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Radiation Polarisation page 1

1.1) Introduction

  It is generally accepted in most antenna text books that simple antennas, such as monopoles, produce radiation that is electrically polarised either in the vertical or horizontal plane depending on the orientation of the transmitting antenna. It is also generally accepted that there can be as much as 20 dB increased loss between antennas with one orientated vertical and the other horizontal. It is therefore, important that the transmit and receive antennas have related polarisations in order to ensure the maximum gain. One polarisation of interest is the Helical antenna which has a basically circular polarisation but can be clockwise or counter clockwise depending upon the direction the helix is wound. Again it is claimed that using opposite helical types can give up to 20 dB increased loss compared to using matching types, but there is little attempt to explain the mechanism that causes this. This article will investigate the traditionally accepted polarisation of monopoles and question if the convention of electrical polarisation is adequate for modern antenna studies. The results of these studies should enable engineers to understand the polarisation of the radiation around a monopole and from this deduce the polarisation around a helical antenna and then appreciate why matching opposite types will give an appreciable loss of signal.

  As charged particles (electrons) pass along a conductor the poles of the electrons line up as is describes at and this fundamental statement of alignment can be found in several other articles on the Internet, although there does not appear to be a single article that defines the correct orientation of the particles in relation to current flow. This process of particle alignment is the cause of the conductor becoming magnetised in one particular direction when a dc current is passed and as will later be proven within this article, of any resulting radiation from a single current in a single plane being polarised in one particular direction. The alignment of the poles of the charged particles in a conductor has to be with the magnetic poles in line with the direction of current flow. If the electrons were aligned according to their electric fields, there would be many possible magnetic orientations and the total magnetic field could possibly be in any direction regardless of the direction of current flow. It is even possible that there would be random alignment of magnetic fields of the charged particles which would result in virtually no total magnetic field. It has already long been proven by Maxwell that this is not the case and there is a sum magnetic field produced from a direct current which is always oriented in relation to the direction of current flow and follows Maxwell's right hand grip rule.

As we have already proven in the antenna current distribution article in relation to a well grounded monopole, there is more current flowing in the INCIDENT direction than in the REFLECTED direction. This means that the predominant polarised radiation will originate from the INCIDENT current and will be represented by the sum radiation measurement. We do not have to consider at this point if the radiation from the INCIDENT and REFLECTED currents produce opposite polarisations.

2.1) Establishing if Upwards and Downwards Radiations Cancel

  One simple method of establishing if an upwardly polarised antenna radiation can cancel a downwardly polarised radiation is to have two antennas mounted pointing in opposite directions i.e. diametrically opposed with the ends close together. This ensures that ground planes are at the furthest distance and that each ground plane is mainly effective for it's own antenna and does not significantly affect the opposite antenna. The two antennas are fed with identical phases thus ensuring that there is no electrical phase cancellation.

2.2) Diametrically Opposed Antennas Measurement Test Circuit

The test equipment is connected as per figure 1 using three grounded antennas that are matched to the same frequency.

Figure 1. Measurement Configuration

[Drawing of test set up]

  The vertical length of coaxial cable to the Upper Tx Antenna (labelled A to B) has to be physically placed away from both the Tx antennas to ensure that it does not interfere with the measurement. The cables from the splitter must be equal lengths and of similar type cable in order to ensure that the antenna input levels are similar and are in phase. The 0° Splitter may be a 3 dB transformer type or a 6 dB resistive type. The amplifier is used to bring the transmit level above any external interference with the levels set such that the amplifier is not driven into compression and the power rating of the splitter is not exceeded.

2.3) Sample Antennas

  Three simple monopole antenna are used and there should be ground planes that are sufficient to ensure that each antenna is well matched to the 50 Ohm coaxial cables. The aim is to ensure that the current distribution in both Tx antennas is similar to that obtained in the previous antenna current distribution measurements. Thus, the upper antenna will have mainly a downward current flow and the lower antenna will have mainly an upward current flow. Combined with the matching cables, the radiation generated from both Tx antennas will then have similar power levels and phase at their ends.

2.4) Measurements

The antennas are placed so the the Upper Tx and Lower Tx are inline with the ends close but not touching. The Rx antenna is located in the far field which is generally regarded to be at least three wave lengths. In order to prove that the measurements are valid and that the VNA gating times are correct, a measurement is first made in the near field with only the Lower Tx Antenna connected and the output to the Upper Tx Antenna terminated. A reading at the antenna matching frequency is taken and the near field gate time established. The Rx antenna is then moved to the far field position, the difference in distance from the Tx antenna measured and the new gate time established. A reading is then made with just the lower antenna connected and then the Upper Tx Antenna is connected and a further reading taken.

3.1) Diametrically Opposed Antenna Results

  In this case, with the lower antenna already connected, when the upper antenna was also connected, the level dropped from -43.4 dB to -56.6 dB showing that there was a significant cancellation. This confirms that the upward and downward antennas have two different radiation polarisations as the cancellation cannot be due to radiation phases or levels which are identical at the antenna ends where the maximum radiation is present. It is apparent therefore that there is an upward and downward polarisation difference and the classical convention of there being simply vertical or horizontal electric polarisations may be too simplistic to enable polarisation to be fully understood.


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