Antennacraft FMSS FM Turnstile (two Folded Dipoles at 90-degrees to each other) analyzed using 4nec2. [Similar to Wade CFM-FM, Wade OFM-1 and Winegard HD-6010, except non-Folded is 75-ohm output.]
Per FMSS Specs, Folded Dipole Elements = 55-in long with overall structure Height = 8-in. Separation between Elements appears to be about 2-in, leaving about 4-in between the Crossed FD's. Per FMSS Manual, Feedline is 9-in long and appears to be 450-ohms open ladder twinlead, although modeling with 300-ohm twinlead revealed minimal differences, assuming it was the same ELECTRICAL length (shorten by velocity factor).
Antenna Pattern maximums are on two diagonals, 300-deg for High Freqs and 335-deg for Low Freqs. There are significant Gain Holes on the opposite Diagonals (40-60 & 220-240 degrees).
Across all angles and frequencies, Raw Gain ranges from -14 to +1.3 dB (Horizontally Polarized). SWR (300-ohms) was 2.9, dropping to 1.5 at high frequencies.
FYI: FMSS Folded Dipole lengths are not quite optimum for Min SWR. Longer, 62.5-in FD Elements minimized SWR, which was under 1.9, but decreased Raw Gain by a few tenths of a dB.
Date(s): 22 Dec 2011. Album by holl_ands. 1 - 17 of 17 Total. 6971 Visits.
enlarge 57KB, 1024x663 1 FM Turnstile - Antennacraft FMSS 3D View
Study this diagram very carefully to determine diagonal direction for Max Gain.
enlarge 90KB, 1152x278 2 FM Turnstile - Antennacraft FMSS "Front" View [1 big box = 5-inches]
enlarge 252KB, 1024x973 3 FM Turnstile - Antennacraft FMSS Top View [1 big box = 5-inches]
enlarge 141KB, 1152x584 4 FM Turnstile - Antennacraft FMSS FM Raw Gain FORWARD (90-deg) Gain Loss on Low Frequencies
enlarge 142KB, 1152x584 5 FM Turnstile - Antennacraft FMSS FM Raw Gain on DIAGONAL (45-deg) Large Gain Loss on ALL Frequencies
enlarge 137KB, 1152x584 6 FM Turnstile - Antennacraft FMSS FM Raw Gain to SIDE (0-deg) Gain Loss on High Frequencies
enlarge 156KB, 1152x584 7 FM Turnstile - Antennacraft FMSS FM Raw Gain on DIAGONAL (300-deg) Gain Loss on Low Frequencies
enlarge 157KB, 1152x584 8 FM Turnstile - Antennacraft FMSS FM Raw Gain on DIAGONAL (335-deg) Gain Loss on High Frequencies
enlarge 192KB, 1152x584 9 FM Turnstile - Antennacraft FMSS FM SWR (300-ohms) Under 2.9
enlarge 149KB, 1152x584 10 FM Turnstile - Antennacraft FMSS FM Impedance
Thanks much for your significant effort in providing such detailed and useful information on this classic old antenna!
I'd known for years about its directional properties but, until now, never understood the exact location of nulls and the proper orientation of the antenna to maximize it characteristics. I have installed two of them, and one will be re-oriented! - W9VG, Sat, 30 Jan 2021 3:34PM
I'm not even a hobbyist of antennas. Just want really good reception. I found this site trying to figure out if the AntennaCraft FMSS omni can be mistakenly placed upside down, or should it be, or should it be placed vertically so the x axis is pointing toward the stations I like (college,PBS, 88 - 92Mhz). I know it's advertised as omni-dir. Does that mean spherically? or 360* on the horizontal plane? The diagrams are all based on the antenna in the horizontal position (z axis vertical). So I set mine that way and it does work fairly well, but not excellent. Can anyone help point me in the right direction? :) please! - edward, Sun, 25 Aug 2013 10:22AM