Inverse Doppler Effect

While researching and learning about the Doppler effect I came across the idea some people have that there is an inverse Doppler effect. Researchers from Britain did an experiment in the early 2000's trying to prove that the Doppler effect could be reversed. They did this by creating a pulse that acted like a siren and then using radio frequency to see the waves. This was done on structure called a nonlinear transmission line that used magnetics and created a new type of wave. They observed that instead of the waves getting looser like the Doppler effect shows, the waves actually got tighter and frequency increased. They claimed this proved that there may be an inverse Doppler effect. I don't believe that in real life the inverse Doppler effect can really be shown but the Doppler effect is seen everyday.