Radar Basics

Typical Search Radar Loss Budget

Every radar system has got miscellaneous losses. Some of these are preventable, or at least reducible by a well-designed radar. Some losses can even be minimized by maintenance.

But unfortunately, most of these losses are inevitable. The sum of losses in Table 1 is declared very hard width the value of 21.1 Decibels. Well designed radars have a fairer loss of about 13 to 15 Decibels mostly.

Atmospheric Losses

These are losses due to atmospheric absorption by the atmosphere. They are dependent upon the radar operating frequency, the range to the target and the elevation angle of the target relative to the radar. These losses are insignificant at low frequencies less than 3 Gigahertzes by clear weather conditions.

Beamshape Loss

This loss term accounts for the fact that, as the beam scans across the target, the signal amplitudes of the pulses coherently or non-coherently integrated varies. Because of the Beamshape Loss, the full integration gain of the integrator can’t be realized. From the Skolnik Radar Handbook typical values are:

• 1.6 dB for a scanning, fan beam radar
• 3.2 dB for a thinner beam, scanning radar
• 3.2 dB for a phased array radar wherein the beams of a search sector overlap at the 3-dB beam positions.

For phase array radars the beam doesn’t move continuously (in most cases) but in discrete steps. This means that the phased array radar may not point the beam directly at the target. This means, in turn, that the antenna gain used in the radar range equation will not be its maximum value. As with the other cases, these phenomena are accommodated through the inclusion of a loss term called, in this case, beam shape loss.

Beamwidth factor

The azimuth beamwidth of radar antenna has not the same value in all elevation angles. This is summarized in an additional loss factor.

Fluctuation Loss

This relatively high loss is a result of the fluttering in the values of radar cross-section. The gaps are frequency depending!

In order to overcome some of the target size fluctuations, many radars use two or more different illumination frequencies. Frequency diversity typically uses two transmitters operating in tandem to illuminate the target with two separate frequencies.

Miscellaneous Signal Processing Loss

If the radar uses an MTI with a staggered PRF waveform, and a good MTI and PRF stagger design, it will suffer up to 3 dB signal processing loss.

Transmit Line Losses

Typically associated with the waveguides and other components between the power amplifier and the antenna. These are typically 1 to 2 dB in a well-designed radar.

Receive Losses

Typically associated with the waveguides and other components between the antenna and RF amplifier. These are also typically 1 to 2 dB for a well-designed radar. If the noise figure is referenced to the antenna terminals, receive losses are included in the noise figure.