Hi-VHF 8-Element Yagi Antennas, using DL6WU Spacings per K7MEM Javascript: http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.html Ch8-10 8-El Yagi has 93-in Boom and Ch11-13 8-El Yagi has 85-in Boom.
Following shows performance of two antennas, one optimized for Ch8-10 (190 MHz design frequency) and the other optimized for Ch11-13 (208 MHz design frequency). Rapid drop in Gain on next lower channel(s) (e.g. CH7 or Ch10) could cause reception problems. This is due to loss of Raw Gain and ALSO excessive SWR.
The Front-to-Rear Ratio (F/R) was "moderate", rising to "good" only across part of the center channel. In this respect, these are truly "Single Channel Yagi's" and the design frequency readjusted for best performance.
These results are for quarter inch copper tubing (aka refrigerator plumbing) with a non-metallic boom. A run with 1-inch O.D. tubing did not help, it severely restricted the bandwidth (not shown).
A 300-ohm input 8-El K7MEM Folded Dipole with 3-in separation between elements was compared to the "stick" Yagi, showing a significant improvement in SWR, although there was negligible difference in Raw Gain. The flatter SWR curve illustrates the broader operating bandwidth of the Folded Dipole configuration.
4nec2 antenna simulation files are included below if anyone wants to try different element sizes. They also list the element lengths and spacings to assist DIY construction.
These 75-ohm antennas need a 1:1 Balun, either of the Choke, 1/4-Wavelength Matching or Transformer variety. A 300-ohm antenna using a 4:1 Balun can be constructed by substituting a Folded Dipole for the Driven Element.
CONCLUSION: DL6WU type Hi-VHF Yagi's can provide good Gain, but only over a bandwidth of three adjacent Hi-VHF channels. And Front-to-Rear Ratio drops off away from the center channel. These Gain results are consistent with Ken Nist's HDTVPrimer NEC Sim results for Wade "Single Channel" 10-Element Yagis: http://www.hdtvprimer.com/ANTENNAS/WadeSCY.html
Either K6STI's 5-El Yagi, a LPDA (Log Periodic Dipole Array) or Zig-Zag LPA is recommended for DIY Full Band coverage: http://imageevent.com/holl_ands/zigzaglpa
Date(s): May 27, 2009. Album by holl_ands. 1 - 26 of 26 Total. 11293 Visits.
enlarge 0KB, 1024x671 1 Hi-VHF 8-Element Ch8-10 Yagi: 3D View
Keep clicking on image until see full rez version. Click on "X" as usual to exit view window.
enlarge 0KB, 1024x747 2 Hi-VHF 8-Element Yagi Antennas: Raw & Net Gain Yagi only covers three adjacent channels.
Optimized for Ch8-10 (190 MHz Design Freq) and Ch11-13 (208 MHz Design Freq).
enlarge 0KB, 1024x748 3 Hi-VHF 8-El. Yagis vs 14-El. LPDA: Gain & F/R Ratio Both are approx. 100-inches long.
LPDA has 2 dB lower Gain, but covers entire band with much higher Front-To-Rear (F/R) Ratio.
enlarge 179KB, 1152x710 4 Hi-VHF 8-Element Yagi Antenna Raw Gain, F/B & F/R Ratios Only good across Ch7-9.
enlarge 191KB, 1152x710 5 Hi-VHF 8-Element Yagi Antenna SWR (75-ohms) only good across Ch8 & Ch9.
