sw radio

[scientifico]

Hello, I need help for building a very simple shortwave radio (cristal like) and, I think, another circuit to amplify the signal and make possible connect a speaker instead of high-impedence headphones, would you suggest me something since I'm not very skilled in electronic projects ?

Thanks

 

[(*steve*)]

Essentially all you need is to change the coil and tuning capacitor on a crystal radio and you'll be able to tune it to a different frequency range.

The issue is that the signal level from most short wave stations is going to be a lot lower than for local broadcasters.

An area where you would need to take more care is in the choice of rectifier as the frequency you're interested in is significantly higher.

 

[duke37]

If you are happy with a battery, look up the ZN414 or the modern equivalent. The output power is very low but it is more sensitive than a crystal set which gets all its power from the aerial.

Something better would be needed for short wave in order to separate the different stations. You are then into receivers with reaction or superhets.

 

[scientifico]

Hi, I'd prefer something essential without integrated circuits...

 

[(*steve*)]

As pointed out, a crystal radio also has selectivity issues (trouble separating stations on closely spaced frequencies).

If you're looking for something without this problem, you might look for something with a fet front end and perhaps regenerative gain.

Selectivity can be greatly increased with a design that converts the signal to an intermediate frequency (IF) first, but I've not seen a circuit using discrete components for a long time. There are also some technical issues to add complexity given that you probably don't want to produce a million units and have to rely on off-the-shelf components.

I'm not suggesting that it can't be done, but it is a task that would be easier if you can make sensible design changes to suit the components you have available or can make yourself.

 

[scientifico]

Selectivity isn't a problem because I just have to build this up as a project...

 

[(*steve*)]

You might get some ideas here: http://www.portabletubes.co.uk/sitefiles/50FETProjects.pdf

 

[scientifico]

why cristal receivers need to be grounded and where should I connect the ground wire ?

 

[duke37]

The crystal set takes all its power from the aerial. The power can only be extracted efficiently by inserting the set between aerial and ground. Sets with amplifiers can manage a much smaller input.

The connection will be to the common line in the set, often termed ground. You could show us a circuit diagram so we can advise the correct position.