed, 1/2-wave dipole in free space has a radiation resistance of 73 ohms, and a peak gain of 2.15 dBi. The sum of the original, or intrinsic radiation from that dipole, and the reflection of it when the dipole is mounted near a reflecting surface may produce a higher net, peak radiated field in some directions than when no reflecting surface is present. But the gain of the dipole, itself, when near a reflecting surface is still 2.15 dBi.

A 1/4-wave, series-fed monopole with its base at ground level has a radiation resistance of 36.5 ohms, exactly half that of a 1/2-wave dipole in free space. Therefore for the same power as applied to the dipole, twice the current will flow on the monopole as flows on each arm of the dipole, and the peak gain intrinsic to the monopole is the same as that of the free-space dipole: 2.15 dBi.

As in the case of the dipole near a reflecting surface, the monopole also has a reflecting surface - the earth. If the earth was a perfect conductor then the peak, net field produced by the monopole and its earth (or image) reflection would be 3 dB higher than the original field radiated by the monopole, or 5.15 dBi. But the peak intrinsic gain of the monopole is still 2.15 dBi.

A radiated field can consist of a direct ray and one or more reflected/refracted rays. These components must be satisfactorily resolved by the analyst if the performance of the system is to be accurately, and fully understood.

RF

 

[rfry]

The link you gave was a rule from 2001. Here's what the 2009 rules say: http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=47&PART=15&SECTION=225&YEAR=2009&TYPE=TEXT

Thanks. I used the link below, which I expected would be kept current, but apparently not. However the higher field allowed by the latest version of 15.225 still won't permit a useful field (say, greater than 10 µV/m) over the horizon by any propagation mode..

http://www.access.gpo.gov/nara/cfr/waisidx_01/47cfr15_01.html

As for the 20µV vs. 20µV/m argument... one of the designers of that Si4734 chip (Scott Willingham) is a participant on the ultralightdx yahoo group. I may see about asking him there what that means.

Hope you do -- the answer to that will be very interesting. Maybe you could refer him to the tests of the calibration of my PL-310 meter at http://i62.photobucket.com/albums/h85/rfry-100/Tecsun_PL-310_Signal_Readings.gif

//

 

[Ermi Roos]

Mr. Fry's explanation of his use of the word, "intrinsic" is not correct. It is well-established that a half-wave dipole in free space has a gain of 2.15 dBi, and a quarter-wave vertical monopole over ground has a transmitting gain of 5.15 dBi. This is 3 dB higher than for the half-wave dipole. Anybody can prove this for himself. It's in all the books.

 

[Tom Wells]

I havent the familiarity with figures and calculatons to interject real data, but I do but recall that efficiency varies ( and not on a linear slope) for all dipoles, depending on spacing above ground according the fraction of wavelength the antenna is elevated above the reflecting ground. This muddies the concept of a dipole in free space, which seldom exists at longer wavelengths, for pratical purposes.
If I recall, efficiency varies in what appears to be a sine-wave response, relative to height above ground, becoming less variable as distance increases and the free space conditon is approached.

There are "relative" minima and maxima in the gain dependent on height and wavelength used.
Meaning at different elevations above ground, the +/- differences in gain between the two types of antennas are not absolute,
but dependent on ground conductivity and "real" depth of the earth reflecting "element".

Theory yields to practice here, and at some heights of dipole, and/or some vertical 1/4 wave antennas depending on ground reflection,
it will be found that the dipole will do better, or the 1/4 wave vertical will do better.


Which is one reason why amatuer radio operators love to play with antennas.
Minor adjustments can result in major improvements. 3 db is a lot of difference in results.

 

[rfry]

... It's in all the books.

It is true that support for the statements I made about the gain of a 1/2-wave dipole and a 1/4-wave monopole can be found in antenna engineering textbooks.

The link below is a pasteup from a few pages in ANTENNAS FOR ALL APPLICATIONS (3rd Edition), by John Kraus and Ronald Marhefka of The Ohio State University.

It shows the radiation resistance of the dipole to be 73 ohms, and that of the monopole to be 36.5 ohms -- as I stated earlier in this thread.

The rest of my comments flow logically from those values.

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

RF

 

[Ermi Roos]

It is true that the radiation resistance of a dipole in free space is twice that of the vertical monopole at ground level, but that has nothing to do with the relative gains of these two antennas. Mr. Fry is playing with us. In most cases Fry is right, but, occasionally, he is wrong, and he has sufficient technical knowledge to know that he is wrong in this case. I have observed, after years of replying to his posts, that he will defend a wrong position to the end. Perhaps this is to protect his "guru" status on the discussion boards. He is doing the readers of this board, who depend on him for knowledge, a disservice by posting utter nonsense, like the defense of his use of the word "intrinsic" on this thread.

 

[rfry]

... Mr. Fry is playing with us. ... He is doing the readers of this board, who depend on him for knowledge, a disservice by posting utter nonsense, ...

It is unfortunate to see such personal attacks here, or elsewhere.

I have posted comments here along with the documentation from recognized antenna engineering references to support them.

At this point, most readers of this thread have been referred to enough scientifically validated information to prove this.

RF

 

[Tom Wells]

The engineering community should scrupulously avoid any personal attacks or attempts to discredit another's views.
Disagreement should be a stimulus for more information and input from the community.

That there would be an argument over a few db is disappointing.
Numbers exist in equations, antennas exist in real space.
Two different places, one theoretical, the other actual.

Theory and practice DO support each other, even in disagreement over a few db. Too many variables exist to ignore one or the other.
It dos not mean either either aspect is "utter nonsense".

 

[pianoplayer88key]

Going back to the original topic.... : It looks like I'll just need to do some experimenting myself. :-\ Anyone know of some sources for circuits for 13.56MHz I could possibly get started with? (I'll probably need to figure out how to adjust the power to compensate for a smaller antenna.) As for checking compliance... I live near San Diego, CA, where there's an FCC field office. I wonder if I could have it checked if I took it there?