neophyte receiver tunnig capacitor

[seldon]

neophyte receiver: tunnig capacitor

Hello,

I've deceided to build my first radio receiver and I found an article in qst 1988 magazine with circuit you must all be familiar with : neophyte

And my question is will this particular air capacitor will work as a tunning capacitor for neophyte receiver?
**broken link removed**

original cap suggested in the article is 365-pF air-dielectric unit. Other than that I find these parts are very expensive another alternative is http://www.spectrumcomms.co.uk/Components.htm DUAL GANG VARIABLE CAPACITOR

Thanks.

 

[BradtheRad]

The dual-gang type is useful because you can hook them up to obtain different range of values, making it useful to dial various bands.

The air capacitor is easy to work with. Easy to fine-adjust. It occupies larger space. The smaller miniature type is convenient for pocket transistor radios, and may require more work on your part to fit a tuning knob properly.

Or you might like to try an inexpensive homebrew. Check the internet for articles on making an air capacitor from sheet metal and wood.

After reading one article I tried a fun experiment. I cut two pieces of aluminum foil, each several square inches, and rolled them between sheets of paper. Clipping my jumper wires to the aluminum created a capacitor. Value about 1 nF, which can be adjusted somewhat by squeezing and relaxing pressure. Cheap and educational.

 

[c_mitra]

You use a dual ganged identical air-core capacitor for selectivity. One capacitor is used for tuning. So you can scan all the signals coming to your antenna. The other capacitor is connected to the Local Oscillator. The local oscillator frequency is mixed with the antenna signal to produce a difference signal that is at a fixed frequency- commonly called the IF (intermediate frequency). Because the two capacitors are ganged, the local oscillator and the antenna signal change the same way and the difference shall stay constant.

We use a fixed frequency IF because they can be amplified using narrow band amplifiers.

In this particular case, the two (sections) capacitors are not identical. That means you will need to struggle a lot. But modern digital tuners are actually simpler.