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Comparison Charts - Parabolics (24 May 2013)Comparison Charts for 7-ft CM4251 and 124-in Repurposed "C-Band" Parabolic Antennas analyzed.
Fol. is an On-Line Gain Calculator for Parabolic Antennas, as well as a discussion on Illumination Efficiency:
https://www.everythingrf.com/rf-calculators/parabolic-reflector-antenna-gain
http://www.w1ghz.org/antbook/chap4.pdf
So there is about a 5 dB Efficiency Loss (at 698 MHz) for the 124-in Parabolic Antenna
due to uneven Illumination by the Feed Antennas.
And about a 4 dB Efficiency Loss (at 698 MHz) for the 7-ft CM4251. |
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 | 2.
CM4251 7-ft Parabolic + 2-Bay Refl Feed (17 Nov 2012)Channel Master CM4251 7-ft Diameter Parabolic Antenna with 2-Bay Bowtie+Reflector Feed
(as delivered) analyzed using 4nec2. This old antenna was designed for the OLD UHF Band (470-806 MHz).
Files from <300ohm> for CM4251 (Coarse Model) and 2-Bay Bowtie feed:
http://www.wuala.com/300ohm/Documents |
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 | 3.
CM4251 Parabolic + 4-Wire FBmod (21 Mar 2013)UHF Channel Master CM-4251 Parabolic Reflector Antenna with 2-Bay Triangular-Bowtie, minimally modified
with 4 additional wires in middle of Reflector (smaller radius material would be acceptable).
This old Antenna is STILL designed for the OLD UHF Band (470-806 MHz).
Raw Gain was only minimally affected and as expected, there was significant improvement to F/B & F/R Ratios,
but very little if any increase on the lower channels.
This minimal modification illustrates that significant improvement in F/B & F/R Ratios are possible with
minimal increase in wind resistance. Optimal placement of additional wires should be investigated
to determine whether it is better to add say 2 or 3 more wires above and 2 or 3 more below or
to extend to the sides more...or both. |
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 | 4.
CM-4251 Parabolic+FBmod - Bigger 2-Bay (23 Mar 2013)CM-4251 7-ft Parabolic Reflector + 4-Wire FBmod (Front/Back Ratio Mod) with two alternative mods:
a) Bigger 2-Bay Bowtie (Rescale Factor = 1.2), but same size Reflector Grid. [Easiest to build.]
b) Bigger 2-Bay Bowtie & Reflector Grid (Rescale Factor = 1.2) but NOT the Separation between them. [More difficult.]
For example see: http://s1286.photobucket.com/user/chapelrun/media/TEST2_zps5ebf36fe.jpg.html
c) Bigger 2-Bay Bowtie & Reflector Grid AND the Separation between them (Rescale Factor = 1.2). [Even more difficult.] |
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 | 5.
CM4251 Parabolic+FBmod - Opt. 2-Bay 9-RR (23 Mar 2013)UHF CM4251 Parabolic+FBmod (4-Wire Front/Back Mod) with OPTIMIZED 2-Bay Bowtie with 9 Reflector Rods
analyzed using 4nec2. [Additional runs needed to optimize number of RR's.]
Nikiml's Python Optimization Scripts were used to find the "best" set of SEVEN variables:
Bowtie Whisker Length, Tine Separation, Feedline Separation, Separation from Bowties to Reflector Rods,
Reflector Rod Length, Rod-Rod Separation and Separation between Bowties and Center of Parabolic Reflector.
For this initial run, Bowtie-Bowtie Spacing was fixed at 9.0-inches.
Note how the 9 Reflector Rods don't cover the entire vertical height of the Bowties. This is unexpected
and may be due to either a trade-off re amount of incoming signal blockage or perhaps an inherent
resonance (note location of highest & lowest RR directly behind the center of each Bowtie)...T.B.D.
Addendum: Or perhaps the Optimizer was trying to adjust the Antenna Pattern for a more
EVEN Illumination of the ENTIRE Parabolic Reflector. Highest Gain was NOT the "Best". |
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 | 6.
CM-4251 Parabolic+FBmod+8-El LPDA (24 May 2013)CM4251 Parabolic Reflector with 4-Wires added to Reflector (FBmod) and 8-Element LPDA
(Log Periodic Dipole Array) Feed analyzed using 4nec2.
CM4251 2-Bay Feed is replaced by 8-Element LPDA, see fol. analysis, which was fabricated by oldsparks:
http://www.qsl.net/va3rr/hdtv/HDTV.htm
GHZ24 offset & reversed the LPDA for insertion into a Parabolic Reflector 4nec2 File:
http://ghz24.freeshell.org/antenna/models/newermodels/parabolas/feeds/lpda8.nec
Note glitches near 698 MHz for Raw Gain, F/B and F/R Ratios. Although probably not large enough to cause
problems, this can be cured by adding a shorting stub. |
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 | 7.
CM-4251 Parabolic+FBmod+9-El LPDA (21 Mar 2013)CM4251 Parabolic Reflector with 4-Wires added to Reflector (FBmod) and 9-Element LPDA
(Log Periodic Dipole Array) Feed analyzed using 4nec2.
CM4251 2-Bay Feed is replaced by 9-Element LPDA, see fol. analysis:
http://imageevent.com/holl_ands/zigzaglpa/uhflpda/uhf9ellpcad34
Note uniformity of Raw Gain with small loss at higher freqs, low SWR and excellent
F/B and F/R Ratios across entire UHF Band.
When 9-El LPDA is used with CM4251, the small loss in Gain with frequency stayed about
the same with a small improvement in SWR.
However LPDA's excellent F/B & F/R Ratios were reduced across all freq, esp. on lower channels.
Hence this reduction is attributed to inadequacies in the CM4251's Reflector. |
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 | 8.
UHF 2-m Parabolic + 8-El LPDA Feed (20 May 2013)UHF 2-meter Diameter Parabolic with 8-Element Log Periodic Dipole Array (LPDA) Feed
analyzed using 4nec2. Shortest LPDA Element to Parabola x=32.26452-inches.
Dimensions and 4nec2 file from GHZ24's website:
http://ghz24.freeshell.org/antenna/models/newermodels/parabolas
BTW: Based on trying different Feed Antennas with CM-4251 7-ft and 124-in (10.3-ft) Parabolics,
it is likely that a differentt Antenna will provide significantly higher Gain, esp. on lower Freqs. |
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