Ramsey Electronics AA7 - Manual (Page 5)

AA7 • 5
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