Active Antennas Circuitry with a Purpose

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

AA7 • 5

AA7 specifications

Ramsey Electronics has long been a name synonymous with quality in the realm of amateur radio equipment and communication technologies. Among its standout products is the Ramsey AA7, a high-performance audio amplifier that encapsulates advanced features and specifications aimed at both hobbyists and professionals in need of superior sound amplification.

One of the main features of the AA7 is its robust power output. Capable of delivering up to 7 watts of audio power per channel, it is an ideal choice for driving speakers across various settings, whether in a home audio system, workshop, or small public address system. This level of power ensures that users can experience clear and impactful sound, with performances that fill the room without distortion.

The AA7 employs Class AB amplifier technology, which strikes a fine balance between high fidelity and efficiency. This design is particularly beneficial as it provides reduced heat generation when compared to Class A amplifiers while still retaining audio quality. The result is a unit that remains cool to the touch during extended use, allowing for reliable and consistent sound performance.

Another key characteristic of the AA7 is its wide frequency response, which spans from bass-heavy lows to crisp highs. This extensive range allows for versatile applications, accommodating a variety of music genres and other audio formats without compromising quality. The built-in tone controls allow users to fine-tune the sound to their liking, offering adjustments for bass, treble, and overall volume to ensure an optimal listening experience.

User-friendly design is also a hallmark of the AA7. Its compact size allows it to fit seamlessly into any audio setup, while the straightforward front panel controls make it accessible even for those unfamiliar with audio equipment. The amplifier supports multiple input sources, making it versatile for connecting a variety of devices such as CD players, smartphones, and other audio sources.

In conclusion, the Ramsey Electronics AA7 stands out as a powerful and efficient audio amplifier that combines superior technology with user-friendly design. Its impressive power output, emphasis on sound quality, and convenient features make it a valuable tool for anyone looking to enhance their audio experience, whether at home or in a more public setting. Whether used by hobbyists or professionals, the AA7 offers a compelling solution for high-quality sound amplification.