I built this antenna(1/2 wave for 125MHz) http://www.alpharubicon.com/elect/vertdipoleniet.htm
The 75 ohm coax is one continuous 25 ft. peice, that runs from roof down to my receiver.
There was about 8ft left over, so I coiled it up into 6-7( 5inch) turns at the bottom of the antenna.
Do I need a balun or choke?
Thanks ,sam

 

Looks fine to me, no need for balun.

JimB

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You don't need a balun, that is the beauty of this design. Are you getting good reception?

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Thanks JimB
sam

 

Yes, the reception is great compared to the 22" wire I had mounted on the radio.I am hearing almost continuous airtraffic now. But I also hear some FM stations in a couple of spots on the dial ( very low ,but there) which I dont want. But thats the receivers fault I guess. I am very happy with the radio/ power supply and antenna and have learned alot from this project.
Thanks sam

 

Sounds like poor image rejection from a badly designed (or cheap) radio.

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Well Nigel, you did describe it as fairly simple and crude but its a start.
I am just happy that it works as well as it does.
Electronics is the most amazing and fun thing.
Thanks,sam

 

It's not simple to design a radio for good image rejection, the simple design you built only uses a bandpass filter - using a proper tuned input stage (or two) would greatly improve image rejection.

One thought?, have you got it mounted in a screened metal box?, if not it may be picking up strong FM stations directly rather than through the aerial.

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Nigel, or anyone ealse, what could I do to the receiver to remove those unwanted FM stations?
Thanks, sam

 

Thanks Nigel, you answerd my question before I asked it.
Sam
No , its in a plastic project box. I have a metal box , but what is a screened metal box?

 

Once you have this working well, you might be interested in picking up Weather Satellite images at 137.62 MHz or 137.5 MHz. Many amateurs do this and there are lots of websites showing how. You end up with weather satellite photos on your PC. Cool.

Here's one information link:

http://www.hobbyspace.com/Radio/index.html#WeatherSat

scroll down to the heading "Image Reception from
Weather and Other Satellites ".

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Very cool site. Thanks RadioRon
sam

 

I got rid of 99% of the FM station noise by turning the slug down in the coil( local oscillator coil) .At full volume I can just barely hear something at one end of the dial. I will try a metal box next.
Thanks, sam

 

It's fully sealed all the way round, and connected to the ground of the PCB - the only way signals can get to the board is then via the aerial socket.

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You might be able to purchase an FM broadcast filter that is designed to attenuate FM broadcast but pass frequencies above and below. You'd want to understand the cutoff frequency. I'd google on FM broadcast trap or FM broadcast filter for more detail. This would be done in addition to the sheilding.

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