Finco FM-4

The Finco FM-4 is dual-drive Yagi with six elements on a 92″ boom. It was once manufactured by the Finney Company. The design dates from the early 1960s. Model FM-4G added gold-colored “corrodizing.”

I modeled the antenna with the AO 9.53 Antenna Optimizer program. This image shows the antenna geometry. The shorting straps on the driven elements broadband the response.

This image shows the segmentation of the driven elements and phasing line. Blue dots mark analysis segments. The red dot is the feedpoint.

I found this FM-4 at a garage sale in the early 1970s. I still use it today. An instruction sheet is here.

Modeling Results

Calculated performance is for 28 analysis segments per conductor halfwave. 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     212 + j65    1.54     0.20      0.01      6.17     17.00
    89     216 + j74    1.55     0.20      0.01      6.15     18.12
    90     220 + j76    1.53     0.20      0.01      6.16     18.97
    91     220 + j74    1.52     0.19      0.01      6.19     19.72
    92     218 + j69    1.51     0.18      0.01      6.25     20.49
    93     213 + j62    1.52     0.19      0.01      6.31     20.92
    94     205 + j56    1.56     0.21      0.01      6.36     21.22
    95     194 + j51    1.63     0.25      0.01      6.41     21.47
    96     179 + j49    1.74     0.33      0.01      6.45     21.70
    97     166 + j50    1.88     0.42      0.01      6.47     21.96
    98     150 + j51    2.07     0.56      0.01      6.46     22.33
    99     138 + j57    2.26     0.71      0.01      6.44     22.51
   100     128 + j65    2.47     0.86      0.02      6.42     22.68
   101     121 + j74    2.67     1.01      0.02      6.40     22.81
   102     115 + j85    2.85     1.14      0.02      6.39     21.74
   103     112 + j95    2.97     1.23      0.02      6.42     20.16
   104     113 + j104   3.02     1.26      0.02      6.49     18.86
   105     118 + j112   2.96     1.22      0.02      6.63     17.81
   106     129 + j114   2.72     1.05      0.03      6.84     17.09
   107     140 + j106   2.46     0.85      0.03      7.05     16.53
   108     144 + j80    2.27     0.71      0.04      7.13     16.23

Antenna File

Finco FM-4G
Free Space
98 MHz
41 aluminum wires, inches
rp = 0
de1p = 20.5
de2p = 37.5
d1p = 49.5
d2p = 70.5
d3p = 91.5
r = 33.25				; Actual length: 34
de1 = 30
de2 = 23
d1 = 24.75				; Actual length: 25.5
d2 = 24.75				; Actual length: 25.5
d3 = 22.75				; Actual length: 23.5
de1s = 10
de2s = 12
d = .43					; Actual diameter: .375
w = .125
x1 = 31.5
xm = 33.5
x2 = 35.5
f = 25					; Another boom hole at 28.25
	
1    rp   -r    0    rp    r    0    d  ; Reflector

1   de1p -de1  -2   de1p -de1s -2  .375 ; Rear driven element
1   de1p -de1s -2   de1p -1    -2  .375
1   de1p  1    -2   de1p  de1s -2  .375
1   de1p  de1s -2   de1p  de1  -2  .375
1   de1p -de1   0   de1p -de1s  0  .375
1   de1p -de1s  0   de1p -2.3125 0 .375
1   de1p -2.3125 0  de1p  2.3125 0  1.1	; Diameter equivalent to 0.5" tubing + bracket
1   de1p  2.3125 0  de1p  de1s  0  .375
1   de1p  de1s  0   de1p  de1   0  .375
1   de1p -de1  -2   de1p -de1   0  .375
1   de1p  de1  -2   de1p  de1   0  .375
1   de1p -de1s -2   de1p -de1s  0  .375
1   de1p  de1s -2   de1p  de1s  0  .375

1   de2p -de2  -2   de2p -de2s -2  .375 ; Forward driven element
1   de2p -de2s -2   de2p -1    -2  .375
1   de2p  1    -2   de2p  de2s -2  .375
1   de2p  de2s -2   de2p  de2  -2  .375
1   de2p -de2   0   de2p -de2s  0  .375
1   de2p -de2s  0   de2p -2.3125 0 .375
1   de2p -2.3125 0  de2p  2.3125 0  1.1
1   de2p  2.3125 0  de2p  de2s  0  .375
1   de2p  de2s  0   de2p  de2   0  .375
1   de2p -de2  -2   de2p -de2   0  .375
1   de2p  de2  -2   de2p  de2   0  .375
1   de2p -de2s -2   de2p -de2s  0  .375
1   de2p  de2s -2   de2p  de2s  0  .375

1   d1p   -d1   0   d1p    d1   0    d  ; Directors
1   d2p   -d2   0   d2p    d2   0    d
1   d3p   -d3   0   d3p    d3   0    d

1   de1p  -1   -2    f    -1   -2    w	; Phasing line
1   f     -1   -2    x1   -1   -2    w
1   x1    -1   -2    xm    0 -1.125  w
1   xm     0 -1.125  x2    1   -2    w
1   x2     1   -2    de2p  1   -2    w
1   de1p   1   -2    f     1   -2    w
1   f      1   -2    x1    1   -2    w
1   x1     1   -2    xm    0 -2.875  w
1   xm     0 -2.875  x2   -1   -2    w
1   x2    -1   -2    de2p -1   -2    w
1   f      1   -2    f    -1   -2    w

1 source
Wire 41, center

This models the Finco FM-4G receiving antenna for the FM broadcast band. I
shortened each parasitic element half-length by 0.75" to account for the inner
0.5"-diameter reinforcement tubing section, the rectangular element-mounting
bracket, and the 1"-square boom. The bracket and reinforcement sections
increase element effective diameter to 0.43". The driven elements are modeled
using actual lengths and diameters, plus an inner 1.1"-diameter section
equivalent to the reinforcement section and mounting bracket. I calculated all
corrections and equivalents with the YO Yagi Optimizer program.

April 25, 201688–108 MHz