ETS-Lindgren 3140B Hybrid Log Periodic and Bowtie (BiConiLog) - Manual (Page 8)

The unique feature of the Model 3140B is the T bow tie elements. A T bow tie
increases the equivalent dipole electrical length, thereby decreasing resonant
frequency and increasing efficiency in the 20 MHz to 60 MHz range. Similarly, a
regular bow tie has a lower resonant frequency than an equal length single-wire
dipole. The T bow tie has the first resonance at a frequency where the length is
about 0.22
λ
, a regular bow tie at a length of 0.3
λ
, and a tuned dipole at about
0.48
λ
. Thus at 50 MHz the 1.4 meter long T bow tie of the Model 3140B behaves
like a 2.8 meter tuned dipole. Cross-polar radiation is minimized because current
flow on one of the T end frames is almost exactly cancelled by the
oppositely-phased current on the other T end.
The standard self-balun feed of the log periodic also provides a matched
balanced feed to the bow tie elements. To prevent cable pickup below 100 MHz
and to improve matching to the bow tie elements, the Model 3140B contains a
balun transformer which acts as a common-mode choke to keep unbalanced
current off the coaxial feed cable outer shield, as well as adding some additional
inductance to improve impedance matching to the bow ties. Even though the
Model 3140B is highly balanced (symmetry +/- 0.5 dB), in vertically polarized
measurements cable position can effect results, so it is recommended that the
cable be suspended horizontally back from the antenna at least one meter before
any vertical drop. Below 150 MHz bow tie radiation dominates with a dipole-like
pattern, while above 150 MHz the radiation in the plane of the elements is
directional.
The Model 3140B is designed only for immunity testing. The large size
of the antenna makes it impractical for emissions testing where height
scanning is required, and the bow tie end plates increase the
measurement uncertainty when the antenna is polarized vertically. For
that reason, individual calibrations are not provided for the
Model 3140B.
8 | Introduction