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active antenna

VHF Active Antenna

A simple vhf active antenna using a single BFG65 transistor which is a low noise type.
The telescopic antenna and the wide band rf amplifier combination is known as an active antenna.
The antenna is a 1.6 meter telescopic dipole which works in lambda/2 from 60 MHz to 187 MHz or as a multi-lambda dipole as far as 900 MHz.

L1 is made of 10 turns of 60 Ω coaxial cable with 2.5 mm diameter, coiled on 10 cm long ferrite core with 10 mm diameter.

If the receiver is at 2 meter distance from the active antenna, connect it at K1 with an ussual cable like RG58. If the radio signals are weak increase a little the R3 value. Do not do this if there are strong radio signals.

With R3 = 560 Ω, the current consumption is 20 mA. The fm dipole antenna gain is around 12 dB, with a noise factor of 1 dB. THis ensure results comparable with Yagi antenna.

Active antenna circuit diagram

active antenna

Active antenna PCB layout

active antenna pcb

3 Comments

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  • estefan2020

    Its known as a poor mans 1 to 1 voltage balun, its purpose is to keep unwanted rf off the transmission line, and not acting as an antenna. using 3 clamp on ferrite chokes will do the same thing. but 5 or 6 turns of coax with a diameter of 4 or 5 inches will do, and not spend money on chokes.

  • Jeff

    Hi, What purpose does L1 serve exactly?
    Impedance matching perhaps?

    • Jim Keith

      Good question- -it appears that it may be a balun coil to transform the balanced antenna to unbalanced transmission line- -no impedance matching is necessary because a dipole is typically 72Ω, or in this case very close to the 60Ω transmission line.

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