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Parabolic Antennas
Parabolic Antennas analyzed using 4nec2 (excluding Circular Polarized).
Date(s): 17 Nov 2012. 1 - 8 of 8 Total. Shared
Comparison Charts - Parabolics
1. 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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CM4251 7-ft Parabolic + 2-Bay Refl Feed
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
13626 Visits
29 Images
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CM4251 Parabolic + 4-Wire FBmod
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.
5573 Visits
15 Images
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CM-4251 Parabolic+FBmod - Bigger 2-Bay
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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CM4251 Parabolic+FBmod - Opt. 2-Bay 9-RR
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".
4459 Visits
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CM-4251 Parabolic+FBmod+8-El LPDA
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.
4603 Visits
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CM-4251 Parabolic+FBmod+9-El LPDA
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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UHF 2-m Parabolic + 8-El LPDA Feed
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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