As
supplied, our active antenna described in PE magazine Aug 1997, will cover
100 KHz to 30 MHz. While this frequency range is adequate for most SWL
use, some SW receivers cover frequencies down to as low as 10 KHz. In
this 10-100 Khz range also lies WWVL on 60 KHz, and reception of this
frequency is desirable for use as a frequency standard. The low frequency
limit of the active antenna is dependent not so much on the length of
the pickup wire (24 inch typical), but on the characteristics of the toroidal
transformer used (T1 in the schematic.) The inductance of T1 sets the
amplifier low frequency limit. However, for obvious reasons the physical
size of the core must be limited to fit into the preamp assembly. The
fact that DC current runs through the transformer causes core permeability
to drop, lowering the effective inductance of T1. If no DC was present,
the core would have an effectively higher permeability and hence higher
inductance. Reducing the collector current of Q2 will help but also reduce
large signal handling capability. A higher permeability core is of dubious
value as it would be more likely to saturate. At medium and high frequencies
this is not important as the transformer acts as a transmission line transformer
and the core is not a very significant factor. There is an easy way out
of this dilemma. Referring to the schematic, Note that DC is fed through
the tap on the transformer and primarily flows to the collector. This
is two thirds of the total turns. A small amount also flows through the
other third, but this is mainly the drain current of Q1 (2 to 4 ma). The
magnetizing force (H) expressed in ampere-turns in the core is proportional
to the current in the winding times the number of turns, and the flux
per unit area (B = U x H) is the product of the magnetizing force and
the permeability of the core. The total flux F equals the core area times
B. Since the inductance is defined as the number of flux linkages per
ampere of current, the coil inductance L is NF/I. Since F is proportional
to U, L is proportional to U. However, U decreases due to the DC in the
winding. If the effective magnetizing force can be reduced, we can increase
L and therefore get better LF response. Note that if the current flowing
through the side of the transformer T1 feeding Q1, R1, and R3 was twice
the collector current of Q2, the effective magnetizing force would be
cancelled since this current would flow through half the number of turns
and in a direction opposite to the collector current of Q2. This can be
accomplished by reducing R2 and R3 to 33 and 150 ohms respectively so
they draw 60 ma. This is inefficient and wastes DC power, but increases
the LF performance. In lab tests, the lower 3 DB point of the amplifier
was improved to 10-20 KHz depending on the core and normal spread in collector
current of Q2. This makes the active antenna useable down to the lower
limit of the radio spectrum, generally taken as 10 KHz. Another way to
modify the antenna is to connect a 180 to 200 ohm resistor from the plus
side of C2 to ground. The resistor R3 should be a 1 watt unit as R3 will
dissipate 600 mW. If the 180 or 200 ohm resistor is used it should also
be a 1 watt unit. If you want to get fancy, a LED or a 60 mA lamp could
be used and will serve as a power indicator if a suitable hole is drilled
in the preamp housing. With this modification R8 in the DC block should
be changed to 10 ohms, and R7 to 47 ohms. C4 can be increased to a 1 uf
unit to reduce signal loss at the lowest frequencies. Use at least a 25V
electrolytic, with plus side to R7 and J2, negative to J1. |