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Sometimes receiving antennas, such as those used in EMC testing, may be placed so close to the source that they are influenced more by the near field effects than the far field radiation. In this case, the receiving and transmitting antennas are coupled by capacitance and mutual inductance.
Why does the current in an antenna flow in the same direction?
The current in each part of the antenna flows in the same direction. Such current is called antenna mode current. This condition is special because it results in radiation. As the signal applied to the two halves of the antenna oscillates, the field keeps reversing and sends out waves into space.
How are antennas made to pick up electromagnetic fields?
Depending on the type of field being detected, the antenna takes on a particular construction. Antennas designed to pick up electric fields, like the antenna of Figure 1(a), are made with rods and plates while antennas made to pick up magnetic fields, as in Figure 1(b), are made from loops of wire.
How is cable loss measured in an antenna?
Cable Loss can also be measured using the Return Loss measurement available in a cable and antenna analyzer. For this measurement, a precision short circuit is placed at the end of the cable. A signal is then sent down the cable. When the signal is reflected back, the energy lost in the cable can be computed.
Can a short antenna be tuned with capacitance?
So, if you want to tune a long antenna with capacitance or a short antenna with inductance that’s fine but be aware of how the resitance (a projection of the resistance of free space) will change significantly and how the cycle repeats itself at 3/4 of a wavelength.
Can a dipole antenna be a capacitive load?
In many circumstances it is not possible to make a dipole antenna that has the optimum length of λ / 2 (or monopole with ground plane of length λ / 4 ). Car antennas are for example shorter than they should be, which would result in an antenna that behaves like a capacitive load.
How is the length of an antenna dependent on the frequency?
Since the wavelength of a signal is dependent on the frequency, at certain frequencies the antenna length is equal to key fractions of a wavelength. The current on a dipole for frequencies resulting in ½ and 1 wavelength is shown in Figure 5 (a) and 5 (b), respectively.