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Below is most of a post I made on this topic on another board. Thought it might be useful here as well. /Rich
________Below is most of a post I made on this topic on another board. Thought it might be useful here as well. /Rich
________Radiating 11.4 nanowatts from a linear, 1/2-wave dipole produce the maximum Part 15 FM field in free space — both in theory and practice.
But if the antenna is not located in free space, then reflections of its radiation together with the direct signal could produce fields 3 meters away from it that are above the compliant level.
Even if the antenna is vertically polarized, and mounted essentially in free space such as on a pole on top of a building, and met the Part 15 FM limit at 3 meters, it should be noted that the FCC typically measures Part 15 FM systems from several hundred meters away. This again means that reflections of the originally compliant radiated signal can add to the direct signal to show non-compliance at the location where the FCC measures it.
The peak gain of a 1/4-wave whip working against a perfect r-f ground is the same as that of a 1/2-wave dipole. The r-f ground seen by a whip attached to a Part 15 FM transmitter consists of whatever conductors comprise the r-f ground on the circuit board, along with whatever un-decoupled conductors lead away from the transmitter (program input cable, power source, etc), and their physical orientations. Such an r-f ground is far from perfect.
So the true gain and pattern of this whip + other conductors probably never is known. But to make a conservative guess, if this whip antenna system had only 1/1,000th of the peak gain of a 1/2-wave dipole, that would mean that the highest transmitter output power possible without exceeding Part 15 FM (not counting reflections) would be 1,000 X 11.4 nanowatts, or 11.4 microwatts — which is 1/877th of the output power rating of a 10 mW transmitter.
Typically there is no means for Part 15 FM operators to accurately measure power levels or radiated fields in these ranges. Neil (radio8z) has addressed this by shortening the whip to the height where the system can’t be heard on a typical receiver much beyond 200 feet, which is the range the FCC expects for it per their OET Bulletin 63. Reducing radiation this way is the result of the poor impedance match of that shortened whip and r-f ground to the impedance the transmitter is designed to drive efficiently. Neil’s pragmatic approach probably has a good chance of keeping Part 15 FM operators away from FCC problems.
For FCC compliance, almost all of the output power of a typical Part 15 FM transmitter needs to be absorbed by system losses, rather than being radiated either by a dipole or a whip.
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Total posts : 45366A 100% efficient 1/4 wave vertical monopole over a perfect ground plane has 3 dB more peak gain than a 100% efficient 1/2 wavelength dipole in free space.
Total posts : 45366The intrinsic gain is the same. The 3 dB difference results from the fact that all of the radiation from the 1/4-wave monopole over a perfect ground plane is confined to one hemisphere.
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Total posts : 45366The gain of an antenna is the ratio of the maxiumum field strength produced by the antenna at a given distance compared to a 100% efficient reference antenna with the same power input. The gain is the directivity of the antenna multiplied by its efficiency. If the efficiency is 1, or 100%, the gain is the same as the directivity.
The 3 dB difference noted in the previous two posts (which, as Rich correctly says, results from the fact that all of the radiation from the 1/4 wave monopole over a perfect ground plane is confined to one hemisphere) is the difference in the gains of the two types of antennas. Because of the 3 dB difference, the gains cannot be the same. The 3 dB difference is a difference in gain of 3 dB. It does not matter what the reason for this 3 dB difference is for there to be a gain difference.
The reference antenna is usually accepted to be the isotropic radiator, which is postulated to have completely uniform radiation in all space directions. Its gain is 0 dBi, by definition. The field strength at 1 km for 1 kW of radiated power is about 173.2 mV/m.
The half-wave dipole has a particular doughnut-shaped radiation pattern and has a gain of about 2.15 dBi. The field strength at 1 km for 1 kW of radiated power is about 221.5 mV/m.
The quarter-wave monopole above a perfect ground plane has a radiation pattern shaped the same as for the half-wave dipole, except that the dougnut-shape is cut in half. Its gain is about 5.15 dBi. The field strength at 1 km for 1 kW of radiated power is about 313.2 mV/m
The editions of Kraus (1950) and Jasik (1961) that I have both give the definition of gain I give here.
Total posts : 45366Ermi Roos wrote; The 3 dB difference noted in the previous two posts (which, as Rich correctly says, results from the fact that all of the radiation from the 1/4 wave monopole over a perfect ground plane is confined to one hemisphere) is the difference in the gains of the two types of antennas. Because of the 3 dB difference, the gains cannot be the same. The 3 dB difference is a difference in gain of 3 dB.
