TransferJet Antenna / Coupler

- details of TransferJet antenna or coupler and the way in which it acts as a longitudinal electric induction coupler.

TransferJet is a near field form of data communications.

As such the antenna or coupler needs to operate in a manner that will enable the form of communications required to take place.The TransferJet antenna or coupler has to ensure that a link can be reliably set up over a short distance, but not radiate or be susceptible to other devices a little further away.

Normally antennas are designed to give the best operation over the greatest distances. This is not the case with TransferJet where the range needs to be limited so that security is maintained, and also interference is limited. When separated by even a few centimetres, the devices should not respond, not even detect each other. Limiting the range in this way is a key attribute provided by the TransferJet coupler or antenna.

TransfetJet coupler / antenna concepts

In order to be able to ensure that the TransferJet coupler limits communication to just short ranges, the correct type of field must be excited.

There are two types of field that are available. There is what is sometimes termed the far field effects. These are the electromagnetic waves that are in everyday common use by everyone. The field intensity from these ways falls with the inverse of the range. This far field effect is the conventional TEM or Transverse Electro-Magnetic wave which consists of both electric, E, and magnetic, H, elements combined within the single wave. Real power is transmitted by this wave from one place to another.

However there are also what are termed near field effects. This field is inductive. It does not radiate power as the TEM wave does but instead it stores power within the near field. The inductive power is only dissipated if another TransferJet coupler appears in the near field.

The longitudinal component of this near field is utilised because it is not polarized. This is important because it means that the two TransferJet devices do not need to be aligned in any particular orientation. This particularly importat where devices may be randomly orientated, and had the far field TEM wave been used, it would be an aspect that would need to be addressed, because it is polarised.

TransferJet antenna coupler implementations

One key element of any TransferJet coupler is that it must be small. The antenna or coupler must be able to fit easily within any enabled product and as many products will have a very small outline, this will place stringent requirements on the antenna / coupler.

One of the many advantages of frequencies as high as 4.8 GHz is that wavelengths are very small and as a result antennas can be made much smaller.

Antennas are being manufactured by a number of companies and some have dimensions as small as about 5 x 5 x 2mm with the height above the board being the smallest dimension.

The antenna bandwidth is also important. The TransferJet antenna must have a sufficiently wide bandwidth to ensure the complete signal is transmitted. An insufficient bandwidth will result in the loss of data.

The antenna must be able to cover 4.2 - 4.76 GHz as this will accommodate the centre frequency of 4.48 GHz and the signal bandwidth of 560 MHz. A flatness of better than 3dB is required although many antennas are easily able to meet this specification.

By Ian Poole

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