Add External Antenna Jack to FM Radio

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Tinkertek

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Sep 6, 2003
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Palmer, MA
Hi,

I have an FM boom box that has the telescopic antenna. I added an external antenna jack so that I can use something like a wideband scanner antenna on it. My FM radio reception in the house is not great, but I think I could do a little better by going with a magent mount scanner antenna attached to a metal filing cabinet. I know an outside antenna would be better, but that is not really possible in the room where it's located.

So, I pulled apart the radio and simply soldered in an F chassis barrel type connector. Center conductor goes to the wire that used to go to the telescopic antenna. The shield goes to ground. I then attached the scanner antenna to the radio (the RG59 coax had an F connector on it). The reception did indeed improve somewhat.

Question, should I have used a 75 ohm to 300 ohm matching transformer inside the radio where I soldered in the connections? I wonder if that may have improved it even more? I have no idea what impedance I'm trying to match.

Ideas out there on this?

Thanks,
Paul
 

Mike_NZ

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Jan 10, 2007
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Location
South Island, New Zealand
Gidday Paul,
No, you only need a balun (75/300ohm) when you are using something like twin-lead ribbon cable from a dipole antenna, this matches the impedance of the dipole and twin-lead (300 ohms) to the 75 ohm input that TV recievers used to have before cable and what-not.
Having said that, most baluns in this situation would be up near the antenna, and coaxial cable run down to the equipment in the house.

I like your idea of the F-type connector though, as opposed to the more popular BNC, some people don't realise that there is a difference in size between the centre pin of a 75 ohm and a 50 ohm BNC male connector.
Only slightly, but when you go to plug the wrong one in, you splay the centre contact in the socket out, the right one will only make intermittent contact after that.

Good stuff! :cool:
 
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kb2vxa

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Point Pleasant Beach, N.J.
You did OK but only up to the point of choosing the wrong kind of antenna. A scanner antenna was never intended for the FM broadcast band so you have plenty of room for improvement.

You can buy an indoor folded dipole made from TV twin lead just about anywhere. TV/FM baluns work both ways so all you have to do is solder the antenna and balun lugs together, wrap them with insulating tape so they don't short to each other and connect it to the radio with a short F connector coax jumper also available just about anywhere. Just spread the antenna out horizontally and tack it to the wall preferably broadside to the stations you listen to and you're done.

Oh and BTW a boom box is also known as a ghetto blaster for good reason bro! (;->)
 
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