______________Differentiation is required between the intrinsic gain of a monopole radiator, and the effect of the propagation environment on that gain in specific directions.
All antennas have a far-field radiation pattern/gain subject to modification by reflections and obstructions in the propagation environment.
In the case of a perfect 1/4-wave monopole over a perfect ground plane (not that either can be a physical reality), the ground reflection contributes 3.0103.. dB to the intrinsic radiation of that monopole in the horizontal plane.
But it is NOT true that a 1/4-wave, ground-mounted vertical monopole itself has ~3 dB more intrinsic gain than a 1/2-wave dipole in free space.
RF
Total posts : 45366Rich,
I will admit that I had some difficulty understanding the point in your last post because your argument seems to me to be rather abstract.
I will try to reduce your argument into its elements to understand it better. You seem to, first of all, call for a separation between the antenna and its propagation environment. For the case of the half-wave dipole, this is conceptually easy. There is the dipole (the thing in itself) with its characteristic, or inherent (intrinsic) properties. It is contained in empty, three-dimensional, space extending in all directions to infinity.
It’s a little difficult to similarly separate the “thing in itself” and its operational environment (because they appear inseparable) if one is talking about the vertical monopole above an ideal ground plane. The ground plane, which extends to infinity, is an integral component of the antenna. Without the ground plane, the antenna would not exist. The operational environment is semi-infinite free space with a boundary at the ground plane.
You also speak of “obstructions” on the operational environment, as if they should be considered separately from the antenna. Suppose the obstructions are warious other vertical monopoles on the ground plane. Such monopoles, if close enough to the powered monopole (and big emough) would affect the gain in some directions. These monopoles are actually not separate from the “antenna,” but parts of a larger composite antenna.
In the situation discussed here, there are no pesky obstructions to complicate the problem, but only a simple, infinite, ground plane. The ground plane itself is the “obstruction” you speak of. You say that reflections from the ground plane add 3 dB to the “intrinsic” radiation from the monopole. Without the ground plane and its reflections, however, there would only be an end-fed 1/4 wave monopole with a voltage source with no return path for the displacement current from the monopole. In other words, in the absence of the ground plane, there would be no radiating antenna at all.
For this reason, Rich, I consider your argument about an “intrinsic gain” to be unpersuasive.
The theoretical and experimental fact is that the gain of the quarter-wave monopole above an ideal ground plane is 3 dB greater than the gain of the half-wave dipole.
Total posts : 45366Before posting my previous comments about the comparison between the half-wave dipole in empty space and the quarter-wave monopole over a ground plane, I tried to find out if anyone else had expressed a viewpoint in print that was similar to what Rich stated in his posts. Not finding any such source, I did the best I could in answering Rich.
Since posting, I have found in an article by John W. Ames and William A. Edson in the Nov./Dec. 1983 and Jan./Feb. 1984 issues of R.F. Design that a debate similar to the one between Rich and myself has happened before. The name of the article is “Gain, Capture Area, and Transmission Loss for Grounded Monopoles and Elevated Dipoles.” The article criticizes “artificial constructs” such as “gain calculated as though the antenna were in free space.” Emphasis was given to the words, “as though.” Unfortunately, the authors did not cite any sources of where these supposed “artificial constructs” were expressed.I think that some authors had felt the need to equate the the half-wave dipole in empty space and the quarter wave monopole over a perfect ground plane because there are certain theoretical difficulties with the quarter wave monopole over ground. These difficulties are distracting when one is trying to teach the very complicated subject of antenna theory, and it makes pedagogical sense to simplify the subject by equating the two types of antennas.
One difficulty with the quarter-wave monopole above ground is that the reciprocity theorem does not hold exactly. The reciprocity theorem states that the receiver gain of an antenna is the same as the transmitter gain. For the monopole above ground, the two gains are the same only if the source of the received signal is a skywave. If the source of the received signal is a groundwave, the receiver gain is only -.86 dBi. A quarter-wave monopole above ground generates groundwaves, but it is a poor receiver of groundwaves.
Another difficulty with the monopole above ground is that two monopoles above ground (one a transmitting antenna and the other a receiving antenna) cannot be properly thought of as two separate antennas because the infinite ground plane, and the distance between the antennas, are components of both antennas.
