WW2 Barrage Balloon warning "squeakers"

[Tommy Tipee]

I am currently reading the excellent "Journeys into Night", an account of wartime training and Bomber Command operations by RAAF navigator Don Charlwood, in which he refers to "squeakers" which were heard over the radio and gave warning of nearby balloon barrages.
I have read many books on WW2 flying, but this is the first time that I have heard of this system.
Can anyone provide any more information on what this system was called and/or how it worked?
P.S. I highly recommend the book. Don Charlwood is a highly accomplished writer, probably best known for "No Moon Tonight", whose highly evocative works graphically describe the great courage shown and dangers faced by the crews of Bomber Command.

[Atcham Tower]

They were short range transmitters on the common frequency used by aircraft on cross-countries. A series of squeaks would be heard when in proximity to the hazard. They were also sited on high ground in various parts of the UK.

[henry crun]

I also Googled Squeakers and found the interesting fact that by about 1941 the balloon barrage had brought down nearly twice as many RAF aircraft as German !