radio-shac: FM Antenna Oscillations

ID: 202864
radio-shac: FM Antenna Oscillations 
20.Oct.09 18:11

Joe Sousa (USA)
Articles: 669
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Joe Sousa

Hello Radiophiles,

This very nice tuner has a tendency to oscillate if the FM antenna twin-lead wire is brought too close to the 6U8 RF front end preamp pentode.


The 6U8 is not shielded, and the input antenna connection is single ended. This means that a twin lead wire connection will be "hot", and will be able to radiate or receive signal.

The proper connection for a 300 Ohm twin lead is with a balanced transformer input.

Balanced input has both inputs equally active, so that any signal arriving at the twin lead is cancelled.


The single ended 300 Ohm connection useful if some of the expected use is to use short length of wire as the antenna, or if the antenna is the power cord that is capacitivelly coupled with a metal clip.



An isolation 1:1 VHF transformer at the antenna terminals can cure the problem.

Or just keep the twin lead away from the left side of the chassis as shown by the arrow. Also remember that the bottom of the chassis is open, and can be a path for undesired feedback.

I added a piece of discarded PCB as a groundedshield to the bottom of my tuner.




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a good solution 
20.Oct.09 22:06

Hans M. Knoll (D)
Articles: 2163
Count of Thanks: 16
Hans M. Knoll

Hello Joe.


Please look on this Tuner.          Pilot T601A from USA

   They has make a good solution for the antenna leads.

pic. ID 154016

I think, is simple but good!

This is my own tuner.

regards hans

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