Using an Anritsu Sitemaster to tune a Magnetic Loop Antenna

The Anritsu Sitemaster family of measuring equipment is used a lot at cell phone sites, and more general, at places where transmitters are operating.

When measuring antenna circuits with a Sitemaster it sweeps the selected frequency range with his signal generator. At the antenna this signal,  together with the RF picked up from the surrounding transmitters, is returned to the Sitemaster and processed. To avoid erroneous results due to the surrounding RF the frequency of the Anritsu signal generator is varied a bit around every measuring point.  This will spread any RF coming back from the surrounding transmitters in the measurement result. This is a technique which works fine with antenna's where the Bandwidth is a lot larger then the jitter in the RF signal frequency, i.e. yagi's, phased array's etc. etc.

Now a Magnetic Loop is a very high Q antenna with a bandwidth of tens of khz. Then this jittering technique does work against you since you are sweeping over the resonance frequency.

The Anritsu manual lists very little about this frequency variation.

FIXED CW — Toggles the fixed CW function ON or OFF. When ON, F1 is

set equal to F2 and a CW frequency is fixed at the selected value. When OFF,

a narrow band of frequencies centered on the selected frequency is generated.

Turning Fixed CW on produced the expected result (Plot to be added)

Thanks go to Theo de Bont for moving my head in the right direction when trying to understand this behaviour.