VHF Scanner Antenna

I used the AO 9.59 Antenna Optimizer to design this antenna for 30–174 MHz reception. It uses arms of EMT conduit and sloping wires of galvanized steel. Height is 10 feet (11 feet including wire support). A highpass L-network accomodates 50Ω coax. The red dot is the feedpoint.

At low frequencies the antenna acts as a thick dipole. The shunt L-network coil permits a short antenna. At higher frequencies the wire slope concentrates radiation at the horizon and the 90° twist promotes an omnidirectional pattern. The antenna receives above 174 MHz, but gain drops and the pattern becomes highly directional.

Modeling Results

Results are for the vertical field using 28 analysis segments per conductor halfwave. Avg Gain is gain averaged over 360° azimuth. It includes mismatch and conductor losses. ΔG is peak-to-peak gain variation.

Frequency  Impedance    SWR   Mismatch  Conductor  Avg Gain    ΔG
   MHz        ohms             Loss dB   Loss dB     dBd       dB
    30    8.26 + j9.08  6.26     3.23      0.10     -3.62     0.00
    36     178 - j166   6.77     3.48      0.07     -3.78     0.00
    42    25.2 - j56.4  4.81     2.44      0.06     -2.65     0.00
    48    18.6 - j9.0   2.79     1.10      0.05     -1.19     0.00
    54    20.6 + j27.8  3.28     1.45      0.04     -1.41     0.01
    60    28.8 + j65.5  5.10     2.61      0.04     -2.42     0.01
    66    48.3 + j112   7.05     3.61      0.03     -3.23     0.02
    72     102 + j174   8.35     4.18      0.03     -3.58     0.02
    78     274 + j208   8.71     4.33      0.04     -3.48     0.03
    84     382 - j106   8.23     4.13      0.04     -3.01     0.04
    90     148 - j164   6.80     3.50      0.04     -2.09     0.04
    96    65.3 - j99.3  4.89     2.49      0.05     -0.82     0.05
   102    38.4 - j49.5  3.02     1.26      0.05      0.60     0.06
   108    27.9 - j11.5  1.93     0.46      0.06      1.47     0.06
   114    24.9 + j21.5  2.47     0.86      0.06      0.95     0.04
   120    27.1 + j54.2  4.32     2.14      0.05     -0.65     0.01
   126    36.7 + j90.1  6.35     3.28      0.05     -2.26     0.05
   132    62.5 + j134   7.64     3.88      0.05     -3.35     0.13
   138     143 + j182   7.73     3.92      0.05     -3.83     0.25
   144     298 + j86    6.48     3.34      0.06     -3.50     0.40
   150     186 - j83    4.52     2.27      0.06     -2.55     0.58
   156    81.8 - j51.6  2.50     0.88      0.07     -1.19     0.78
   162    48.2 - j7.6   1.17     0.03      0.07     -0.36     0.98
   168    35.9 + j29.0  2.10     0.59      0.07     -1.07     1.18
   174    33.3 + j59.7  4.06     1.98      0.07     -2.77     1.38

The curves show maximum and minimum gain over 360° azimuth.

The antenna is suitable only for strong-signal reception at UHF.

Antenna File

VHF Scanner Antenna
Free Space
30 37 44 52 110 122 135 146 160 174 MHz
9 zinc wires, inches
s = 1.832512				; EMT half-spacing
a = 23.81871				; EMT arm length
h = 60.31255				; wire height
w = 4.656758				; wire offset
1   0  0 -s   0  0  s    #16
1   0  0  s   0  a  s   .706
1   0  0  s   0 -a  s   .706
1   0  w  h   0  a  s    #16
1   0 -w  h   0 -a  s    #16
1   0  0 -s   a  0 -s   .706
1   0  0 -s  -a  0 -s   .706
1   w  0 -h   a  0 -s    #16
1  -w  0 -h  -a  0 -s    #16
1 source
l = 346.4386
c = 46.75296
Wire 1, center l nH c pF		; L-network

Construction

Support with a nonconductive pole. Separate two 47⅝″ lengths of ½″ EMT conduit by 3⅝″ center-to-center and mount them at right angles. At each end attach #16 galvanized steel wire with a screw or small clamp, or by wrapping and soldering it. Free wire length should be 61½″. Tie the wires to the pole with dacron cord so that their ends are 4⅝″ from the antenna centerline. Install a clamp at the center of each EMT. Between them connect a 346-nH coil made of four turns of the galvanized wire with an inside diameter of 1″ and an outside length of 1⅛″. Solder the center conductor of 50Ω coax through a 47-pF capacitor to one side of the coil. Solder the coax shield the other side. Keep all leads as short as possible and waterproof the coax. Install a ferrite choke at the feedpoint. To suppress shield current, add four more chokes at 17″ intervals along the coax and route it down the support pole.

Sensitivity Analysis

The following table shows the largest performance degradation over the antenna file frequencies in dB when altering a symbol value by Tol.

Symbol      Tol   Gain     ΔG
     s   0.0197   0.09   0.01
     a   0.0394   0.03   0.01
     h   0.0394   0.03   0.01
     w   0.0394   0.01   0.00
     l  34.6439   1.34   0.00
     c   4.6753   0.46   0.00

December 10, 201688–108 MHz