measured between the two ends of the wire with the expression describing the output
voltage given by Equation 3-110. In practice, since the antenna is constructed from
coaxial cable, it is convenient to use coaxial cable connectors (such as BNC or SMA
connectors) and measure the voltage difference across the connectors at the two ends
of the cable. The output voltage would then be the difference between the measured
voltages across the connectors at the two ends of the cable.
Vout(t) = (vic (t) Vol (t)) (Vic2(t) Voc2(t)) (3-111)
The subscripts ic and oc refer to the inner and outer conductor, respectively. The
subscripts 1 and 2 refer to the two ends of the coaxial cable. This configuration is the
basis of a differential output coaxial loop antenna.
However, this configuration poses a problem since Equation 3-111 can be
rearranged to yield
Vout(t) = (Vicli(t) Vic2(t)) (Vocl(t) Voc2(t)) (3-112)
As mentioned previously, the induced voltage on the inner conductor and the
outer shield are almost identical, therefore the output voltage of the differential coaxial
antenna, as shown in Equation 3-112, would be close to zero. However, if the shield
from the two ends of the cable is soldered together at the output of the antenna, then
Vocl(t) will be equal to Voc2(t) and Equation 3-112 reduces to
Vout(t) = Vic (t) ic2(t) (3-113)
This is simply the voltage difference between the two ends of the inner conductor
of the coaxial cable. Although soldering the shield together at the base of the antenna
alleviates one problem, it introduces another. Once the shield is soldered together, the
outer shield of the cable forms a closed loop. If any current is induced on this loop by
either an external electric or magnetic field, an unwanted magnetic field will necessarily