Couldn't believe the difference this simple mod made to HF reception, eliminating the background hash/noise which has always plagued the lower bands 80m (3.5MHz) & 40m (7MHz). The latter especially now sounds like a breath of fresh air; when first installed I thought the aerial had become unplugged as it was so quiet!
It's technically an air choke or coil (described and nicknamed in this article), and is connected between a coax feeder and a dipole aerial to reduce unwanted radiation or reception by the feeder.
Waveforms on 40m & 80m and MP3 recording on 40m before adding coil
40m no coil mp3
Ditto after adding coil
40m with coil mp3
The coil is made with coax cable and is simply connected between the feeder and the dipole - no ferrite toroids required. N.B. no longer have a licence so unable to check transmission qualities - see original article for more info.
Drill 2 small holes at one end to thread string for hanging and 2 larger holes at either end to thread coax. I used tie-wraps to hold the coax in place so needed 2 more small holes near the large ones. Photos below show outside & inside the tube.
Side view; dipole is 15.3m long between the gable ends of the loft, so runs front-to-back in this photo.
Bottom view; dipole is running left to right.