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As for "twin EM signals, 90 degrees out of phase", the right-angle
relationship is spatial, in the plane of the wavefront, and it is not between twin signals. It is between the lines of the E and M fields. It appears that I am having difficulties explaining this "out of phase" business. Let me try once mo Take a snapshot of the dipole when the charges are crowded at the ends of the dipole: all negative charges are at one end and all positive charges are at the other end. At this very moment, in the proximity of the antenna there is a maximum electric field with vectors oriented from the "positive" half of the dipole to the "negative" half. This electric field has just reached its peak. But in the same time there is close to zero flow of charges in the antenna, implying that the magnetic field is close to zero. Indeed, the magnetic field is perpendicular to the electric field; however, it is zero exactly when the electric field is maximal. This is called a "90 degree out of (time-)phase". However - in theory - an accelerated point of charge creates an EM front with an electric field and a magnetic field in (time-)phase, i.e. they peak and die together, perpendicular on each other. Moreover, in free space a varying electric field generates a magnetic field in (time-)phase with the originating electric field (without the need of any charge). So, the problem is that the dipole generates two fields (E&M) out of (time-)phase; whereas EM propagation in free space implies two fields in phase. The "contradiction" can not be more clear than this. In order to explain this wonder, I was suggesting the following scenario: The dipole give us these two ortogonal fields, E1 and M1 : an electric field and a magnetic field out of time-phase. E1 is a time varying electric field, hence it creates its own companion - say - M2 which is a magnetic field in time-phase! with E1. In the same time, M1 is a time-varying magnetic field, hence it creates its own companion - say - E2 which is an electric field in time-phase with M1. Doing the arithmetic, we conclude that we have four fields altogether, which represent two separate EM emissions: E1&M2 is one emission, and E2&M1 is the other one. These two emission are obviously out of time-phase(they inherited from their originators), i.e. when [E1 and M2] both peak, [E2 and M1] both die. This is like saying that the two EM radiations are 90 degrees out of time-phase. From here on, I do not know how to continue. One way may be that these two EM radiations - as any behaving waves - sum up vectorially, and lead to a unique EM radiation with an amplitude a bit higher that the initial ones. The problem I am having with this explanation is that the wavelength is also a bit shorter - which does not sound right. So, I am looking for a better continuation - or for a better explanation of the following : how comes that we start with two out of time-phase E and M fields and end up with an EM propagation, i.e. with E and M in time phase ? Cordially, Nic. |
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