Still another difficulty with the monopole above ground is that an isotropic source is defined as being in free space, but, to generate a grounwave, an isotropic source should be above the ground plane. This is a difficult situation conceptually, because an isotropic source is already a fiction, and to redefine this fiction as another kind of fiction is simply hopeless. Nobody has been able to clearly define what is meant by an isotropic source above ground. Ames and Edson have characterized the idea of an isotropic source above ground as another “artificial construct.”
The situation would certainly be simpler if we looked at the monopole above ground as being equivalent to a dipole in free space, but this notion is not consistent with physical reality. “Nature is what it is, not what we would want it to be.” E.T. Whittaker. I think that it is best to simply use the definitions of receiver gain and transmitter gain and accept the results that are obtained by measurement (or, for the ideal case, by calculation.) The tramsmitter gain for the quarter-wave monopole above ground is 3 dB higher than the transmitter gain of a half-wave dipole in free space. The receiver gain of a quarter-wave monopole above ground is the same as the transmitter gain when receiving a skywave, and -.86 dBi when receiving a groundwave.
Total posts : 45366Ermi wrote: One difficulty with the quarter-wave monopole above ground is that the reciprocity theorem does not hold exactly. The reciprocity theorem states that the receiver gain of an antenna is the same as the transmitter gain. For the monopole above ground, the two gains are the same only if the source of the received signal is a skywave. If the source of the received signal is a groundwave, the receiver gain is only -.86 dBi. A quarter-wave monopole above ground generates groundwaves, but it is a poor receiver of groundwaves.
_________Sorry, but this concept is not supported in antenna engineering texts such those of Johnson & Jasik, Balanis, Kraus and Terman.
The shape of the elevation pattern of a perfect monopole over a perfect, flat, infinite ground plane is the same whether it is used to transmit or receive. Therefore its receive gain for the groundwave (at zero degrees elevation) is the same as its transmit gain.
The groundwave gain of a resonant, 1/4-wave monopole for these conditions will be the 2.15 dBi gain of a 1/2-wave dipole in free space plus 3 dB due to the perfect ground reflection, which is 5.15 dBi in total. Maximum elevation gain for monopoles up to 5/8 wavelength in height over a perfect ground plane always occurs in the horizontal plane. These statements are easily confirmed by simple NEC models.
The loss of intrinsic receive gain suggested in the quote above may be an interpretation of monopole elevation patterns published for an ~ infinite distance, over other than a perfect, flat, infinite ground plane. In such cases the ground reflection results in an apparently zero field from the monopole in the horizontal plane, at that distance. Also in the real world the groundwave is traveling over a spherical surface (the earth), so at great distances there are obstruction and diffraction losses, as well as those related to the imperfect conductivity of the earth.
Note that these published patterns are not the result of the loss of the ground reflection, as may be inferred from the -0.86 dBi groundwave gain quoted above. And even if was, that would reduce the 1/4-wave monopole groundwave gain of 5.15 dBi by 3 dB — which is 2.15 dBi, not -0.86 dBi.
A monopole over an imperfect, spherical ground plane will have less relative field in the groundwave than it has at some elevation angles above the horizontal plane, as a function of the distance between the monopole and the constant radial length where that field is measured. But that relative loss in the groundwave is the result of propagation conditions, and not because the relative field of the monopole was intrinsically less for groundwave fields. And such field loss will be the same for a real-world (and theoretical) monopole whether in transmit or receive mode.
A further consideration here is that the maximum amount of power that can be extracted from a radiated EM wave by a perfectly matched receiving antenna is only 1/2 of the power in the wave received by that antenna. This is related to the fact that the r-f current induced in the receiving antenna also produces radiation from that antenna, and therefore is not delivered to its output terminals. But whatever that loss is, it applies equally to waves arriving from all directions, and does not cause a 3 dB relative loss for groundwave signals as compared to skywave signals.
Probably you and I are the only ones reading this thread at this point, Ermi. So if you want to continue it, maybe we should do so off-forum.
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Total posts : 45366The reason that a vertical quarter-wave monopole above ground has 6 dB less gain when receiving a groundwave compared to when receiving a sky wave is that there is no reflection from the ground plane when the groundwave is received.
