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Ermi 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.*

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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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