Schwarzbeck XSLP 9142 Dual Polarized Logarithmic Periodic Antenna

Assignment of Polarisation Planes

The XSLP 9142 has two orthogonal polarisation planes, which can be accessed independantly via N-connectors. The assignment of the N-connector to the corresponding polarisation plane is shown in the above picture. The Inner conductor of the N-connector stands always perpendicular on the corresponding polarisation plane. Example: Connector "B" should be used for horizontal polarisation. The antenna is rotated around its longitudinal axis until the elements assigned with "B" are horizontal. The connector "B" faces either upwards or towards ground.

Decoupling of the Polarisation Planes

Although the two polarisation planes are exactly orthogonal to each other, there is no 100% decoupling between them. A horizontal polarised field contributes also to the indication of the vertical polarised antenna section, but with reduced effect. Typical decoupling values for the cross polarised fieldstrength indication are 15 to 20 dB. The decoupling of the polarisation planes depends a lot on environmental reflections. Best decoupling results can be achieved under free-space conditions. The cross polarisation decoupling decreases at higher frequencies due to the element displacement at the antenna tip, which approaches to the element length itself.

Equality of Polarisation Planes

Small differences between the polarisation planes are recognizable for construction reasons. The feeding point distance to the active element is somewhat different. Normally the deviations between the planes are less than 1 dB, as worst case 1.5 dB difference can be assumed. Especially at higher frequencies and for best accuracy it is recommended to use the data which is explicitly assigned to the respective polarisation plane (Sections A or B).

General Hints

For highest accuracy requirements a suitable fixed attenuator (3 dB to 10 dB) may be useful under certain circumstances. Inserting a fixed attenuator at the antenna terminal improves impedance matching, but also reduces the gain and increases the antenna factor by the attenuation value. With an SWR < 2 the attenuator may be omitted in most applications. Antenna measurements in the microwave frequency range suffer from environmental reflections, which may even occur at nonmetallic surfaces as e.g. plastic. Therefore it is recommended to avoid large mast adapters and other big parts in the near surrounding of the antenna.

Antenna reference point

The center between antenna tip and longest element was used as antenna reference point during calibration. This leads to accurate results on most frequently used measuring distances. For measurements on short distances (< 0.7 m) the accuracy may be improved, if the actual position of the phase center is considered. The phase center position is located near the element in halfwave resonance. Example: The wavelength at 1 GHz is 30 cm, the corresponding element would be 15 cm long (the location of the phase center is the approx. 15.5 cm behind the antenna tip)