Small radio receiver project

[DarkBabylon]

Hello, I have recently had a lab experiment as part of my physics degree, to measure RF signals in the AM band (starting from around 650 kHz) and got inspired to build my own one, and hopefully a small one. I've done some digging and the only problem I am having is the Antenna. It looked simple at first, in the lab it was a simple ferrite rod with wire wound around it, but all of a sudden I see many antenna designs with requirements of hundreds of meters for the same frequency ranges.

In short, I need help in at least getting some idea of how an antenna works and which should I go with. Knowing maxwell's equations already, I can either detect the Magnetic field, or use the Electric field. One thing that boggles my mind is what exactly do I need the antenna to do? what kind should I be looking for if I want to make it short?

[vk3yedotcom]

If you have a nearby signal generator then a ferrite rod (coil in parallel with variable capacitor) should pick up a signal.  On the other hand if you want to receive broadcast stations then you'll need something bigger. Eg an outdoor wire 20 to 40 metres long.  Ideally you'd also want an earth - eg a lead connected to a tap (if metal pipes), stake in the ground or radial.

If  you're in a reasonably good reception area you can, instead of using a ferrite rod or external wire antenna, wind an antenna on the side of a large cardboard box.  Reception may be better outside if using this.   This will be directional so you'll need to turn the box for maximum signal.  But these sorts of crystal sets can be quite selective and you should be able to hear several stations.  Details of one of these is at

http://home.alphalink.com.au/~parkerp/projects/projcry.htm

To make reception easy you might wish to use a small audio amplifier module (eg LM386 etc) to allow reception on low impedance phones or speaker.

[G0HZU]

You can also consider making an active antenna. This would be quite broadband compared to a tuned ferrite rod but you would have to allow for the fact that it will have fairly limited dynamic range compared to a passive antenna.

A short monopole antenna will look like a very small capacitance down at these frequencies and one way to enhance an active (monopole) antenna is to make the active stage appear as a negative or inverted capacitor. This will cancel the capacitance of the monopole and give extra bandwidth and you can make a fairly reasonable longwave receiving antenna from just a couple of inches of wire and the active/inverting capacitance interface to it.

But this circuit can be fiddly to build and set up as it can go unstable if not adjusted correctly. I've used it in the past with a 3 inch monopole looking for signals across 50-500kHz. It can easily be designed for the medium wave band as well.
 

[cdev]

Good suggestion!

The keyword to use to get started is pa0rdt mini whip, because I think that is where the design originated or was first popularized. but there are many variations.