It is reflection that creates the virtual quarter-wave element below the level of the ground surface that adds to the real quarter-wave element to creates an antenna that is something like a half-wave dipole. Since a sky wave reaches the antenna at an angle with the ground plane, there is reflection from the ground plane, and the virtual quarter-wave antenna element is functioning. A groundwave is a surface wave that travels along the surface of the ground plane, and there is no reflection at the receiving antenna, and the virtual antenna element is not present when the receiving antenna intercepts a groundwave. The lack of the virtual antenna element causes the effective height of the receiving antenna to be half of what it is when receiving a sky wave. Halving the effective height of the antenna causes the effective aperture of the antenna to be reduced by a factor of 4. This is the cause of the 6 dB difference between the gain of the receiving antenna when receiving a sky wave and when receiving the ground wave.
A quarter-wave transmitting monopole above ground depends on reflection from the ground plane to function, so its gain has a single fixed value, which is 3 dB higher than for a half-wave dipole remote from earth.
It is this the 6 dB reduction in the gain of the quarter-wave monopole when receiving a groundwave that accounts for the fact that two quarter-wave monopoles above ground (one is a transmitting antenna and the other is a receiving antenna) have the same systen gain as two parallel half-wave dipoles remote from the earth, separated by the same distance as the two monopoles. For the monopoles, the transmitting antenna has a gain that is 3 dB greater than for a dipole. The receiving antenna has a gain that is 3 dB less than for a dipole. The transmitting antenna creates a groundwave, and the receiving antenna receives the groundwave. So, the system gain for the two monopoles is the same as for the two dipoles.
For both the dipoles and the monopoles, the system gain remains the same if the transmitting and receiving antennas are interchanged. So there actually is reciprociry for these two antenna systems.
Total posts : 45366Ermi: The reason that a vertical quarter-wave monopole above ground has 6 dB less gain when receiving a groundwave compared to when receiving a sky wave is that there is no reflection from the ground plane when the groundwave is received. It is reflection that creates the virtual quarter-wave element below the level of the ground surface that adds to the real quarter-wave element to creates an antenna that is something like a half-wave dipole.
A base-driven, 1/4-wave monopole working against a perfect, infinite, flat ground plane has 1/2 the feedpoint impedance of a 1/2-wave dipole in free space (37.5 ohms instead of 73 ohms). Therefore for equal applied power, twice the current will flow in the 1/4-wave monopole as in each arm of the 1/2-wave dipole, which means that the peak power “launched” by each of these radiator forms is the same. The ground plane reflection (or “image”) adds 3 dB to the net far-field radiation of the monopole system, so that its peak, far-field system gain for the groundwave is 5.15 dBi.
The base impedance of a 1/4-wave monopole is a constant, whether it is in receive or transmit mode. Therefore a 6 dB difference in its receive gain is not possible, even if the monopole in receive mode was unaffected by reflections from the ground plane.
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Total posts : 45366Rich’s comments about the gain of the monopole as a transmitting antenna, and the the input impedance of said monopole, do not address the issue of the monopole used as a receiving antenna.
The gain of a receiving antenna is its capture area for electromagnetic radiation, which is called its “effective aperture.” Many textbooks give the capture area of a half-wave dipole in free space as .13 wavelengths squared. The antenna books I have (Kraus, Everitt, Jasik, and two different titles by Terman) do not give the capture area for a vertical monopole above ground. Rich says that the capture area for the monopole should be 3 dB more than for a half wave dipole in free space, which would make it .26 wavelengths squared. I agree with this capture area for sky waves. For groundwaves, I maintain, for reasons I gave in my previous post, that the capture area is .07 wavelengths squared.
The question is not what the input impedance of the monopole is, but what is its capture area.
Total posts : 45366Quoting from paragraph 3.8.2 of Antenna Theory, Analysis and Design, 2nd edition, by C. Balanis:
“The radiation pattern is a very important antenna characteristic. Although it is usually most convenient and practical to measure the pattern in the receiving mode, it is identical, because of reciprocity, to that of the transmitting mode.”
Note that Balanis does not restrict this reciprocity condition from applying to monopoles, or any other configuration(s).
Ermi has agreed that a perfect, 1/4-wave monopole in transmit mode over a perfect ground has a radiation pattern with a groundwave (horizontal plane) elevation gain of 5.15 dBi. This gain value also is calculated by NEC, and very closely approached for short paths over real earth in thousands of accurate groundwave measurements of operating AM broadcast stations — going back many decades.
According to the Balanis quote above, such monopoles have the same gain in receive mode.
