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Title: The YooFab™ Active Antenna Receiving Loop Kit Instructions Author: G.D. Cowne Description: Instructions for assembling the YooFab Active Antenna Receiving Loop Kit, with educational explanations to further electronic understanding. This knowledge-sharing aligns with YooFab's ethos. Date: 14.10.2019

YooFab Active Antenna Receiving Loop#


  • The RFLEGO™ Active Antenna Receiving Loop Kit, when assembled correctly, offers a sensitive HF Antenna that operates dominantly in the magnetic domain rather than the electrical. It is effective across the entire HF spectrum.

Electromagnetic wave

  • This module utilizes a 2.1 mm Power Jack for a 6 - 16V connection. Ensure the power supply is devoid of noise or ripple from the mains. For testing, a battery is recommended.

  • Its design offers a level of immunity to nearby electrical QRM (noise), making it ideal for urban dwellers, especially those in apartment complexes.

  • WARNING: This is a receive-only antenna. DO NOT TRANSMIT using it. Remember this if connecting to a transceiver.

  • The antenna's design allows for discreet operation, benefiting those restricted from installing large antennas. It performs exceptionally on terraces, balconies, or near windows, and is suitable for outdoor use due to its lightweight and compact nature.

  • The employed differential amplifier design has low impedance inputs, suitable for a 1-1.5 meter loop wire connection. It provides immunity to common mode signals, utilizing a unique common base low impedance input design advantageous for low impedance input loop wire. Designed by M0OOZ 2.

  • Gain: The amplifier boasts a voltage gain of about 20dB (10X). If this gain is excessive for your receiver, refrain from altering the Differential Amplifier circuit. Instead, use a 10dB attenuation pad at the coaxial cable's receiver end.

  • The antenna operates across all HF bands, functioning similarly to a Low Noise Amplifier (LNA).

  • Its compact nature ensures swift setup, enabling easy reception of HF HAM radio signals, akin to using a standard VHF radio antenna.

  • This antenna pairs well with most HF receivers, particularly direct conversion (DC) receivers, due to its immunity to emissions from the DC receiver's Local Oscillator, which can cause interference with end-fed long wire antennas.

  • High-quality components are procured from reputable EU and USA suppliers. All components are affixed to a single-sided, pre-drilled printed circuit board.

  • The PCB style is favored by many RF design professionals.

  • The loop, connected via two terminals, can be adjusted. Ideally, it should measure between 1 and 1.5 meters. Larger loops may not improve reception and might introduce instability and more unwanted signals.


  1. Power supply: The circuit uses a 6 - 16 Volt DC supply, connected via a 2.1mm Power jack. The center pin is positive.

  2. RF output: The Active Antenna Loop output employs an RCA socket.

  3. Wire Loop A & B: Both feature two Banana sockets for connecting a 1 - 1.5 meter wire loop. The kit provides galvanized wire, which can be molded into a loop. Though slender, this wire is rigid enough to maintain shape.

Circuit Diagram#

Below is a simplified version of the schematic circuit diagram. This representation is essential as unauthorized reproductions, like the "Yoo Loop!", have emerged in the market.

schematic RFLEGO™ Active Receiving Antenna Loop

Circuit Operation - A Brief Explanation#

RF signals are induced across the wire loop connected to the LOOP terminals. These signals are then relayed to the emitters of the differential pair amplifier.

Respect The Difference!#

Differential pairs predominantly reject common-mode signals, ensuring that identical signals on both inputs are dismissed.

The animation below illustrates a 5uV (p-p) input signal from the wire loop, which is amplified due to its differential nature.

YooFab Antenna Loop Differential pair amplifier

Notes On Simulation#

The animation above uses two 10 MΩ resistors grounded for simulation purposes only. They are not required in the actual assembly.

While simulations offer valuable insights, they represent an idealized environment, differing from real-world conditions.

No Signal!#

In the simulation below, an identical signal is injected into both differential inputs. Consequently, no difference is detected, resulting in no amplification.

YooFab Differential amplifier for loop antenna No amplified signal

Remember This When Testing#

Always recall the differential nature of this amplifier during tests. If a signal generator is connected to both sides, minimal to no output will be observed.

Common Base Differential Amplifier#

The amplifier circuit uses two NPN transistors, with their bases biased slightly below 2V (assuming a 12V supply; about 1V for a 6V supply). Each transistor has an 11 mA quiescent current. These specifications, combined with low impedance inputs, provide headroom for handling large signals, accommodating both weak and strong signals.

The Output#

Output is derived from a transformer, which converts balanced output to an unbalanced form suitable for 50Ω coaxial cables. Note the DC path to ground via the transformer's output winding. No capacitor coupling is present, assuming the receiver input contains a capacitor. If desired, add a capacitor, but maintain its value at 100nF or higher. The other side of the transformer is linked to the differential amplifier's collector load resistors through 100nF capacitors.


  1. The term "RFLEGO™" highlights the modularity and interconnectivity of an educational and functional system, an intellectual property of YooFab. YooFab™ and RFLEGO™ are not affiliated with the famous toy brick company that begins with "L" and ends in "EGO". 

  2. The circuit was designed by M0OOZ, a YooFab™ founder.