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.
The dimensions are not too critical; I used a piece of plastic waste pipe
4.25" (11cms) diameter. The coil is approx 8m of the same RG58 coax used
for the feeder. This made 24 close-spaced turns occupying 13cms of the
18cms overall pipe length. The length of wire used for the coil (not the
no. of turns) governs the low frequency limit.
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