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 a 92″ boom.
I modeled the antenna with the AO-Pro 8.50 Antenna Optimizer program. This image shows the antenna geometry.
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-4G at a garage sale in Pasadena in the early 1970s. The antenna still works today.
An instruction sheet is here.
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/R is the ratio of forward power to that of the worst backlobe in the rear half-plane. The SWR reference impedance is 300Ω.
88.000 MHz: Impedance 217 + j64 ohms
SWR 1.50
Mismatch Loss 0.18 dB
Wire Loss 0.01 dB
Mismatched Gain 6.21 dBd
F/R 17.14 dB
93.000 MHz: Impedance 218 + j60 ohms
SWR 1.48
Mismatch Loss 0.17 dB
Wire Loss 0.01 dB
Mismatched Gain 6.35 dBd
F/R 20.90 dB
98.000 MHz: Impedance 156 + j54 ohms
SWR 2.00
Mismatch Loss 0.51 dB
Wire Loss 0.01 dB
Mismatched Gain 6.51 dBd
F/R 22.23 dB
103.000 MHz: Impedance 117 + j100 ohms
SWR 2.88
Mismatch Loss 1.17 dB
Wire Loss 0.02 dB
Mismatched Gain 6.47 dBd
F/R 20.02 dB
108.000 MHz: Impedance 145 + j76 ohms
SWR 2.24
Mismatch Loss 0.69 dB
Wire Loss 0.04 dB
Mismatched Gain 7.12 dBd
F/R 16.22 dB
The FM-4G feedpoint is across the phasing lines at a point 25" from the reflector. There the boom supports an insulated terminal block with a single bolt. My antenna had another hole 28.25" from the reflector. These graphs compare the antenna response for the two possible feed locations.
Mismatch loss due to SWR lowers the gain in the upper part of the band for the 25" feedpoint. Paralling it with 5 pF reduces calculated SWR to 1.81 and mismatch loss to 0.38 dB worst case. The dashed blue curve shows the modified response.
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 shift x -45.75 ; Center geometry display 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. The FM-4G 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-4G that lacked assembly instructions. For best accuracy disable bent-wire correction and use at least 25 segments/halfwave.
88–108 MHz