Antenna-amplifier with BFT 12 for FM-range

The only way to improve the reception of FM-signals or particularly of stereo FM-signals under disadvantageous receiving conditions is to amplify the antenna signal. This should be made directly at the antenna, since the loss is increased meter by meter of any cable. In most cases higher line attenuations can be expected than usually calculated. Even receivers with an additional noise figure of zero do not show any improvements of the signal-to-noise ratio becomes too low because of a long transmission line. Therefore it is practically useless to require extremely low noise figures only for the receivers.

To avoid interferences of signals received from stations of other bands it is propitious to amplify the FM-band only. The band amplifier shown in Fig. 1.3 is connected directly to the 60 ohm output of the FM-antenna, i. e. it is placed at the top of the antenna pole. The matching is achieved by the input band-pass connected to the base of transistor BFT12, being a typically linear silicon transistor for broadband amplifiers. The output filter matched accordingly to a 60 ohm coaxial line is connected to the collector. The operating voltage of 15 V can be supplied by the coaxial line.

Fig. 1.3

Antenna-amplifier with BFT 12 for FM-range

Technical data:

  • Power gain Gp = 22 db
  • Noise figure F = 3.5-4.0 db or 2.2-2.5 KT0
  • Input and output reflexion coefficient |r1| and |r0| <= 0.3
  • dim(60db) at Vout = 680 mV
  • dim(50db) at Vout = 1000 mV

Optimum operating point of minimum intermodulation

Ic =~ 80 mA Vce =~ 7-7.5 V
Supply voltage 15 V

Coil data:

Vogt coil former, ordering code: Sp 3.5/16.6-2048C
Core: U17

  • L1 : 5 turns Cu 0.6 mm \o
  • L2 : 3 turns Cu 0.6 mm \o
  • L3 : 3+2 turns Cu 0.6 mm \o

Choke Ch1 = Ch2: 20 turns Cu L 0.3 mm \o, cross section of winding = 4 mm

Credits –

Antenna-amplifier with BFT 12 for FM-range
from Design Examples of Semiconductor Devices – Siemens

VK5JST Amateur Radio Ideas with Photos

The new Ham Project Pages are VHF/HF Aerial Analyser, A 600MHz Frequency Counter.  Then there is a an ESR Meter for Electrolytic Capacitors.

VK5JST Amateur Radio Ideas with Photos

With a two tone tester, a dummy load and an oscilloscope, a transmitter can be really put through its paces. Two tone testing consists of applying two clean non-harmonically related sine waves of approximately equal amplitude to the audio input of an SSB transmitter.

Simple Two Tone Oscillator


The sine waves are typically around 600 – 700Hz and 1800 – 2000Hz i.e. about 300Hz from either end of the audio pass band.

A Simple Two Tone Oscillator

This project was published in the September 2004 issue of the Australian magazine “Amateur Radio”.

Other Readworthy Pages

  • Easy PCB’s Made With Kitchen Appliances
  • How to Make a Q Meter
  • Making a High Current 13.8 VOLT Supply For SSB Transceivers
  • VK5MM’s excellent amateur radio homepage
  • Transformer Construction Details
  • Shortform Method of Designing Power Transformers

The photos at VK5JST Homepage are very educative and express the idea better than words. – delabs

RASON Projects Page

Designing Bipolar Transistor Audio PreAmps, Designing JFET Transistor Audio PreAmps, Designing Op Amp IC Audio PreAmps, Switching Regulator Basics, Using Transistors As Switches.

RASON Projects Page – Amateur Radio

  • 10 Amp, 13.8 Volt Power Supply
  • 12 Volt Gel Cell Charger
  • Thermal Fan Controller
  • Transistor Audio Amplifiers
  • 12 Volt Gel Cell Charger
  • Switching Regulator Basics


A Potpourri of Audio Amplifiers  By N1HFX

“I intentionally avoided the use of IC amplifiers to provide a real learning experience for audio amplifier design. Although many IC amplifiers provide excellent performance at low cost, we need to learn the basics first. I will address IC audio amplifiers in a future article.”

The Radio Amateur Society of Norwich
P.O. Box 329 – Norwich, Connecticut 06360