Scanner Antenna

I used the AO 9.66 Antenna Optimizer to design this antenna for 30–174 and 440–500 MHz reception. It uses arms of EMT conduit and sloping wires of galvanized steel. Height is 10 feet. 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 intermediate frequencies the wire slope concentrates radiation at the horizon and the 90° twist promotes an omnidirectional pattern. A short dipole provides UHF reception.

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    7.64 + j10.4  6.83     3.51      0.10     -3.90      0.00
    36     202 - j180   7.35     3.75      0.07     -4.06      0.00
    42    26.1 - j59.6  4.96     2.53      0.06     -2.75      0.00
    48    19.4 - j9.1   2.68     1.02      0.05     -1.12      0.01
    54    22.3 + j30.3  3.20     1.39      0.04     -1.37      0.01
    60    32.4 + j72.3  5.22     2.68      0.04     -2.50      0.01
    66    58.0 + j126   7.36     3.76      0.04     -3.40      0.02
    72     134 + j200   8.91     4.40      0.03     -3.84      0.02
    78     377 + j190   9.48     4.62      0.03     -3.83      0.03
    84     359 - j181   9.04     4.45      0.04     -3.42      0.04
    90     140 - j181   7.69     3.90      0.04     -2.62      0.05
    96    69.6 - j115   5.74     2.96      0.04     -1.44      0.06
   102    44.9 - j65.4  3.64     1.70      0.04      0.00      0.06
   108    35.6 - j25.2  1.96     0.49      0.04      1.29      0.06
   114    34.2 + j10.8  1.58     0.23      0.05      1.52      0.04
   120    40.2 + j48.8  2.89     1.17      0.05      0.40      0.00
   126    59.5 + j95.3  4.88     2.48      0.05     -1.20      0.06
   132     118 + j155   6.71     3.45      0.05     -2.51      0.16
   138     300 + j154   7.62     3.87      0.05     -3.25      0.29
   144     305 - j133   7.28     3.72      0.06     -3.38      0.47
   150     123 - j138   5.79     2.99      0.06     -2.90      0.68
   156    64.6 - j80.1  3.79     1.80      0.07     -2.00      0.92
   162    46.1 - j34.9  2.05     0.55      0.07     -1.16      1.18
   168    41.8 + j2.0   1.20     0.04      0.06     -1.22      1.44
   174    45.9 + j35.5  2.07     0.57      0.05     -2.49      1.71
   440    60.3 + j108   5.72     2.95      0.03     -2.32      2.00
   450    73.8 + j124   6.15     3.18      0.03     -2.90      2.43
   460    92.9 + j129   5.78     2.99      0.02     -3.07      1.63
   470     108 + j125   5.35     2.75      0.02     -3.07      0.84
   480     119 + j119   4.98     2.54      0.02     -3.10      0.47
   490     127 + j107   4.52     2.27      0.02     -3.21      0.42
   500     122 + j91    3.95     1.90      0.02     -3.29      0.69

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

Maximum and minimum UHF gain over 360° azimuth.

Antenna File

Scanner Antenna
Free Space
30 37 44 52 110 122 135 146 160 174 440 470 500 MHz
11 zinc wires, inches
s = .9487533				; EMT half-spacing
a = 24.8958				; EMT arm length
h = 59.413916				; wire height
w = 4.2531083				; wire offset
u = 7.0126703				; UHF dipole half-height
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   0  0  u   0  0  s    #16
1   0  0 -u   0  0 -s    #16
1 source
l = 344.1866
c = 46.77906
Wire 1, center l nH c pF		; L-network

Construction

Support with a nonconductive pole. Separate two 49¾″ lengths of ½″ EMT conduit by 1⅞″ 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. Free wire length is 62⅞″. Tie the wires to the pole with dacron cord so that their ends are 4¼″ from the antenna centerline. Install a clamp at the center of each EMT. Between the clamps connect a 344-nH coil made of four turns of the galvanized wire with an inside diameter of 1″ and an outside length of 1⅛″. Extend the coil wire above and below the clamps to form the UHF dipole, with 14″ between wire ends. 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. If possible, route the coax away at right angles to the antenna, avoiding the EMT arms. If not, run it out a foot or more, drop it vertically, and add five more chokes at 18″ intervals along the coax.

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   0.08
     a   0.0394   0.03   0.04
     h   0.0394   0.03   0.04
     w   0.0394   0.01   0.01
     u   0.0394   0.04   0.16
     l  34.4187   1.33   0.00
     c   4.6779   0.46   0.00

December 11, 201788–108 MHz