By Dave Clingerman W6OAL
We have been dealing with round rod or tube construction type halo or other omni antennas used for horizontal polarization for years in the marketplace. These are usually Gamma matched with a lossy Gamma capacitor. They are said to work over the entire band of interest (in this case) the two meter band! Why would anyone want to cover the entire two meter band (144 to 148 MHz) with a horizontally polarized omni in the first place? Horizontal polarization is generally used for weak signal work or SSB/CW in the portion of the band between 144.0 to 145.0 MHz centered around the calling frequency of 144.2 MHz.
So, the philosophy behind the Flat Halo Antenna has to do with the following;
- Narrow bandedness,
- Less metal in the vertical plane,
- Directly driven,
- Sturdy construction, DC grounded and easily removable from the vehicle.
Due to the construction of the Flat Halo it is designed to cover only the lower porting of the two meter band (144 to 146 MHz). The fatter the material used in an antenna the greater the inductance and thus the greater the bandwidth. The radiating portion of the Flat Halo and the Gamma arm are 0.125” thick and 1.0” wide constructed of T-6 or similar aluminum plate.
We know from experience, practice and philosophy that manmade noise is predominately vertically polarized. So, why have material in the vertical plane if it is going to introduce unwanted signal/noise into the front end of our receivers? Further, we don’t intend to propagate our signal in the vertical plane so why make it possible?
Most of the horizontal omnis in the market place today employ a Gamma arm and a gamma capacitor. The series inductance of the Gamma arm and capacitance of the Gamma capacitor are supposed to pass the two meter RF to the 50 ohm point above ground, of the one quarter wavelength side of the one half wavelength radiator or dipole that is bent onto a circle. The series L/C circuit therefore is a ratio, of L in µH to C in pF, of 1.218:1. (e.g. 0.1 µH and 12 – 18 pF). The L is generally fixed leaving the C to be of the variable variety subject to the elements (rain, wind, snow, dirt & bugs,). To eliminate the problems of a Gamma capacitor, the Flat Halo employs a Gamma arm that is 45 electrical degrees long at 2 meters and attached to one of the quarter wavelength sides of radiation element and the Gamma capacitor is dispensed with. Tuning of the antenna is done at the open, opposing ends of the circular dipole with a couple of movable tuning tabs and not some lossy variable, element whipped Gamma capacitor.
By having the radiating element and the Gamma arm constructed of flat material wind resistance is quite negligible rendering the structure very aerodynamic. It has been tested in practice at road speeds of 90+ MPH and has not left the test vehicle. Even though the antenna sports a 12” mast, to the structures advantage of integrity, is a ‘killer’ 5” magnet that requires at least two hands to be removed from a vehicle.
The Flat Halo is very sturdy of all metal construction and all parts DC grounded. The mag-mount affords fairly easy removal for in vehicle storage, in assurance, when traveling that in the morning you will have an antenna if not left on the vehicle overnight.