Introduction
MODEL 3140 BICONILOG™ ANTENNA
2
© EMC TEST SYSTEMS, L.P. – MARCH 2002
REV C – PN 399258
useful low frequency range of the typical LPDA's from
100 MHz down to 26 MHz. At 26 MHz, an efficient single
dipole type antenna must be over 5 meters long, whereas
suitable performance is obtained here with a 1.6 meter long
bow-tie. A simple wire outline bow-tie antenna is
narrowband compared to a sheet bow-tie or biconical, thus
struts are added to the Model 3140 bow-ties to better
simulate the broadband sheet bow-tie.
The unique feature of the Model 3140 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 20-60 MHz range.
Similarly, a regular bow-tie has a lower resonant frequency
than an equal length single-wire dipole. The T bow-tie has
its first resonance at a frequency where its length is about
0.22
λ
, a regular bow-tie at a length of 0.3
λ
, and a tuned
dipole at about a length of 0.48
λ
. Thus at 50 MHz the 1.4
m long T bow-tie of the Model 3140 behaves like a 2.8 m
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 3140 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
