Finco FM-4G
The Finco FM-4G is an outdoor directional receiving antenna for the FM broadcast band once manufactured by the Finney Company. It is a dual-driven Yagi design with six elements on an eight-foot boom.
I modeled the antenna with the AO-Pro 7.03 Antenna Optimizer program. This image shows the antenna geometry.
This image shows the segmentation of the driven elements and phasing line. Blue dots indicate analysis-segment boundaries. The red dot indicates the feedpoint.
This is the FM-4G I've used since the early 1970s. It came with the feedpoint 25" from the reflector. An instruction sheet is here.
Modeling Results
Below are calculated performance figures for a segmentation density of 40 segments per halfwave. Mismatch loss is due to SWR. Wire loss is due to conductor resistance. Mismatched gain is forward gain including wire and mismatch losses. F/B is the ratio of forward power to that directly to the rear. The SWR reference impedance is 300 ohms. The feedpoint is 28.25" from the reflector for the modeling results and patterns.
88.000 MHz: Impedance 267 + j116 ohms
SWR 1.53
Mismatch Loss 0.19 dB
Wire Loss 0.01 dB
Mismatched Gain 6.11 dBd
F/B 17.27 dB
90.000 MHz: Impedance 287 + j118 ohms
SWR 1.50
Mismatch Loss 0.17 dB
Wire Loss 0.01 dB
Mismatched Gain 6.11 dBd
F/B 18.87 dB
92.000 MHz: Impedance 293 + j98 ohms
SWR 1.39
Mismatch Loss 0.12 dB
Wire Loss 0.01 dB
Mismatched Gain 6.24 dBd
F/B 19.95 dB
94.000 MHz: Impedance 281 + j73 ohms
SWR 1.29
Mismatch Loss 0.07 dB
Wire Loss 0.01 dB
Mismatched Gain 6.41 dBd
F/B 21.18 dB
96.000 MHz: Impedance 254 + j55 ohms
SWR 1.29
Mismatch Loss 0.07 dB
Wire Loss 0.01 dB
Mismatched Gain 6.60 dBd
F/B 22.68 dB
98.000 MHz: Impedance 222 + j52 ohms
SWR 1.43
Mismatch Loss 0.14 dB
Wire Loss 0.01 dB
Mismatched Gain 6.75 dBd
F/B 24.04 dB
100.000 MHz: Impedance 195 + j62 ohms
SWR 1.64
Mismatch Loss 0.27 dB
Wire Loss 0.01 dB
Mismatched Gain 6.88 dBd
F/B 24.27 dB
102.000 MHz: Impedance 180 + j80 ohms
SWR 1.85
Mismatch Loss 0.40 dB
Wire Loss 0.01 dB
Mismatched Gain 7.00 dBd
F/B 22.96 dB
104.000 MHz: Impedance 180 + j99 ohms
SWR 1.93
Mismatch Loss 0.46 dB
Wire Loss 0.02 dB
Mismatched Gain 7.20 dBd
F/B 21.11 dB
106.000 MHz: Impedance 202 + j100 ohms
SWR 1.75
Mismatch Loss 0.34 dB
Wire Loss 0.02 dB
Mismatched Gain 7.53 dBd
F/B 19.49 dB
108.000 MHz: Impedance 220 + j31 ohms
SWR 1.40
Mismatch Loss 0.12 dB
Wire Loss 0.03 dB
Mismatched Gain 7.77 dBd
F/B 18.13 dB
Patterns
Feedpoint Location
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 = 28.25 ; Mounting holes at 25 & 28.25 shift x -45.75 ; For better geometry display 1 rp -r 0 rp r 0 d 1 de1p -de1 -2 de1p -de1s -2 .375 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 ; 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 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 1 d2p -d2 0 d2p d2 0 d 1 d3p -d3 0 d3p d3 0 d 1 de1p -1 -2 f -1 -2 w ; Transmission 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. Each parasitic element half-length was shortened 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. All corrections and equivalents were computed with the YO Yagi Optimizer program. The FM-4G has two possible mounting holes in the boom for the feed bracket. This model was developed to determine where to place the bracket on a used FM-4G that lacked assembly instructions. For best accuracy disable bent-wire correction and use at least 25 segments/halfwave.
More is here.
Updated February 26, 2008
