- April 12, 2007 at 1:03 pm #6888RichParticipant
Total posts : 196
Below are some comments I posted on another site, and I thought they might be of interest to Part15us readers. The statements I commented on are paraphrased, and shown in bold face.
Below are some comments I posted on another site, and I thought they might be of interest to Part15us readers. The statements I commented on are paraphrased, and shown in bold face. There is little radiation from a long ground lead. The height of a 3-m whip above the earth produces better signals than if it is lower. The 3-m whip is a point source of radiation on medium wave frequencies These beliefs are popular among Part 15 AM equipment suppliers and operators, but they are not supported either by theoretical physics, or decades of field experience. Here is a quote from RADIO ENGINEER’S HANDBOOK by F. E. Terman, 1st Edition, page 799, in the section about grounded vertical radiators: “When the length of the grounded antenna is of the order of one-eighth wave or less, the radiation is almost exactly proportional to the cosine of the angle of elevation.” The 3-m whip portion of a Part 15 AM antenna system is much shorter than one-eighth of a wavelength even at 1700 kHz, so we can construct a short table showing its relative field in the vertical plane: Elevation Angle = Relative Field [cos(angle)] 0 deg = 1.000 (always maximum radiation in the horizontal plane) 15 deg = 0.966 30 deg = 0.866 45 deg = 0.707 60 deg = 0.500 75 deg = 0.259 90 deg = 0.000 (always zero radiation toward the zenith, directly above the radiator) These characteristic patterns also can be seen in the paper at this link (figure on page 3). Conclusion: these electrically short antennas are far from being point sources. Adding a long, vertical ground lead/conductor to a Part 15 AM tx+whip extends the radiating length of the antenna system. All of the r-f current that exists at the base of the elevated 3-m whip flows along that “ground” path at about the same value as at the base of the 3-m section. The flow of r-f current along the ground wire makes it radiate, just as that current makes the 3-m whip radiate. This can be seen in the figures on pages 1 and 2 of the paper referred to above. So it is the added radiation from the ground conductor of an elevated Part 15 AM system that produces the increase in signal strength/coverage radius, not because the 3-m whip is an isolated point source, which when elevated has better line of sight paths to nearby receivers. The majority of the radiation from such systems occurs from the ground lead, not the whip. A well-designed and impartial measurement will prove this. These are fine points of engineering that can be debated What we are discussing here are not fine points that are open to debate. They are fundamental to physics, and to an understanding of how Part 15 AM systems operate. Anyone, but especially a Part 15 AM tx manufacturer wanting to provide accurate information about the installation and operation of Part 15 AM systems should not dismiss them, and offer advice to the contrary — which only misleads others, and may lead to FCC problems for those readers (at least). //April 12, 2007 at 5:06 pm #15243radio8zGuest
Total posts : 45366
Rich, or anyone else who might know,
Concerning the radiation from the ground lead, I have read a couple of reports on other boards where people claim to have shown that the field strength near the ground conductor is very small compared to that near the antenna. My take on this is there is possibly an error in the measurement technique and present it here to ask if it makes sense.
One reported experiment involved a low sensitivity field strength meter (FSM) held near the radiators giving a high reading near the antenna compared to a negligible reading near the ground. It seems to me that the voltage to ground on the ground lead would be low compared to that on the radiating antenna and that the meter being held near the conductors was responding to the capacitively coupled voltage rather than the radiated field. The current in the ground lead can cause appreciable radiation even at a low potential to ground.
If I touch the antenna of my AM tx. with a neon light, it lights: not so if I touch the ground, thus illustrating the difference in voltage but not giving any indication of the current nor the field strength.
Is it reasonable to question that the measurement technique described is flawed and misinterpreted and the experimenter did not measure the radiated field?
NeilApril 12, 2007 at 9:00 pm #15244RichGuest
Total posts : 45366
radio 8z wrote: Is it reasonable to question that the measurement technique described is flawed and misinterpreted and the experimenter did not measure the radiated field?
Yes, it is very reasonable to question it. The accuracy of such measurements will depend on the sensitivity of the typical field strength meter (FSM) to radiated fields, and the horizontal distance from its antenna to the Part 15 AM antenna components.
The field of a radiating antenna system should not be measured very close to it, because that does not give usable results with a FSM. Such is possible only with exotic and expensive near-field probing techniques, where the complete radiation pattern can be synthesized from the near-field data measured at various locations along the antenna aperture.
A better approach for Part 15 AM would be to locate a sensitive FSM a few hundred feet horizontally distant from the Part 15 antenna system, over level ground. The meter should be tunable, to be sure that only the frequency under test is being measured. The total radiated field observed from a Part 15 AM system with a long ground lead will be higher than when the 3-m whip is mounted with its base at/very near ground, using a short conductor connected to the same r-f earth ground as used in the elevated system (and other things equal).
As the 3-m whip in the elevated Part 15 antenna system then will be virtually the same distance away from the FSM, and with no better line-of-sight path to the FSM than from the earth-based system, this will conclusively show that it isn’t the height of the 3-m whip that is important, it is the total length of the radiating conductors used (3-m whip plus the long conducting path from the tx chassis to the earth r-f ground).
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