Quoting from Section 11, paragraph 8 of Radio Engineers’ Handbook, 1st edition, by F. E. Terman:
“…the relative response of the antenna to waves arriving from different directions is exactly the same as the relative radiation in different directions from the same antenna when acting as a radiator. Also like the antenna directivity, the effective height and the impedance of the antenna are the same in reception as in transmission. These reciprocal relations between transmission and reception properties make it possible to deduce the merits of a receiving antenna from transmission tests, and vice versa.”
Again, there is no qualification — this applies to all forms of antennas.
As far as I have discovered, there is no authoritative reference stating and conclusively proving that reciprocity fails for monopole antennas.
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Total posts : 45366What I have claimed differs from the cited references only for vertical monopoles over ground used in the receiving mode where the elevation angle of the incident radiation is zero degrees, in which case I assert that the receiving gain is lower than the transmitting gain by 6 dB. I think I can prove my assertion by using the development of the effective aperture in Kraus.
I do not claim any credit for what I said. I am only retelling what others have said.
The demo copy of EZNEC I use does not permit analyzing an antenna in the receiving mode. I would therefore greatly appreciate finding out what result NEC gives for the following receiving antenna situation:
Plane waves are applied to a vertical quarter-wave monopole above ground, with the electric field parallel to the monopole. Said plane waves travel in a horizontal direction along the surface of the ground plane. The height and width of the plane waves are very large compared to the height of the antenna. The field strength is 10 mV/m, and the frequency is 1.7 MHz.
I would like to either find out the induced open-circuit voltage between the base of the antenna and the ground plane, or the power applied to a 36.5 ohm load resistor between the base of the antenna and ground. Either of these results would tell me if NEC agrees with my concepts or not.
Corrected on 4/10/08:
Since posting, I found out that it is difficult to use simulated plane waves in the NEC program if a ground plane is used. The ground plane interferes with the plane waves, and it is difficult to get the desired field strength.
So, I am proposing another method for finding out if the the gain of a quarter-wave monopole is the same as the transmitter gain when receiving groundwaves, or if it is 6 dB lower.
I propose simulating two quarter-wave monopoles over a ground plane; one a receiving antenna and one a transmitting antenna. The two antennas are separated by a long enough distance so that the receiving antenna is in the far field of the transmitting antenna. 1 km is a good separation to use. 1.019 watts of radiated power provides a field strength of 10 mV/m at 1 km. 6.1 V applied to NEC transmitter antenna terminals gives 1.019 W of radiated power.
From the information about effective height in Kraus, which gives the relationship between the field strength applied to an antenna, and the induced open-circuit voltage at the antenna terminals, at 1.7 MHz, 10 mV/m incident on a quarter-wave monopole induces .565 V (open circuit) if the receiving gain is the same as the transmitting gain, and .282 V (open circuit) if the receiving gain is 6 dB less than the transmitting gain. With NEC, it is easier to find short-circuit current than open-circuit voltage. The corresponding values of short-circuit current are 15.48 mA and 7.74 mA. In the simulation, the receiving antenna can be connected directly to ground, and the induced short-circuit current can be read from the segment of the receiving antenna connected to ground.
What I have described can even be done with the demo EZNEC program that I use. I am going to do the simulation. It would be very good if Rich would also do the simulation on his NEC-2 program so that the results can be compared.
Total posts : 45366For a baseline check on how well NEC might calculate receive parameters for a simple dipole, I constructed a NEC-2 model consisting of two, vertical, 1/2-wave dipoles at the same elevation in free space, and separated horizontally by 3 km. One dipole was driven with 1 kW, and the other had a terminating resistor at its center feedpoint, equal to the Zo of the dipole. The frequency was 1 MHz. The maximum radiated field at that 3 km distance would be about 73.9 mV/m.
NEC showed 0.01564 A in the resistor at the receive antenna feedpoint, which equates to a power of about 17.5 mW (12.43 dBmW).
By other means I calculated the 1 MHz field strength necessary to generate 12.43 dBmW of dissipation in that resistor, which was 23.5 mV/m.
So it doesn’t appear likely that NEC will be useful toward proving/disproving the more complex case of a 6 dB gain difference expected by Ermi for a 1/4-wave monopole when used in receive mode.
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Total posts : 45366Kraus agrees with your “other means” calculation, and gives gives a value of 172.4 mW where your NEC simulation gives 17.5 mW. This looks a lot like a transcription error consisting in a shift in the decimal point.
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