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Page updated: 4-Nov-2014

The 'Ugly Balun'

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
Waveform 40m no balun  Waveform 80m no balun  40m no coil  mp3

Ditto after adding coil
Waveform 40m with balun  Waveform 80m with balun  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.
balun close-up      balin top hanger
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.
balun outer view of tie-wrap fixing      balun inner view of tie-wrap fixing
 
Side view; dipole is 15.3m long between the gable ends of the loft,
so runs front-to-back in this photo.Balun side view

Bottom view; dipole is running left to right.Balun bottom view