For a lossless electrically short vertical monopole over a perfect ground plane:
The field strength at 1 km for 1kW radiated power is 300 mV/m.
Because field strength varies as the inverse of distance, the 65 mV/m at 100 m measured by the FCC is equivalent to 6.5 mV/m at 1 km.
Field strength varies as the square root of radiated power.
Therefore, the field strength of 65 mV/m at 100 m corresponds to a radiated power of [(6.5/300)^2]X(10^6) = 469 mW of radiated power from a lossless short vertical monopole over a perfect ground plane.
For a lossless quarter wave vertical monopole over a perfect ground plane:
The field strength at 1 km for 1 kW radiated power is 313.2 mV/m. [edit...unit correction]
Therefore, the field strength of 65 mV/m at 100 m corresponds to a radiated power of [(6.5/313.2)^2]X(10^6) = 431 mW from a lossless quarter wave vertical monopole over a perfect ground plane.
The FCC agrees with Rich's statement that an elevated antenna does not necessarily have more efficiency than one that is at ground level. Section 73.189 of the FCC Rules and Regulations considers an AM BCB antenna to have the same effective height whether it is mounted on the ground or on top of a building. However, there is likely to be some conductive radiating path between an elevated antenna and earth ground unless there is a deliberate effort to eliminate one. For example, a battery-powered transmitter feeding a vertical dipole on top of an insulated pole would not have a conductive path to ground, provided that there are no audio or control wires going to the transmitter. The audio source to the transmitter should also be battery powered, and on on the insulated pole. If unlikely extreme measures like this are not taken, there will probably be a spurious conductive path to ground by some route.
We agree on the difference in gain and fields for a short monopole compared to a 1/4-wave monopole, for the same system losses.
BTW, there is a typo in your statement for the 1/4-wave field at 1 km for 1 kW -- the unit of measure should mV/m instead of mW/m.
The FCC agrees with Rich's statement that an elevated antenna does not necessarily have more efficiency than one that is at ground level.
But the efficiency of an elevated Part 15 AM antenna system IS greater than one at ground level. My post attempted to show what was responsible for that.
To clarify, the increase in Part 15 AM antenna system efficiency and field strength when elevated is the result of radiation by conductors OTHER than the 3-m monopole, and not the result of greater radiation or better path clearances from the elevated 3-m monopole, itself.
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Thanks for pointing out the typo. Unfortunately, I no longer have the edit function available for that post. Perhaps the moderator can make the correction for me.
An elevated antenna will not have increased efficiency if there is no conductive path from the the antenna to the ground. In the case of licensed broadcasting, audio wires will not have sufficient RF current-carrying capacity to act as effective radiators. The CFA promoters, who are trying to get into the licensed AM broadcasting industry, placed their rooftop antenna on a copper ground plane and attached heavy metal straps along the side of the building to earth ground. The straps provided the needed RF current handling capacity from the antenna to the ground to increase the antenna efficiency.
Ermi Roos wrote: "An elevated antenna will not have increased efficiency if there is no conductive path from the the antenna to the ground.
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But then an elevated "Part 15 AM" monopole with no, or an ineffective conductive path to an r-f ground reference or counterpoise has poorer radiation efficiency than when it is placed at the surface of the earth, and uses even a rather low-resistance r-f ground.
The reason that elevated (whip-mast) Part 15 AM antenna systems are so popular, and so vigorously defended by some is the result of the increased field strengths they radiate, other things equal.
If they didn't produce better results, they would not be so preferred and defended.
And unfortunately, such elevated installation configurations even are suggested/recommended by some manufacturers of Part 15 AM transmitters, whether or not such transmitters have been awarded Part 15 AM certification by the FCC.
The reason they do so is not generally understood in the Part 15 AM community.
Caveat emptor.
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What astonishes me the most about the first enforcement action discussed in this thread is the very low efficiency of the system that was cited. I already posted about that. I wonder how typical the efficiency of this installation is of Part 15 AM installations in general. If this poor efficiency is typical, Part 15 AM is next to useless as it presently exists. A fundamental technical reevaluation is needed.
