A Classic Active Antenna for 0.5–30 MHz

Shortwave radio enthusiasts often face the challenge of weak or noisy signals, especially in urban environments. This Classic Active Antenna is a powerful solution designed to enhance signal reception across the 0.5 MHz to 30 MHz range. This blog post explores the working of this circuit and guides you on building your own version for effective SWL.

                                                                                             Fig1 – Classic Active Antenna for 0.5–30 MHz

Circuit Overview

The Classic Active Antenna circuit consists of a FET front-end amplifier and a bipolar transistor buffer stage. This two-transistor arrangement forms an active antenna amplifier, converting a high-impedance signal from a short whip into a low-impedance 75-ohm coaxial output suitable for most shortwave receivers.

Key Components:

• Q1: 2N5485 (FET)
• Q2: 2N4121 (BJT)
• Power Supply: 18–24V battery or DC source
• Inductors: RFC1 – 680 µH, RFC2 – 560 µH
• Coax Output: 75-ohm impedance for standard receiver input

The Core Concept: A Two-Stage Amplifier

At its heart, the Classic Active Antenna is a buffer amplifier connected to a short rod antenna. The goal is to take the very high impedance of the antenna and convert it to the standard $75ohms$ impedance that your radio receiver expects. This is achieved with a two-transistor design:

• Q1 (a JFET like the 2N5485): This transistor provides a very high input impedance. This is crucial because it allows the amplifier to connect to the antenna without “loading” it down and losing precious signal strength.
• Q2 (a BJT like the 2N4121): This transistor is configured to provide a very low output impedance.

The Signal Path and Amplification

Signals picked up by the aerial are coupled through capacitor C1 and resistor R1 to the gate of transistor Q1. This input stage is essentially untuned, meaning it’s a broadband amplifier, ready to receive a wide range of frequencies.

An interesting feature is the feedback loop from the collector of Q2 back to the source of Q1. This design choice improves the amplifier’s linearity and limits its response at the top end, preventing unwanted behavior at very high frequencies (UHF). Resistor R4 is also included to suppress any parasitic oscillations that might occur.

Clever Powering Over Coax

One of the most elegant parts of this design is how it’s powered. The DC supply voltage is sent to the remote antenna unit using the same coaxial cable that carries the radio frequency (RF) signal back to the receiver!

• At the Receiver End (Fig. 2): A Radio Frequency Choke (RFC2) lets the DC voltage pass up the coax to the antenna but blocks any RF signal from getting into the power supply.
• At the Antenna End (Fig. 1): RFC1 does the opposite. It presents a high impedance to the RF signal, directing it into the amplifier, while providing a low-impedance path for the DC voltage to power the transistors.

Capacitors C2 and C3 are used as DC-blocking capacitors, ensuring the DC voltage stays where it’s supposed to and only the AC signal (the radio waves) travels to the next stage.

This is a fantastic example of efficient and clever electronic design, creating a high-performance active antenna that has been a favorite of hobbyists for years. Happy listening.

Performance and Application

This active antenna performs well for broadband shortwave reception. It is especially useful in situations where long wire antennas aren’t practical. The short whip combined with high gain amplification ensures even weak signals are clearly received.

Best used for:

• Shortwave listening (SWL)
• DXing in noisy environments
• Portable or urban receiver setups

Alternatives for Q1 (2N5485 N-Channel JFET)

The most important characteristic of Q1 is its high input impedance, which is typical for a JFET (Junction Field-Effect Transistor). It’s used as a high-frequency amplifier. When looking for a substitute, you want an N-channel JFET with similar characteristics.

Top Recommendations:

• • 2N5484: This is a very close relative and often interchangeable. It has slightly lower gain but will work well in this circuit.
  • BF245A/B/C: This is a popular and widely available family of N-channel JFETs designed for VHF/UHF applications. They are an excellent replacement.
  • MPF102: A classic “utility” N-channel JFET that is often cited as a suitable replacement for many similar transistors. It’s a great choice if you have one on hand.
  • J113: Another common N-channel JFET that can be used here

Alternatives for Q2 (2N4121 NPN Transistor)

Q2 acts as a buffer to provide a low impedance output. It’s a general-purpose NPN Bipolar Junction Transistor (BJT). Finding a replacement for this is much easier, as many common NPN transistors will work perfectly.

Top Recommendations:

• • 2N3904: This is perhaps the most common and widely available general-purpose NPN transistor. It’s an excellent, all-around substitute for the 2N4121.
  • 2N2222A: Another extremely common and robust NPN transistor. It can handle more current than the 2N3904 and will work flawlessly in this circuit.

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