HF Omni

Although a directional HF antenna usually is preferable, an omni can be useful. Use it to determine where to point a directional antenna when the band is open in several directions. Use it for domestic contests from the middle of the country, roundtable conversations, net or emergency operation, or just casually tuning around.

A simple vertical antenna is omnidirectional, but it restricts coverage by suppressing high-angle radiation. In addition, verticals are susceptible to local noise and their performance varies greatly with ground quality.

An inverted-V dipole provides horizontal polarization broadside to the wires and vertical polarization off the ends. This is a simple way to obtain somewhat omnidirectional coverage. While it may be good enough in many cases, response off the ends can be as much as 13 dB down for a 90 apex angle depending on elevation angle, antenna height, and ground quality.

An inverted-V turnstile consists of two inverted-V dipoles mounted at right angles and phased 90. The azimuth pattern is within a few tenths of a dB of perfectly omnidirectional at all elevation angles.

Modeling Results

This compares the pattern of an inverted-V turnstile and a single inverted V. The apex of each antenna is at 70 feet. The apex angle is 90. I modeled the antennas with NEC-2 using Sommerfeld-Norton ground and average ground quality. Plotted is the total field, which is the response for matched polarization.

This compares the turnstile and a groundplane with four sloping radials whose feedpoint is at 25 feet.

Turnstile polarization is elliptical with the horizontal field dominant. At high elevation angles it is almost perfectly circular. The maximum polarization-fading depth for linearly polarized signals is equal to the axial ratio.

Turnstile dipole currents should be equal in magnitude and phased 90. A simple way to do this is to lengthen one dipole until its phase is +45, shorten the other to −45, and connect them in parallel. No phasing line is needed. The resulting impedance is close to that of a single dipole.

When designed for 7.15 MHz, SWR is less than 1.5 across the band, but azimuth response variation rises to 4.7 dB at the band edges.

Turnstile
70' high
7.05 MHz
Sommerfeld-Norton 7050.GND
Dielectric constant 13, conductivity 5 mS/m
20 elevation
60 segments/halfwave (121 segments total)
 
Gain          2.45 dBi @ 20
F/R          -0.38 dB
Impedance    43.6+j0.8 ohms
SWR           1.15
Ohmic Loss    0.12 dB

Antenna File

Turnstile
70' High
7.05 MHz
5 copper wires, inches
ang = 45			; leg tilt
f = 1				; half-distance between balun terminals
a = 429.5			; a & b are leg lengths to balun 
b = 405.5			; swap a & b for left-circular polarization
shift z 70'			; 7.15 MHz: a = 423, b = 400
1   0  f  0   0 -f  0   #12
shift y f
rotate z -45 x ang
1   0  0  0   0  a  0   #12
rotate z 45
1   0  0  0   0  b  0   #12
shift y -f
rotate z -45 x -ang
1   0  0  0   0 -a  0   #12
rotate z 45
1   0  0  0   0 -b  0   #12
1 source
Wire 1, center

October 2, 201888108 MHz