Cross Country Wireless Loop Antenna Amplifier
New HF/VHF version now available. See below...
The Loop Antenna Amplifier contains all the electronics needed for home DIY construction of an active loop (magnetic loop) low noise receiving antenna.
The amplifier consists of two units, a weatherproofed outdoor unit for connection to a suitable loop and a base unit to further amplify the signal and to provide DC power up the coaxial cable to the outdoor unit.
The outdoor unit is housed in a polycarbonate box with stainless steel antenna connections and a BNC socket. The indoor unit is a PCB with two BNC connectors and a USB socket to take 5V from a USB socket on a PC or phone charger.
Like our other active antenna products it has RF overload protection to allow it to be used very close to transmit antennas without damaging the amplifier or the attached receiver.
The loop depends on what the user has available. We have tested it with simple wire loops or deltas, coax loops and an alloy loop made from a bicycle wheel rim. We supply a 3m (10 ft) length of wire as a simple loop to make a first loop for testing.
The photograph on the right shows the prototype with a 1m diameter loop of LDF4-50 coax cable as a test loop.
With a simple wire loop or delta and a small USB powerbank it makes a very compact and portable receiving antenna for holiday listening or covert use.
The latest versions can now have the head unit powered directly from receivers with a 5V bias-tee such as the SDRplay receivers or some RTL-SDR dongle receivers with a bias-tee option.
Following customer feedback we now make two versions of the head unit.
The VLF/HF unit has a low impedance input for small loops up to 1m diameter and covers 40 kHz to 30 MHz. The 2020 version uses a different input transformer to improve performance at HF frequencies.
The HF/VHF version has a higher impedance input for use with more inductive wire loops up from 1m diameter and covers 500 kHz to 150 MHz. This version has slightly higher gain and is recommended for use with the 5V bias-tee antenna feed on SDRPlay and some RTL-SDR receivers. It is also available without the base unit for those customers.
Frequency range: VLF/HF version 40 kHz to 30 MHz, HF/VHF version 500 kHz to 150 MHz
Loop amplifier input impedance: VLF/HF version 0.3 ohms, HF/VHF version 50 ohms
Output impedance: 50 ohms
OIP2: >+75 dBm (both versions)
OIP3: >+37.5 dBm (both versions)
Gain (head and base units VLF/HF version): 21 dB at 14 MHz
Gain (head and base units HF/VHF version): 26 dB at 14 MHz
Gain (head unit only HF/VHF version): 12 dB at 14 MHz
Noise figure: 0.79 dB (both versions)
Supply voltage: 5 V from USB socket or charger
Supply current (head and base unit VLF/HF version): 137 mA
Supply current (head unit VLF/HF version fed with 5V bias-tee): 81 mA
Supply current (head and base unit HF/VHF version): 88 mA
Supply current (head unit HF/VHF version fed with 5V bias-tee): 32 mA
Loop antenna outdoor unit connectors: Two M6 stainless steel threaded studs and BNC female (RF out 50 ohms)
Loop antenna indoor unit connections: BNC female (RF input and DC feed to outdoor unit), BNC female (RF output to receiver), USB B connector (for 5V DC power input)
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Here's a video of the VLF/HF version of the Loop Antenna Amplifier in use with a loop made out of a 700C bicycle wheel rim. The video is made during a field test in a very quiet rural environment. Watch the volume control during the early few seconds of the video due to wind noise on the camera phone used. It clears up later.
This video shows how to fit the upgrade kit to modify the 2019 build original and VLF/HF versions of the Loop Antenna Amplifier to the 2020 specification. This upgrade improves the HF performance with a loop but reduces VLF performance below 40 kHz.
Contact Chris Moulding via firstname.lastname@example.org
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