[300ohm]

I did notice the original feed has a swr of 2.86 at 585.
So maybe there will be a detectable difference with a mismatch loss of 1.14 dB net gain drops to 18.42 dBI

Because of that, the difference with digital TV may be even more pronounced as it exceeds holl_ands max SWR of 2.7 for digital TV. Holl_ands can explain the exact details, but basically the ones and zeros get corrupted with too high of a SWR in digital. For analog, that high of a SWR was acceptable.

 

[holl_ands]

Briefly, high SWR has TWO effects: 1) nulls along the coax...IF AND ONLY IF the null on a particular channel falls on the END of the coax, you will see Mismatch Loss (ML), aka Net Gain reduction (so try adding a short length of coax) and 2) signals bouncing up and down the coax (e.g. short delay multipath that the equalizer CAN NOT handle) degrades digital signal detection, as measured by the Error Vector Magnitude (EVM) [Search forum & Google for this term].

 

[Chapelrun]

Picture worth a 1000 words

Adding 4 Wires to the MIDDLE section of the Reflector should be very easy to do with some stiff wire (of same metal type, but should be smaller size to minimize wind resistance) and a (friend with a) Welder.

F/B and F/R Ratios were significantly improved for mid and high freqs, but wasn't very effective on low freqs. Raw Gain was within a dB of each other, with a minor SWR improvement to under 2.9.

The next simple step (exercise for the student???) would be to add 2 or 3 more wires above and 2 or 3 more wires below the existing set and see if it affects the low frequencies. But I suspect that all of these wires will need to be extended further out to the sides, although perhaps not as far as the rim.

Thanks for this because this is very similar to what I did in test2 from my first post. The top and bottom reflectors are indeed longer and I did that only because I thought the edge affect from the bowties would need more at the corners. See picture below.

http://i1286.photobucket.com/albums/a615/chapelrun/TEST2_zps5ebf36fe.jpg

Also, I did purchase the material to build the LPDA but have not had a chance to put it together yet.

 

[Chapelrun]

I finished building the small LPDA antenna to go in front of the CM4251. I did take this little antenna and connect it to my Samsung LCD and got 4 channels from the closest stations. So I know it works. My plan it to mount the LPDA on piece of PVC so I can easily find the best focal point on the CM4251. Raining and dark here now - - -so maybe tomorrow or Saturday.

http://i1286.photobucket.com/albums/a615/chapelrun/LPDA-build_zpscd2c86fd.jpg

 

[300ohm]

It take it you used oldsparks directions, but from the look of the photo, that's not 7/32" square tubing. By using bigger tubing, it changes the impedance of the antenna, which can be compensated for to some degree by using wider spacing. The main reason oldsparks used the 7/32" sq tubing was that it perfectly fit the inside plastic section of the coax, which in effect made the tubing the coax shield on the lower boom. Larger tubing negates that effect.

Of course it still works, but my guess the SWR is high, which for digital isn't good.

 

[holl_ands]

Last November I tried to analyze mods to the CM4251 Parabolic, but was frustrated by solid errors when I tried to run nikiml's Optimization Scripts for even simple one or two Variable searches (like Bowtie-to-Parabola and Bowtie-to-Reflector Separations).

I took another look at it and discovered that in 300ohm's "CM4251_Parabolic_CoarseModel" I needed to perfectly match some numbers that were slightly different in the 5th and 6th decimals (Wire numbers were reported by the nikiml's Opt. Script) and (very mysteriously) the FAT Element Radius numbers in the Parabolic Reflector (0.5 and 0.3125 inches) had to be changed to SYmbols before the Opt. Script would stop declaring a missing value for the first variable in the Target Function.

FYI: Geometry Editor is notorious for generating mismatched decimals. Many times I've methodically gone through and performed a Find/Replace with a value rounded off to only two decimals (TRICK: Only search for a number with the first three decimals...it will find ALL of the mismatches). This ALSO makes the 4nec2 File a LOT easier to read and facilitates aligning the fields into columns. [Of course, I much prefer using SYmbols, completely avoiding all of this numeric nonsense.....]

So I could then find the "optimum" Separation for the 9-El LPDA from the Parabolic Reflector (see above)....it is a more "balanced" antenna, with generally flat frequency response, with about 1 dB higher Gain on lower channels, but loses 1-2 dB on the higher channels.

And the first of several runs T.B.D. for a Generic 2-Bay (Whisker) Bowtie with 9 Reflector Rods (using 7 SYmbol driven Variables with a fixed Bowtie-Bowtie Separation):
http://imageevent.com/holl_ands/parabolic/cm4251parafbmod2bay9rropt
I did not include this Generic 2-Bay in below Comparison Chart, since I don't think I've found the optimum...need to try different numbers of Reflector Rods to begin with.....and let the optimization run to completion, rather than aterminating after running for "only" one day & one night.....

