Active Antenna Modification for VLF Reception (Early Models)

Note: This is NOT required or recommended for active antenna kits purchased after Dec 2003 as they will support reception down to 10 kHz as constructed
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.
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