DRAFT Recommended Practice for Measurements and ...

1/29/98 81 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
(2) The perturbing object (passive reradiator) may have any scattering cross section,
i.e., its size can be much greater than several wavelengths at the frequency being
measured. Small scatterers will introduce lower measurement errors.
(3) The dipole electrical length is less than or equal to 0.4 wavelengths tip-to-tip.
(4) The distance between the probe and the perturbing object is greater than 20 cm
at 300 MHz (0.2 wavelengths), and greater than 2 cm (0.2 wavelengths) at
3000 MHz.
Tell [E] described a test designed to evaluate the possibility that a measurement probe
might, when placed very close to a strong localized RF source, capacitively couple to the
source, thereby altering the source characteristics and, thus, the fields being measured.
In these tests, the electric field near one end of a re-radiating resonant rod was
measured with one instrument while other probes were brought near the opposite end of
the resonant rod. Any interaction of the probe with the re-radiating rod could conceivably
be manifested as changed readings of the field at the other end of the resonant rod. Of
the probes used in these tests, only the magnetic field probe and one electric field probe
produced any observable change in the electric field being monitored at the other end,
and even in this case the effect was minimal. Data related to the magnetic field probe
that produced the greatest interaction are summarized in the table.
Table 5.2 Effect of Probe Interaction with Resonant Dipole Antenna at 144 MHz
Caused by the Presence of a 10 cm dia. Isotropic Magnetic-Field Probe (from Tell
[E])
Distance from Probe Surface to Rod (cm) Change in Square of Electric Field Strength
(%)
20 0
10 0
5 0
4 1.2
3 1.2
2 3.5
1 7.1
0 17.6
These data show that only when the probe surface was placed in contact with the
resonant rod was there sufficient coupling between the probe and the rod to significantly
change the electric field strength at the other end of the rod. In the case of the magnetic
field probe tested, it was observed that the probe has loop windings on its spherical
surface. Hence, the windings were essentially placed in direct contact with the source
and, consequentially, there was no distance isolation afforded as there was with the other
probes tested evaluated.
In the evaluation of a broadband isotropic electric field with a 40 GHz response, direct
contact between the probe housing and one end of the re-radiating rod resulted in a
maximum change of 2.2 percent in the field observed at the opposite end of the rod.
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subject to change.