Suppose you have two scenarios (all using identical with negligible radiated field to the back of the antenna, ignoring splitter losses and phasing, considering the basic relationship of field and power as ). The relative field on any azimuth is defined as the ratio of the field on that azimuth to the field on the major lobe.

Case A: Two antennas, one pointed north, one pointed south. The north-pointed antenna has an input power of 66 Watts, the south antenna has 33 Watts.

Case B: Three antennas, two pointed north, each with 33 Watts input, one pointed south with 33 Watts input.

The calculations are based on the idea of adding fields (which are vectors) instead of powers. The cumulative effect of adding two antennas requires the addition of fields (which are vectors) rather than powers on each azimuth. (Of course, to fully represent the antenna, the fields would have to be added as true vectors, considering the phase of each component. Detailed phase information about the individual antennas is often not available, so the simple arithmetic addition of the field magnitudes is used as an estimate of the field on each azimuth.)

Note that adding powers would not give the same results. The above calculations are based on first computing a field value on a given azimuth from each antenna (and its associated power) and then summing the fields, as in:

The alternative would be to first sum the power on a given azimuth for each antenna, them compute the corresponding field, which would yield a different result, as in:

In Case B, the fields of the two north-pointed antennas are computed from the power and then added together. Comparing Case A and Case B, the input power pointed north (66 Watts) and south (33 Watts) gives two different relative field values for the minor lobe in the two cases.

This would also mean that the major lobe field for Case B (two antennas) would be 3dB higher than for Case A (one antenna): . Would this then imply that the ERP in Case B is also 3dB higher (since the field is 3dB higher), so the ERP in the two cases would be different. Thus the Effective Radiated Power of two identical antennas increases 3dB over a single antenna with the same total input power.

I'm interested! Please click here to jump to an online form which helps us better understand your needs. Then we will be able to respond to your request with information that is most useful to you.

Search SoftWright Website