Ch22 9-Element Folded Dipole Yagi (518-524 MHz) analyzed using 4nec2 [see *.nec.doc file below]. Design Dimensions for 75-ohm Yagis from fol. MTM Scientific UHF TV Antenna Kit Article: http://www.mtmscientific.com/yagi.pdf It is Optimized for 8 MHz Bandwidth (to ensure no problems on the band edges...and if ReScaled Upwards) centered on Ch22 [518-524 MHz].
Ch22 9-El FD-Yagi - 75-ohm MTM Scientific: Ch22 Raw Gain = 11.5 to 11.4 dBi, F/B & F/R Ratio Min. = 17.0 dB and SWR (75-ohms) = 3.3 to 5.0 is EXCESSIVE. BOOM LENGTH = 760-mm.
Unfortunately the MTM Scientific Designs (like similar "Formula Driven" Yagi Designs such as K7MEM), are NOT OPTIMIZED. Although Gain Curve was VERY FLAT, F/B & F/R Ratios were Best on 502-508 MHz [vice desired 518-524 MHz] and SWR Minimum is ACTUALLY near 498 MHz rather than desired 521 MHz. [BTW: Since MTM article assumed NON-METALLIC Boom, no Boom Corrections are involved.]
Compare to OPTIMIZED Designs for similar 9-El Yagis: use pull-down Navigation Menu above:
Ch22 9-El FD-Yagi - 300ohm OPTimized: Ch22 Raw Gain = 12.7 to 12.9 dBi, F/B & F/R Ratio Min. = 26.4 dB and SWR (300-ohms) Under 2.2. BTW: Gain for Optimized 75-ohm FD-Yagi was not quite as Good. And note that F/B & F/R Ratios could be improved at the expense of a small amount of Forward Gain and/or slightly higher SWR.
Ch22 9-El FLF-Yagi [Flat-Loop-Fed] - 300ohm OPTimized: Ch22 Raw Gain = 13.2 to 13.4 dBi, F/B & F/R Ratio Min. = 40.2 dB and SWR (300-ohms) Under 1.7.
Dimensions (all in cm) from below 4nec2 Text File:
' Cumulative Element Spacings (cm): SY S1=0 SY D2=7.2 SY S2=S1+D2 SY D3=5.7 SY S3=S2+D3 SY D4=6.8 SY S4=S3+D4 SY D5=7.9 SY S5=S4+D5 SY D6=9.4 SY S6=S5+D6 SY D7=11.0 SY S7=S6+D7 SY D8=12.8 SY S8=S7+D8 SY D9=15.2 SY S9=S8+D9 ' ' Element Lengths (cm): SY L1=28.8 SY L2=27.3 SY L3=26.3 SY L4=24.7 SY L5=23.8 SY L6=23.6 SY L7=21.6 SY L8=21.5 SY L9=19.7 ' ' Distance of Yagi above & below Center line (NOT USED IN THIS MODEL): SY Zyag=0 ' SY Relem=0.3175 ' Yagi Passive Element Radius (in cm for 1/4-in O.D.) SY RACT=Relem ' Yagi Active Element Radius (in cm for 1/4-in O.D.) SY FW=1.25 ' Width between Long Elements in Folded Dipole (in cm for 1/2-in) SY FD=FW/2 ' Distance between Center of Boom and Folded Dipole Element SY Gap=2.5 ' Feedpoint Gap (in cm for 1-in Gap)
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