2m Amateur Radio Halo Omni-Directional Antenna

Last crawled date: 1 year, 11 months ago

This is a model of a 2m Amateur Radio Halo Omni-Directional Antenna that i designed after a commercially available one wherein i made modifications to remove all ferrous materials to help improve the performance of the antenna.
https://www.amazon.com/FM-Loop-Antenna-Outdoor-Attic-mount/dp/B006SLV25C/ref=asc_df_B006SLV25C/

I also chose NOT to use coax and use a 300 ohm twin ladder line instead. The particular one i used has a larger gauge wire as 18 AWG instead of the typical 20 or 22 AWG and is a 'window' type that reduces the dielectric losses by the cutouts. Typical loss of "type RG-58 coaxial cable loses 6.6 dB per 100 m at 30 MHz, while 300 ohm twin-lead loses only 0.55 dB" (https://en.wikipedia.org/wiki/Twin-lead).

High quality 18 AWG 300 ohm twin-leand cable is available from the 'wireman' : https://thewireman.com/product/18-awg-300ohm-stranded-window-line/#iLightbox[]/0

The halo antenna is basically a folded dipole that is wrapped in a cylindrical shape which is useful for cars, RVs, boats, or other areas where a smaller antenna does well. Since the folded dipole is 4x the impedance of the dipole antenna @ 74 ohms x 4 = 292 ohms ~ 300 ohms, which is what the twin-lead feed line is, so a matching network is not local to the antenna in the NEAR field, but a 300:50 or 6:1 balun should be used close to the ground level. https://en.wikipedia.org/wiki/Dipole_antenna#Folded_dipole

The elements are 1/4" (6mm) aluminum tubing. I choose a gold anodizing passivation for the color.

Design note : there are only (2) fasteners for the assembly : (1) 6mm or 1/4" carriage bolt with a nylock nut and (1) 4mm rounded nylon screw. The carriage bolt can be nylon (if available) or aluminum as well as the nut. I specifically choose NOT to use steel (iron) based fasteners as they will detune the antenna and re-shape the radiation pattern.

Design note : the use of twin-lead reduces line losses *significantly*, however the transition to coax must be done at least 12" (1 foot or 1/3 meter) above Earth ground, otherwise ground coupling will incur losses to the twin-lead cable. A 300:50 or 6:1 Bal-Un (balanced twin lead to unbalanced coax) is required here. There is no issue with coax laying on the Earth ground and in fact it may reduce EMI issues on the outside of the coax better than other choke and magnetic methods. And the coax can also be buried into the earth ground as well !

There are (3) 3D printed parts :
(1) beam support
(1) access cover
(12++) twin feed line standoffs

All of these being 3D printed in a non-conductive material will also help performance.

Halo antennas come in other shapes with a split single loop being the most common :
https://en.wikipedia.org/wiki/Halo_antenna

However NX7U has a 3-coil design that's worth taking a look at :
https://www.nx7u.net/HiPar.html