If this installation is typical, tweaking the power input above 100 mW and elevating the antenna are definitely not enough to make Part 15 AM useful. In the cited system, the power input was 12 times higher than what is allowed, and elevating the antenna six meters caused the radiation resistance to be increased by a factor of about 20 compared to a 3 m antenna at ground level. The total radiated power increase caused by the increased input power and increased elevation is 12X20 = 240. Even with this advantage, the total radiated power is only 22.2 uW.This is a very insignificant radiated power.
Using Rich's example of a 3 m antenna that produces about a half watt of radiated power for 80 watts input to the antenna, the efficiency is .5/80 = .625 %. Rich assumed 20 ohms loss resistance. Let's arrume 25 % transmiter efficiency. In that case, the radiated power produced by Rich's antenna if its input power is 25 mW is 25X.00625 = .156 mW =156 uW. This a lot higher than the 22.2 uW actually measured at the cited system. The loss resistance has to be increased to about 140 ohms to get only 22.2 uW from Rich's antenna fed by a 100 mW transmitter that is only 25 % efficient. Only 22.2 uW was obtained with a system that gives a power increase of a factor of 240 over a system that is compliant with Section 15.219.
It's too bad that almost nobody has an accurate field strength meter to evaluate Part 15 AM installations. A field strength meter would allow determining the efficiency of a system, and then troubleshooting an inefficient system. Two causes that I can think of for the inefficiency of the cited system are very high ground resistance and improper tuning.
Ermi wrote: The total radiated power increase caused by the increased input power and increased elevation is 12X20 = 240. Even with this advantage, the total radiated power is only 22.2 uW. This is a very insignificant radiated power.
What all this points out is just how little radiated power is required to produce a useful Part 15 AM groundwave signal.
One factor in this reality is that the radiated field strength changes only by the square root of the change in radiated power.
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If 22.2 uW is, indeed, a "useful" Part 15 AM radiated power, it can be obtained within the restrictions of Section 15.219. This, after all, is a system efficiency of only .0222/100 =.0222 %. If I am correct in guessing that the causes for the inefficiency of the cited system are very high ground resistance and/or poor tuning, then special attention has to be paid to the ground and the tuning circuit.
I think that the SSTRAN tuning circuit, which is a capacitive input "L" network with a loading coil in series with the antenna, is a good one. I use it myself in my experimental transmitter. I am trying to improve its efficiency and its ease of use. I do not favor using a tuned transformer, which is less capable than the "L" network of obtaining a good impedance match between the transmitter and the antenna.
Ermi wrote: If 22.2 uW is, indeed, a "useful" Part 15 AM radiated power, it can be obtained within the restrictions of Section 15.219.
Just to note that in this NOUO, the measured field strength under 15.209, and the measured DC input power to the final r-f power amplifier in that transmitter under 15.219 BOTH were found to be non-compliant by FCC agents.
A later comment posted on this server states that an excessively long ground conductor was used, however the FCC citation didn't include that finding. But if true, maybe the FCC thought that two causes for their NOUO were enough?
Power radiated by compliant Part 15 systems should not be any greater than the parameters permitted by Part 15 Rules as they now exist (by physics, not supposition).
As a point of fact, such parameters do not support the use of legal Part 15 setups as "community broadcast" services with coverage radii of "1-2 miles" as shown in the www statements of some Part 15 operators, and even those of the manufacturers of some well-known "Part 15" transmitters.
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Here is another rare NOUO for an AM operator:
http://www.fcc.gov/eb/FieldNotices/2003/DOC-280714A1.html
In this case, the field strength reading corresponds to 8.8 W of radiated power from an electrically short vertical monopole above ground. Only field strength readings were made (Section 15.2O9) probably because the field strength was high enough so that there was no chance that the station would comply with Section 15.219.
Here is another rare NOUO for an AM operator:
http://www.fcc.gov/eb/FieldNotices/2003/DOC-280714A1.html
In this case, the field strength reading corresponds to 8.8 W of radiated power from an electrically short vertical monopole above ground. Only field strength readings were made (Section 15.2O9) probably because the field strength was high enough so that there was no chance that the station would comply with Section 15.219.
Comment withdrawn.