You need a capacitor. Either the little square ones that come out of AM portable radios or a vintage type one with vanes that open and close. Either are available on ebay. For my loop I use an AM one I removed from a broken portable radio, but I've also used the type with vanes.
Once you have chosen your capacitor you need to connect a loop of wire across the terminals. I use a loop 35cm in diameter of insulated wire, but for listening it doesn't really matter what type of wire you use. Connect it to the capacitor like this:
If you decide on the AM radio-type capacitor like me, you may have to experiment a little to determine which terminals to connect the loop of wire to. I used a big metal tag on the left and a similar big tag on the right. Use crocodile clips to test the aerial once you've built the pick up loop and then solder once you have determined which ones tune the aerial across your desired range of frequencies.
So you now have something like this (though for now only connected using croc clips):
Next you need to build the pick up loop. There is no physical connection between the pick up loop and the main loop. I didn't understand this when I first tried to build mine. The pick up loop just sits inside the main loop and picks up the received and tuned signal via INDUCTANCE (ie its physical proximity to the main loop). Again you can use any type of wire. I use the braid only of a piece of coax. I read that the pick up loop should be 20% of the main loop, but this looked ridiculously small when my main loop was only 35cm so I settled on 14cm diameter and it worked just as well. Connect one end of this smaller loop to the center conductor of a piece of RG58 coax and the other end of this smaller loop to the braid. I actually don't use coax at all and bought a small 1.5m length of wire on ebay with bare wire on one end and a 3.5mm plug on the other (search for "3.5mm plug to bare wires"on ebay). I've also used speaker wire with a 3.5mm plug soldered on the end.
Depending on the type of capacitor you've used, you probably won't get a huge tuning range from a single capacitor, unless you use a 750 to 1000pF variable capacitor. I used a 1000pF variable capacitor with reduction drive that I got off ebay and it covered around 3mhz fright up to 30mhz! But my AM radio cap only tunes 7mhz upwards, so to get down to 5mhz I've added a small 220pF ceramic capacitor and put a switch in the wiring to cover both the lower end and the upper ends of the band. DON'T PANIC!!! This sounds complicated but it is not. These ceramic capacitors are cheap on ebay and if you only need to drop a couple of MHz down in frequency a 200-300pF one will be fine. The more capacitance you add, the lower it will go. I'm guessing that 500pF might take you down from 7mhz to 4mhz. Maybe 3mhz if you're lucky.
So, you need to buy a 200 to 500pF capacitor and a little switch from either ebay or Maplins. I removed mine from a broken radio. You wire it up as follows:
So I hope this helps a little, and as I said at the beginning, if you want any help or assistance, please feel free to email me via my email on QRZ,COM or via the comments on the blog. It is an easy project. The hardest thing is finding the confidence in yourself to have a bash. I had school teachers that inspired precious little of this in me and I left school thinking I was useless. It's taken 30 years to realise I was wrong!!!!!
My finished loop after dozens of attempts and several years to get it more neat and tidy|: