Ferrite Sleeves for VHF.
It is genereally known that HF antennas need a good Common-Mode Current rejecting BALUN to ensure an optimum antenna radiation pattern and avoid RF feedback and noise pickup.
What is often overlooked is that the same applies to VHF antennas.
While it is known and generally accepted that the “String of Beads” solution with ferrite sleeves slipped over the coaxial cable mostly offers unadeqatly high CMC impedance values to be effective on HF, on VHF ( 50 / 70 / 144 / 435 MHz ) however, this approach is very effective.
Antennas often exhibit a less than advertised or less than simulated clean radiation patterns, simply because stray effects like the undesired feeder radiation is not included in the simplified simulation or calculation models.
Installing some ferrite sleeves over the coaxial cable, right at the feedpoint can overcome this issue and considerably suppress spurious side lobes as well as optimize F/B ratio.
Several ferrite sleeve types are offering a considerable high impedance in the VHF – UHF frequency range.
At ON7FU different shapes and sizes of ferrite sleeves are available.
Contact us for more information.