antenna-theory.com

[messi-d]

if you put a horizontal positioned dipole antenna( horizontally polarized) very close( about 10mm) to a vertically positioned dipole antenna( vertically polarized) both operating at 2.4GHz the isolation in simulation CST is about 30dB. the isolation improved by about 28dB when compared to two vertically positioned antennas placed next to each other(about 10mm) operating at 2.4Ghz.

The isolation in this case is in the near filed but polarization is in the far field. how is polarization something in the far field have an effect on isolation which in this case is in the near field.

[k3nnw]

This is long-winded and I am not sure it helps as much as I had intended. Some of this is based on conversations with Ed Joy, Professor emeritus at Georgia Tech, when I wanted to understand more about the field regions and attended one of his antenna measurement courses some time ago.

Near field regions contain circulating H fields (and E fields) which interact with other conductors in their vicinity. If one antenna is being driven with a current sinusoid at the same time as the other antenna is being driven with the same sinusoid (amplitude and frequency and *phase*) then the one driven antennna's fields align with the other antenna's fields.

Rather like having a push-pull amplifier, both antennas are driving energy into their fields in phase and in equal amplitude with each other. The inpedance seen from one antenna to the other is thus high. If you adjusted their drives to be out of phase, their isolation would worsen. We remember that the phase of the current which flows in the centre of a dipole is no longer in phase with the lower amplitude of current towards the end of the dipole, for each dipole. The currents are a vector in that they have magnitude *and* phase *and* direction, and are thus sometimes known as dyads (three values of "sense" rather than magnitude and direction only which is a vector, like velocity = speed and direction).

Whilst we might not consider driving both antennas to do the measurement, there are nonetheless currents flowing in both even if you do some S21 measurements on a VNA. The driven antenna induces alternating currents in the coupled antenna which resonates and the above qualitative argument can still be used to inform the situation. As you move one away from the other, the phase and amplitude of the coupled current changes and hence the isolation changes.

The *polarisation* is yet another aspect. the orientation of the E and H fields means that when they are co-polar the currents flow in the two dipoles and are coupled closely to one another (and all of the above still applies). The length of parallel conductor is large (half a wavelength) and coupling is much like a microstrip or coaxial transmission line coupler. If you now orientate one antenna to be orthogonal to the other the coupling must reduce because the mutual inductance (for example) is lowered and so is the mutual capacitance. when they are co-polar you effectvely have two parallel plate capacitors (sort of, but in the form of a rod or wire structure) and hence tight coupling occurs compared with turning one of those plates edgeways on.

Polarisation *is* a far field effect but it is the near fields themselves which give rise to the signal power which can be resolved in the far field region. The near fields are still present at great distances from the antenna, well into the far field and extending towards infinity, but they have diminished in magnitude to such a huge extent as to only leave the far field component at all detectable. You could say that the near fields have vanished for all practical intents and purposes.

In other words, it is not the effect of doing something in the far field which changes the isolation (that's how it manifests itself of course), rather it is the near field coupling which effects the isolation and those near fields give rise to far field effects (polarisation in this case).