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. 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.
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. 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.
Ermi: 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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Ermi: 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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Rich'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.
Rich'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.
Quoting 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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Quoting 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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What 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.
What 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.
For 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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For 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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Kraus 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.
Kraus 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.
A review of my NEC data will not show such...
The NEC current across the 71.7 ohm resistor at the center terminals of the receive antenna is 0.015641 amperes.
P = (0.015641)^2 * 71.7 = 0.017541 watts = ~17.5 mW.
Your EZNEC evaluation should show about the same, if your model setup is similar.
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