View single post by sparks
 Posted: Nov 8th, 2011 01:12 PM
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sparks



Joined: Jan 15th, 2008
Location: New Zealand
Posts: 77
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The chokes I was referring to are standard RF Chokes, and not a ferrite bar antenna coil. I think the type you are using in the photo is the correct type.

For the Raudive design, If you want to use a low frequency that is free of radio traffic, then Id use a 4.7mH choke like this one...
http://www.circuitspecialists.com/70f473ai.html
It might have some degree of self resonance, but it will be at a frequency about 260kHz, so it shouldnt be sensitive to medium wave radio stations.

The above idea is for an un-tuned diode receiver. The problem with this idea is to avoid reception of radio stations, which is going to be difficult with this simple design, but may be possible with some careful experimentation.

Another better option is to use a higher Q coil and make it tuned to a frequency like 260kHz that doesnt have any nearby radio stations. For this Id use 2 medium wave ferrite bar coil antennas in series to ensure resonance is below the medium wave band. A tuning capacitor could be added, but may not be needed. The high Q and self resonance should ensure it doesnt pick up medium wave stations. It wont need an external wire antenna of course.
http://www.maplin.co.uk/ferrite-rod-aerial-859

Good luck with this. Another option for a raudive diode concept (as far as the output is concerned), is to just use a portable radio tuned to an unused portion of the AM or LW band. The noise structure there will the same as what youre trying to achieve with the diode, and you wont have the problems of trying to design out radio station voice content.

As I said in my previous post, it may be a lot easier to use a non radio type of noise source.

JEFF