I also analyzed three alternatives for a Bigger version of the current 2-Bay FEED:
a) 20% bigger Solid-Triangle-Bowties, incl. the feedlines (ONLY)
b) 20% bigger Bowties and Grid Reflector, but maintain current Separation: Chapelruns' Example
c) 20% bigger Bowties, Grid Reflector and Separation (a COMPLETE 2-Bay Rescale)
http://imageevent.com/holl_ands/parabolic/cm4251parafbmodbiggerbowties

I did NOT analyze a 20% larger Rescale for the ENTIRE Antenna (incl. Parabolic), which is what "SHOULD" be needed to truly shift the current Gain curve downward to NEW UHF Band....for obvious reasons... I hope this exercise illustrates that putting an Antenna with the highest Gain and F/B Ratio in front of the Parabola doesn't ensure good results....it is heavily interacting with the elements in the Parabola...so we need to do a JOINT Optimization on the entire structure to find the "best" FEED dimensions and placement.

I think a JOINTLY OPTIMIZED YAGI FEED may be needed to finally improve on the original....runs T.B.D.....

BTW: My model for CM4251's 2-Bay Solid-Triangle-Bowties with Reflector Rods (using SYmbol driven Variables) still refuses to be run by nikiml's Opt. Scripts, so I've been frustrated from determining the "optimum" Triangle Length, "Tine" Separation, Reflector Rod Length/Seperations, et.al.....Grrrrr....

Comparison Chart for these Parabolic modifications can be found here (I'll let the Chart do all of the explanations):
http://imageevent.com/holl_ands/parabolic

 

[Chapelrun]

Even though I did not build the little LPDA exactly correctly - - - I went ahead and tested it anyway. I took before numbers again with the 20% larger bowties and 20% larger reflector behind and maintaining the original separation (using holl_ands words above). Then I took 3 different reading with the new LPDA with it set at different focal points. In all cases except one the little LPDA had lower numbers than my current best configuration. That one exception is one of the closest transmitters and the LPDA was receiving the signal directly and not reflected from the CM4251 - - - that is why that one was higher.

I may go back and rebuild the LPDA using the correct 7/32 tubing but my guess is it will still not end up better than my current best configuration. I should have looked harder for the correct materials in the first place and I'm sure 300ohm is correct this did affect the little LPDA negitively.

The 1" PVC fits almost perfectly and was a good way to mount the LPDA and adjust the focal point. If someone else builds one of these, I think 1" PVC is very useful to mount the LPDA onto a CM4251.

http://i1286.photobucket.com/albums/a615/chapelrun/LPDA-Test1_zps8679d946.jpg

 

[holl_ands]

Could you describe each of the five photos in the above Album....and perhaps detailed dimensions?

 

[holl_ands]

Parametric Study for VA3RR's 8-Element LPDA (aka GHZ24, oldsparks) :
http://imageevent.com/holl_ands/parabolic/cm4251parabolicfbmod8ellpda
Separation between Center of Parabolic Reflector and SMALLEST Element in LPDA was varied from x = 30-36 inches (also 25.25 and 28.5 inches not shown cuz they're worse).

My first (and so far only) run using nikiml's Optimization Scripts determined an "optimum" value of x=25.25, using my "standard" Target Function that tries to also optimize F/B and F/R Ratios....which it did quite well....but Max Gain was reduced by a couple dB....so I need to spend more time to see if readjusting the Target Function relative weights will yield a "better"...yet still "balanced" compromise result....T.B.D.

I also need to do the same for the 9-El LPDA previously reported....I may want to reoptimize the LPDA for Max Gain, rather than shooting for 20 dB F/B & F/R....cuz it's going to be degraded by the CM4251's Parabolic Reflector.

 

[holl_ands]

UHF 8-Element LPDA per VA3RR (also modeled by GHZ24 & built by oldsparks):

I uploaded 4nec2 analysis for the 8-El LPDA (see oldsparks posts for pictures of his build), which was also used in te Parabolic Feed above (from GHZ24's file):
http://imageevent.com/holl_ands/zigzaglpa/uhflpda/uhfl8elpdava3rr

Note small glitch near 698 MHz for Raw Gain, F/B and F/R Ratios (also replicated in some Parabolic Reflector runs). Although probably not large enough to cause
problems, this can be cured by adding a shorting stub.

Note that Feed Antenna should receive reflected signals over a range of +/- 45-deg in BOTH Horizontal and Vertical angles.....so a VERY wide beamwidth antenna (like the 8-El LPDA with 60-70 deg Beamwidth) may have an advantage over a very high Gain, but narrowband antenna......hmmm.....

 

[GHZ24]

I did try some narrower band (aimed at the present TV frequencies) LPDA feeds but they didn't exhibit the same SWR stability (at least in front of imperfect parabolic-ish reflectors )
That stability is a good part of the attraction of this feed. (at least to me)
The raw gain doesn't reflect this quality.

