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RFLEGO Active Antenna Receiving Loop#

YooFabRF™ Active Antenna Receiving Loop

M0OOZ - YooFab / CC BY NC ND

Main characteristics#

The RFLEGO Active Antenna Receiving Loop Kit, once correctly built, will provide a highly sensitive HF Antenna that should operate more in the magnetic domain than it does in the electrical domain. It should be effective across the whole HF spectrum.

Electromagnetic wave SuperManu / CC BY-SA

WARNING: DO NOT USE THIS ANTENNA TO TRANSMIT. This is strictly a receive only antenna.

  • This type of receiving antenna has a certain immunity to near-field electrical QRM (noise), which is a clear advantage for those who live in urban areas and particularly for those who live in apartment buildings.

  • This antenna permits stealth operation. Many radio enthusiasts live in places which prohibit the installation of large antennas. This antenna provides excellent results when used on a terrace, balcony or even near a window. It is also great when used outdoors, as it is light and small yet provides great reception.

  • The differential amplifier design employed here has low impedance inputs to which the 1-1.5 meter loop of wire is connected. Please note that this differential amplifier provides a certain immunity to common mode signals. The circuit is based upon a differential pair that has an unusual common base low impedance input design which is an advantage here as the input loop wire presents a low impedance. (Designed by M0OOZ)

  • Gain - The amplifier has a voltage gain of approximately 20dB (10X). This is more than sufficient. If the gain proves to be too much for your receiver, do not modify the Differential Amplifier circuit. Instead, place an attenuation PAD of 10dB at the receiver end of the coaxial cable.

  • This antenna should operate across all HF bands and is a form of Low Noise Amplifier (LNA)

  • Its compact size makes possible a rapid setup. You can then receive HF HAM radio signals with the much same ease as the use of a typical antenna fitted to a VHF consumer type radio.

  • This antenna is great for almost any type of HF receiver. However, it is especially useful for direct conversion (DC) receivers, as it has a certain immunity to emissions sourced from the DC receiver's own Local Oscillator. Such emissions may cause hum when received by a DC receiver on an end fed long wire Antenna (Zeppelin).

  • The LOOP connected to the two terminals is something that can be experimented with. It should be between 1 meter and 1.5 meters in length. A bigger loop will probably not help with better reception and instead may lead to instability and reception of more unwanted signals.

Component List#


M0OOZ - YooFab / CC BY NC ND

The color / style of some hardware components like pins or connectors may vary.

Warning: there are 3 transistors on the picture.

Name Value Quantity Notes
C1, C2, C3, C4, C5, C6, C7 100nF 7 disk ceramic capacitor
C8 100uF 1 electrolytic capacitor more than 25 V
D1 led 1
FT37-43 10T BIFILAR 1
Q1, Q2 2N2222 2 NPN Transistor
R1, R2 120Ω 2 Resistor
R3, R4, R9 1K 3 Resistor
R5, R8 4.7K 2 Resistor
R6, R7 270Ω 2 Resistor
Printed Circuit Board 1 Vinyl protected copper
35 mm stand-off 4 Black plastic
M3 pernos 4 Black plastic
packing ring 5 mm 2 to put around the led
RCA (Phono) connector 1 Gold Plated
Banana connector 2
Power jack 2.1 mm 1
Red and black cable 2
Galvanized wire 1 m length
Sticker 1 To put on the PCB
Polystyrene piece 1 To organize your components

Circuit Diagram#

active antenna loop diagram YooFabRF ™

M0OOZ - YooFab / CC BY NC ND

Circuit Operation#

La señal de RF se induce a través del loop de cable, que está conectado a los terminales LOOP que se ven en el esquema. La señal se acopla luego a los emisores del amplificador par diferencial.

Respect The Difference!#

El par diferencial es en gran medida inmune a las señales de modo común, de modo que cuando aparece la misma señal inyectada en ambas entradas, debe rechazarse.

Below we can see the animation of a 5uV (p-p) input signal from the wire loop. This signal is amplified because it appears to the differential amplifier's inputs, such that as one side becomes positive, the other side becomes negative. The 5uV signal appears on the left side coloured Green and on the right side coloured Blue.

Amplificador de par diferencial de bucle de antena YooFab

M0OOZ from YooFab / CC BY-SA

Notes On Simulation Please note that in the above animated simulation, there are two 10 MΩ resistors that go to ground. These are needed by the SPICE simulation and are not needed in reality.

The real world and the simulated world are different. Simulation is very useful. However, necessarily it presents an artificially 'perfect' world where components behave as idealists. In the real world no component is ideal, which results in differences in the performance when compared to the simulated world.

No Signal!#

Compare the animated simulation shown below with the above. The first thing we notice is there is no amplified output! The blue trace shows the input signal and the orange trace shows the output.

In this simulation the signal has been injected into both differential inputs. That is, the signal appears to go negative and positive on both sides of the differential amplifier (with respect to ground) at the same time. Ergo, there is no difference and so nothing is amplified.

Amplificador diferencial YooFab para antena de cuadro sin señal amplificada

M0OOZ from YooFab / CC BY-SA

Remember This When Testing We must remember the differential nature of this amplifier when testing. If we forget this and connect a signal generator to both sides of the differential amplifier, then at best nothing will appear at the output. In practice the amplifier is not completely immune to common mode signals and so instead of there being no signal we should see a comparatively small signal at the output.

Common Base Differential Amplifier#

On both sides of the amplifier circuit, we see two NPN transistors with the bases biased at just below 2V (assuming a supply of 12V. At 6V supply, the bias is just above 1V)

Each transistor has approximately 11 mA current passing at quiescent state. This, in addition to the low impedance inputs, provides some headroom for dealing with large signals if they are present. Bear in mind that ideally we want to be able to amplify signals of 1uV or less on our wire loop. However, every signal from ELF to upper HF may arrive at the loop, and so it is that we need to allow for this probability together with the possibility of large variance in signal strength. In other words we need the amplifier to have a large dynamic range.

The Output#

The output is taken from the transformer which is used to convert the balanced output to an unbalanced output suitable for feeding along 50Ω coaxial cable. Here we must note that there is a DC path to ground through the output winding of the transformer. No capacitor coupling is used as it is assumed that the input of our receiver will have a capacitor.

We may choose to add a capacitor if DC coupling is not desired. If this is done, we should keep the value at 100nF or above.

The other side of the transformer is coupled to the collector load resistors of the differential amplifier via 100nF capacitors.


  1. Power supply - The circuit is powered using a 6 - 16 Volt DC supply. This is connected to the circuit using a standard 2.1mm Power jack, with the center pin as ++. The supply must be clean (free from noise or ripple from the mains). Indeed, you should use a battery to test the antenna loop.

  2. RF output - The Active Antenna Loop output uses an RCA socket.

  3. Wire Loop A and B - Two Banana sockets allow the connection of a 1 - 1.5 meter length of wire. Your kit includes a length of galvanized wire, which may be fashioned into a loop. This is thin wire, yet has enough rigidity to maintain a form.

Instructions and more#

Click HERE for assembly instructions in SPANISH.

Click HERE for assembly instructions in ENGLISH.

Kit assembly (GIF):#

YooFab™ antenna loop construction sequence

M0OOZ from YooFab / CC BY-SA

Transformer assembly (GIF):#

YooFab™ bifilar toroidal transformer construction sequence

M0OOZ from YooFab / CC BY-SA


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