InnovAntennas FMDX-9

The InnovAntennas FMDX-9 uses one reflector, seven driven elements, and one director on a 4.1 m (161″) boom.

I modeled the antenna with the AO 9.61 Antenna Optimizer program. This image shows the antenna geometry and segmentation detail. Blue dots mark segment boundaries. The red dot is the 75Ω feedpoint. Green dots mark loads for parasitic element mount capacitance.

The lower phasing line extends to support the insulated parasitic elements. I did not model the support truss, which connects to the lower phasing line and is electrically active, as is the mast.

Modeling Results

Click pattern to denormalize

Calculated performance is for 28 analysis segments per element halfwave, phasing line segment length equal to the line spacing, and parasitic element mount capacitance of 10 pF. Forward gain includes mismatch and conductor losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.

Frequency  Impedance    SWR   Mismatch  Conductor   Forward    F/R 
   MHz        ohms             Loss dB   Loss dB   Gain dBd     dB 
    88    62.3 - j2.6   1.21     0.04      0.02      7.35     20.25
    89    62.3 - j4.0   1.21     0.04      0.02      7.46     22.97
    90    61.7 - j5.3   1.23     0.05      0.02      7.54     25.26
    91    60.9 - j7.2   1.26     0.06      0.02      7.60     26.53
    92    59.0 - j9.9   1.33     0.09      0.02      7.60     25.32
    93    53.3 - j14.0  1.50     0.18      0.03      7.38     20.52
    94    35.8 - j10.6  2.15     0.62      0.04      5.55     12.34
    95    40.6 + j18.2  1.99     0.51      0.04      5.44     11.59
    96    59.6 + j16.1  1.39     0.12      0.02      7.29     17.15
    97    64.9 + j10.1  1.23     0.05      0.02      7.63     19.23
    98    66.7 + j5.8   1.15     0.02      0.02      7.74     20.10
    99    67.5 + j2.8   1.12     0.01      0.02      7.75     20.20
   100    68.8 + j0.7   1.09     0.01      0.02      7.69     19.49
   101    72.8 - j1.5   1.04     0.00      0.02      7.51     17.22
   102    78.5 - j23.2  1.36     0.10      0.04      7.00     11.20
   103    49.0 - j18.4  1.68     0.29      0.03      7.71     14.57
   104    49.7 - j10.4  1.56     0.21      0.02      7.75     19.02
   105    49.7 - j9.7   1.55     0.21      0.02      7.59     20.89
   106    45.4 - j11.9  1.72     0.31      0.02      7.17     19.21
   107    29.8 - j6.8   2.54     0.91      0.04      4.76     11.00
   108    34.2 + j19.7  2.38     0.79      0.04      4.55     10.46

This image shows the antenna current distribution for constant source voltage. Note the unequal current in the two phasing line conductors and dipole halves, and the nonzero current in the phasing line extensions.

Mount Capacitance

The reflector and director mount to the lower phasing line extension with plastic bushings. These curves show the effect of mount capacitance over four orders of magnitude.

Antenna File

InnovAntennas FMDX-9
Free Space
98 MHz
33 6063-T832 wires, meters
x1 = 0			; element positions
x2 = .8
x3 = 1.3665
x4 = 1.8875
x5 = 2.367
x6 = 2.808
x7 = 3.2135
x8 = 3.587
x9 = 4.1
y1 = .78		; element half-lengths
y2 = .833
y3 = .765
y4 = .703
y5 = .646
y6 = .593
y7 = .56
y8 = .48
y9 = .51
d = .5"			; element diameter
p = 1.56"		; 1.56" round wires spaced 1.02559" yield same line Z
s = 1.02559"		; in AO as 3/4" square wires with surfaces 7 mm apart
n2 = (x3 - x2) / s	; # segments for phasing lines
n3 = (x4 - x3) / s
n4 = (x5 - x4) / s
n5 = (x6 - x5) / s
n6 = (x7 - x6) / s
n7 = (x8 - x7) / s

1    x1  -y1   0    x1    0   0    d	; elements
1    x1    0   0    x1   y1   0    d
1    x2  -y2   0    x2    0   0    d
1    x2    0   s    x2   y2   s    d
1    x3  -y3   s    x3    0   s    d
1    x3    0   0    x3   y3   0    d
1    x4  -y4   0    x4    0   0    d
1    x4    0   s    x4   y4   s    d
1    x5  -y5   s    x5    0   s    d
1    x5    0   0    x5   y5   0    d
1    x6  -y6   0    x6    0   0    d
1    x6    0   s    x6   y6   s    d
1    x7  -y7   s    x7    0   s    d
1    x7    0   0    x7   y7   0    d
1    x8  -y8   0    x8    0   0    d
1    x8    0   s    x8   y8   s    d
1    x9  -y9   0    x9    0   0    d
1    x9    0   0    x9   y9   0    d

1    x1    0   0    x2    0   0    p	; lower phasing line
n2   x2    0   0    x3    0   0    p
n3   x3    0   0    x4    0   0    p
n4   x4    0   0    x5    0   0    p
n5   x5    0   0    x6    0   0    p
n6   x6    0   0    x7    0   0    p
n7   x7    0   0    x8    0   0    p
1    x8    0   0    x9    0   0    p

n2   x2    0   s    x3    0   s    p	; upper phasing line
n3   x3    0   s    x4    0   s    p
n4   x4    0   s    x5    0   s    p
n5   x5    0   s    x6    0   s    p
n6   x6    0   s    x7    0   s    p
n7   x7    0   s    x8    0   s    p

1    x8    0   0    x8    0   s   #18	; feedpoint jumper

1 source
Wire 33, center
2 loads
Wire 19, end1 10 pF			; capacitance between lower
Wire 26, end2 10 pF			; phasing line and parasitics

April 15, 201688108 MHz