Amplifier for Small Magnetic and Electric Receiving Wideband Antennas

Model AAA-1
Active Antenna Amplifier

The Kit

The amplifier kit consists of the following parts:

  • Amplifier board (mounted and tested)
  • Control board (mounted and tested)
  • 6-pin Female connector for connection of remote control switches
  • Plastic ABS IP55 protected box
  • 3 mounting screws diam. 3.5 mm
  • Load resistor 100 ohms / 2W
  • Short service cable crimped with two RJ45 connectors
  • Spare RJ45 shielded plug connector
  • Spare RJ45 unshielded plug connector

Active Antenna Amplifier

This amplifier is intended for construction of small wideband active receiving antennas from LF to upper HF bands. The mechanical construction of the particular loop/dipole antenna must be performed by the user. This amplifier can use two separate small magnetic loops. These two loops are used also as two arms of a small electrical dipole. Thus 3 different antennas can be used. The amplifier has four remotely switched modes: Loop A, Loop B, crossed parallel loops A&B and dipole. Each mode can be switched immediately and this enables us to use the best antenna for the moment.

This kit consists of two SMD mounted PC boards – an amplifier board and a control board. The amplifier has low noise, high dynamic range and wideband properties. It has separate current and voltage amplifiers for the different antennas. It is a balanced design and uses monolithic pairs of BJT (loop amplifier) and JFET (dipole amplifier). The balanced power amplifier uses medium power BJT with very low IMD and can give approximately 75 mW of output power on a 50 ohms load in order to minimize the non-linear distortions. The small antenna frequency response in all modes is flat in a wide frequency range. Usable frequencies are from 20 KHz up to 55 MHz. The antenna gain in the current and voltage mode is set to be approximately the same to avoid striking differences in the gain when the antennas are switched. Every kit set has been tested.

Control board and cable

Block Diagram for Active Antenna Amplifier
Fig.1 Block Diagram for Active Antenna Amplifier (click for larger view)
Block Diagram for Active Antenna Amplifier

Active Antenna Amplifier - Block Diagram

What the user should prepare
  • Loop antenna
  • FTP cable with crimped RJ45 plugs
  • External DC power supply
  • 3 switches for manual mode control or simple interface for PC control

The amplifier is connected to the receiver via a control box and an FTP (shielded CAT5E) cable with RJ45 connectors which ensure good balance to avoid common mode noise signals. The cable length can be above 100m. There is a possibility to limit the maximal voltage applied to the receiver input which is needed for some direct sampling SDR. The amplifier has jumpers for setting it to the particular environment. It is of SMD design and is mounted and tested.


The amplifier input is protected from high electromagnetic field intensities – it can be used with magnetic and electric small antenna in the immediate vicinity (20 – 30 m) of transmitting HF antennas even if the power is 1 - 2 KW. The amplifier has input filters to reject the frequencies above 55 MHz to avoid the influence of strong stations in the broadcast FM band (80-108MHz).

Power & Other

The amplifier is powered by a nominal 13.8 V (12.3 – 16 V margins) DC power supply @ 145 mA nominal current. The standard existing power supplies (PS) for the transceivers or receivers can be used. The amplifier is mounted in an ABS plastic (IP55 protected) box. It can be used with any receiver which has sufficient sensitivity and 50 ohms input impedance. This amplifier can be used as a building block for more sophisticated antennas such as phased arrays etc.

Tested parameters in production stage

This set of measurements assures that the amplifier has normal gain, frequency response, noise floor and dynamic range.

Minimal additional efforts are needed to complete this active antenna project. For more details look at the Active Antenna Amplifier Technical Information.

Active Antenna

Small wideband receiving active antennas have been widely used in amateur radio and the commercial world for years. Generally there are two types – small electric dipole (or monopole) antenna and small magnetic loop antenna. Recently the interest in these antennas in the ham radio community is increased due to several factors:

To solve all these problems we decided to build a wideband amplifier which is able to use both electric and magnetic small antennas so that the user can choose the best antenna for his/her particular environment and propagation and to solve for himself an ongoing dispute as to which small antenna is better – the electric or the magnetic. Building the appropriate small antenna is left to the user – this reduces significantly the costs and moreover – the user can build antennas with different efficiency: bigger or smaller loops, dipoles and verticals, crossed loops etc. according to his/her interests and needs. Also two mutually orthogonal loops can be switched thus making possible the use of the directional properties of the loop antennas to reduce selectively the man made noise sources or interfering stations. It is always an advantage to have several antennas which can be switched quickly in order to have the best receiving for the moment. This amplifier is also capable to use a novel concept – crossed parallel loops which enables to increase substantially the sensitivity of the small wideband loops.

This amplifier combined with an appropriate small antenna has good sensitivity and a flat frequency response in a range of almost 3 decades. It is not a surrogate antenna and in certain conditions with properly sized loops and dipoles it will fulfill the requirements of the DX-ers. The immunity of this small antenna to a local man made noise is very good and in urban environment very often better reception can be achieved compared to larger sized antennas.

Loop Antenna Constructions and Results from Other Hams

Orders and Pricing