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 9.67 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     220 + j77    1.54     0.20      0.01      6.26      16.57
    89     222 + j90    1.58     0.22      0.01      6.22      17.69
    90     226 + j94    1.58     0.23      0.01      6.22      18.56
    91     227 + j94    1.57     0.22      0.01      6.25      19.31
    92     227 + j90    1.56     0.21      0.01      6.30      20.09
    93     223 + j84    1.55     0.21      0.01      6.37      20.56
    94     217 + j78    1.56     0.21      0.01      6.45      20.86
    95     207 + j71    1.59     0.23      0.01      6.52      21.13
    96     194 + j68    1.68     0.29      0.01      6.58      21.34
    97     181 + j67    1.78     0.36      0.01      6.62      21.61
    98     165 + j65    1.94     0.47      0.01      6.64      21.98
    99     153 + j70    2.10     0.59      0.02      6.64      22.18
   100     143 + j77    2.28     0.72      0.02      6.64      22.39
   101     134 + j86    2.46     0.85      0.02      6.63      22.59
   102     129 + j96    2.61     0.96      0.02      6.64      22.52
   103     127 + j106   2.71     1.04      0.02      6.68      20.82
   104     129 + j116   2.74     1.06      0.02      6.76      19.44
   105     136 + j122   2.65     0.99      0.02      6.92      18.32
   106     150 + j123   2.42     0.82      0.03      7.13      17.56
   107     164 + j110   2.16     0.63      0.03      7.33      16.97
   108     168 + j77    1.95     0.48      0.04      7.41      16.61

Antenna File

Finco FM-4
Free Space
98 MHz
49 6063-T832 wires, inches
rp = 0
de1p = 20.5
de2p = 37.5
d1p = 49.5
d2p = 70.5
d3p = 91.5
r = 34
de1 = 30
de2 = 23
d1 = 25.5
d2 = 25.5
d3 = 23.5
de1s = 10
de2s = 12
d = .375		; Tubing diameter
eqd = 1.35		; Equivalent diameter of 0.5" sheath at bracket
b = 2.3125		; Bracket half-length
w = .11			; Phasing line diameter
x1 = 31.5
xm = 33.5
x2 = 35.5
f = 25			; Another boom hole at 28.25
	
1    rp   -r    0    rp   -b    0    d  ; Reflector
1    rp   -b    0    rp    b    0  eqd
1    rp    b    0    rp    r    0    d

1   de1p -de1  -2   de1p -de1s -2  .375 ; Driven element 1
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   -b   0  .375
1   de1p   -b   0   de1p    b   0   eqd
1   de1p    b   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 ; Driven element 2
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   -b   0  .375
1   de2p   -b   0   de2p    b   0   eqd
1   de2p    b   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    -b   0    d  ; Directors
1   d1p    -b   0   d1p     b   0  eqd
1   d1p     b   0   d1p    d1   0    d
1   d2p   -d2   0   d2p    -b   0    d
1   d2p    -b   0   d2p     b   0  eqd
1   d2p     b   0   d2p    d2   0    d
1   d3p   -d3   0   d3p    -b   0    d
1   d3p    -b   0   d3p     b   0  eqd
1   d3p     b   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 49, center

This models the Finco FM-4 receiving antenna for the FM broadcast band. The
elements use a 0.5"-diameter reinforcing sheath that I included in the mounting
bracket model. It extends about 1.5" past the bracket, which I ignored.

The FM-4 has two possible mounting holes in the boom for the feed bracket.
I developed this model to determine where to place the bracket on a used
FM-4 that lacked assembly instructions.

June 15, 201888–108 MHz