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
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
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.
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.
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
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.
I built a FM band stop filter I found on line and put the power supply and radio in a aluminum box and no more fm noise, and what static there was is very quite now. I dont even have to use the squelch.
It worked great at home but when I set it up at school( inside a metal hanger with a half wave dipole on the roof) I get the AWOS (atomated weather observing system) signal so strong that I can hear it three places on the dial and it blocks out most of the aircraft transmissions.
The AWOS transmitter is only about 200 yards away.
Can I build a filter that would block out just that( 134.75Mhz)frequency. It would be ok to block it out and above, because it is at the very top of the frequencies I want to hear. I can follow a schematic, but dont have any experience designing. Would a stub filter work?
Thanks, sam