Nowadays we generate radio signals as continuous waves, but in the beginning it was generated by a spark, and if you were lucky it was tuned and filtered. You know about electrical sparks; they generate radio-frequency interference (RFI), with the static from the electrical discharge of lightning as the most widely known case.
But even as late as 1922, you could build a spark-gap station from components you could buy from Sears, Roebuck (click on the image to make it big--as big as your screen if you want):
This is a page from the 1922 catalog of Electrical Goods and Radio Apparatus, a specialized sixty-four page publication that featured electrical appliances, and the parts and equipment you would use to build a receiver or your own amateur radio station. These rotary spark gaps are just about the technological peak of spark design; pretty soon it was all over for spark, and the soothing tones of continuous-wave signals blanketed the bands.
Steve Davis found a copy of this catalog, and scanned it; I cleaned up all of the images (64 large pages; I must have been out of my mind) and published it as an e-book. It's now on sale in ePub format (for any ebook reading device) and in Kindle format (for Amazon's hardware).
You can buy it on Amazon, Barnes and Noble, Google Play, and at the Apple iBooks store (search terms: Sears 1922 radio catalog). It's beautiful, if I say so myself, and it's $2.99.
Yes, it's cheaper than a latte.
There's a picture of the cover, and a little more about the fun and excitement of squeezing ten pounds of image data into a two-pound box, on the Early Radio blog. The big version of the image is exactly the image that's used in the book (along with sixty-three other images) so you can get an idea of just how much detail is being shown.