The dual-gate MOSFET amplifier circuit (Q1), selected with S1 in the “out” position is designated the HF preamp circuit, due to the exceptionally low noise level of its operation. This is ideal for enhancing weak CW-SSB signals. This amplifier performs the true “active antenna” function of the AA7.
The 2SC2498 amplifier circuit (Q2), selected with S1 in the “in” position, is designated the VHF preamp circuit, due to the excellent VHF (very high frequency) and microwave (baked-potato frequency) response characteristics of the 2SC2498. With the on-board whip antenna adjustable for peak resonance throughout the VHF- UHF region (Wavelength [ft.] = 234/ f [MHz] - terribly long for a baked potato), this mode of the AA7 is ideal for indoor, outdoor and portable use of VHF scanners and other receivers.
Both modes are usable for HF, while the 2SC2498 offers higher gain but also a higher noise level. The user can easily choose either amplifier for copying any signal of interest. The front panel gain control (R5) is usable with either amplifier.
ACTIVE ANTENNAS: CIRCUITRY WITH A PURPOSE:
There is more to a true active antenna than any old preamp or preselector hooked to a piece of wire.
The AA7 was designed with reference to research carried out by Ulrich L. Rohde and T.T.N. Bucher, who co-authored Communications Receivers: Principals and Design (New York: McGraw-Hill Book Company). Rohde also shares credit for the highly sophisticated Rohde-Schwarz Type HE010 10KHz-80MHz Active Antenna. The book illustrates how proper design of a broadband active antenna can give receiver performance very comparable to a quarter-wave vertical ("passive antenna") for a given frequency. The book's footnotes cite considerable study by professionals on the optimum design of active antennas.
A short rod antenna, such as the whip supplied with your AA7, "receives" signals from the whole radio spectrum. Hence if you were to connect it directly to the antenna connection of your HF SWL receiver or antenna connection, you may not be impressed. The reason is not so much that the antenna is too short, or not resonant, but that its performance is all but shorted-out by the receiver, which is “looking for” a more conventional antenna.
A well-designed broadband active antenna considers field strength of the desired signal (measured in microvolts per meter of antenna length), atmospheric and other noise, diameter of the antenna, radiation resistance and antenna reactance at various frequencies, plus the efficiency and noise
