To: File emi.wp6

From: Jon Hagen

Date: 5-26-99

Subject: EMI susceptibility from various locations

To determine the EMI shielding requirements for cabinets enclosing various drive system electronics, LO generators, monitoring equipment, etc, we must know:

a. the RF flux density emitted by the unit in question

b. the free-space propagation loss from the locations in question to the radio astronomy receiving horn. (assuming the interference enters by way of the horn).

The flux density emitted by the unit can be determined from a lab measurement using a probe antenna (e.g. a dipole) and a spectrum analyzer. With the probe placed, say, one meter from the unit, we find the position where the radiation is a maximum (i.e. worst case). Suppose the power in the strongest frequency component is measured to be P watts. The flux density at this frequency is then given by

Swatts/meter2 = P/Aeff = P/( 2G/4 )

where G is the gain of the probe antenna. (G=1.5 for a dipole).

The propagation loss from the equipment location to the receiver can be measured by locating a signal generator, together with the probe antenna, at the location in question. The generator power is set at a conveniently high level so that the telescope operator can determine the power received without any integration. The antenna is oriented to maximize the received power. The flux density, 1 meter from the probe antenna, is given by

Swatts/meter^2 = Pgenerator (G/4 )

Knowing the propagation loss, we calculate the value of emitted flux density that would inject ,say, -220 dBw into the receiver. (This is the ITU criterion for an interfering signal encountered during 1612 Mhz spectral line observations with a 20 kHz channel bandwidth).

Suppose the equipment in question is the control box for the tertiary drive system and that the propagation test shows that the flux at a distance of 1 meter from the control box must be less than -100 dBm /m2. If lab measurements have shown that the drive electronics, when unshielded, produce 107 times this level, the shielding box will have to provide 70 dB of isolation.

The ITU interference level is so low that it may be impossible to avoid violating it with our own observatory equipment. Consider the radiation level permitted from an interfering source located at the control room, approximately 300 m from the feed. At 430 MHz, the ITU interference level is specified to be no more than -200 dBW/m2 or -170 dBm/m2 ). At a distance of 1 m from the source of interference, the level becomes -170 + 20log(300) = -120 dBmW/m2. If we use a dipole antenna to sense the level one meter from the source, the power level would be -120 -12 = -132 dBm. (The effective area of the 430 MHz dipole antenna is .06 m2 or -12 dBm2). This level, -132 dBm, can be compared with a lab measurement of a 425 MHz signal leaking from the tertiary drive control box (the 17th harmonic of a 25 MHz clock oscillator for the microprocessor in the servo amplifier). At a distance of 1 meter, the level of the 425 signal was observed to be no lower than about -80 dBm.