This may not be as important when feeding a true CM4251 reflector or other close tolerance parabolic reflectors as they don't stress the SWR of the feed as much.
I usually didn't even include SWR in consideration during optimization.
Only after did I look to see how the SWR was because it always seemed to stay good.
Some variations of the KISS reflectors are pretty hard on the SWR of the feed.

 

[LBarouf]

Replace LNB with OTA feed point?

Why couldn't a parabolic antenna be used to concentrate signal in a directional matter, towards an ota antenna? Let's suppose for an instance, that you could secure the OTA antenna, say a small form factor like those shark fins type, at the focal point. Why wouldn't this work?
Nobody manufactures an antenna to replace LNBFs? I would guess you could get signal 80miles away if you were able to concentrate them enough.
Just a tough for people not into C or Ku band TV, but would rather reuse their old dish for OTA. The dish itself can be useful in my opinion, just replace the LNB.
Anyone tried this before?

 

[holl_ands]

Large (7-ft to 2-m Diameter) Parabolic Antennas (such as old BUD for C-Band) have been analyzed using 2-Bay Reflector (e.g. CM4251), 8-Element LPDA and 9-E LPDA as the Feedpoint Antenna:
http://imageevent.com/holl_ands/parabolic
Note that these Antennas extend quite a bit away from the CENTRAL FOCUS of each Antenna.

I have also analyzed Large Parabolics with a SIMPLE, Bi-Directional Stick Dipole or Folded Dipole, but did not post any results due to a HUGE GAIN HOLE in the middle of the UHF Band. Just a taste of the various difficulties encountered when you try to analyzed Parabolic Reflector type Antennas......

Ku/Ka-Band Dishes (e.g. DirecTV & Dishnet) are MUCH SMALLER, using an OFFSET FOCAL POINT. Although I have not modeled this specific configuration, there will be significant losses due to not only the smaller Diameter, but also due to the very wide disparity in sizes between the Dish and the Feedpoint Antenna....it's not longer even "close" to the FOCAL POINT. To see the problem, see the last design on fol. webpage, an M2 2-Bay Bowtie with 18-inch Pizza Platter Reflector:
http://imageevent.com/holl_ands/multibay/2bayrefl/m2refl

I would expect BETTER performance using the usual Bowtie Reflector, whether a FLAT or ANGLED Screen Grid Reflector...which can be as large as or MUCH BIGGER than a small Dish.

 

[majortom]

To me, that would be a complete waste of a dish. Better to keep / use the dish for it's intended purpose.
Unless of course it's warped, damaged and otherwise unsuitable for satellite service any longer, then certainly experiment... probably a better use than trashing it.

 

[300ohm]

Holl_ands, youre a bigger glutton for punishment than I am for even thinking of symbolizing a CM4251, heh.

 

[holl_ands]

Although I Optimized the "SYmbolized" Feedpoint Antenna parameters and Separation from the Parabolic, I haven't (yet) prepared a SYmbol driven Reflector model. I suppose that this could be used to investigate different values for the number/spacing of the Horizontal (ONLY) Bent Reflector Rods, such as are found in the "UHF 2-m Parabolic + 8-El LPDA Feed" model. But I found that it was easy to simply use 4nec2 BUILD utility instead....

But i wouldn't shy away from doing it....it's all in the MATH..."simply" use the Parabolic Equations to generate the X, Y Coordinates for each value of Z. Although they aren't required for the model, Bent Vertical Wires could be generated using the same X,Y,Z Coordinates. I'm already doing something similar to generate the X, Y Coordinates for the VARIABLE Double Angle Reflector, used with FF4 [analysis of larger Reflector Versions should be ready to upload in a week or two....or three]:
http://imageevent.com/holl_ands/multibay/4bayrefl/uhffreeform4baydblanglreflopt

Offset Feedpoint (i.e. most Ku/Ka-Band Dishes) isn't all that much more difficult....start with a standard Parabolic and then "cut-away" (delete) the wires outside the dimensions of the Offset Dish outline. However, I'm not sure whether these dishes MIGHT follow a Parabolic profile ONLY along Vertical lines, since they support a spread of multiple LNB's along the Horizontal, where they MIGHT follow a Circular profile.

 

[Stereocraig]

In some cases, I might agree w/ using dishes for their intended purpose.

Around here though, people give them away free if you're willing to remove them cleanly.

 

[pips]

guys i know this is an older thread but would a fiberglass dish work or would i need to cover it with mesh?i have been looking at ghz24s designs and in looking for the aluminum dishes that have mostly been lost to scrap metal.i was givin 3 fibergass ones ranging from 7 to 10 ft.i have seen guys using the plastic dish tv dishes for wifi.so i was wondering could we use the fiberglass dishes as is or would i need to cover it with mesh for the uhf tv band?

 

[DXer]

Fibreglass dishes have mesh embedded in them. Radio signals can only be reflected off a conductive surface (usually metallic), so fibreglass on it's own wouldn't be reflective.

 

[pips]

thank you i knew there was something more to them.i guess the question now is do dish network round or oval shaped has mesh or metal inside under the plastic?