i wish someone could have measured their CM4251 bowtie alignment with parabolic center to see if they were all made like this ...

to me this misalignment with the focus will seriously reduce the gain, probably by more than 2 dbi...

is my thinking flawed???
No, not at all. You could email Rocket Roberts, he seems to know more about the CM4251 than anyone.

300ohm, do you have the latest compete modeling of the CM4251....id like to try something with it
This is my "coarse" model of it. Check it over and make corrections.
http://www.wuala.com/300ohm/Documents/cm4251_CoarseModel.nec

On ebay when they are listed, CM4251s go for like $150+. Even a bent up one like yours would probably fetch $100. Bet your neighbor didnt know that, heh.

To build the dish in 4nec2, I use Run > Geometry Builder and choose a Round+rectangular grid. Then I knock out and adjust the wires.

I don't believe that the "point" at the focal point of any focusing apparatus, even a perfect (ie diffraction-limited) dish, can ever be smaller than the wavelength being used.
Its got to be pretty small though. Remember those shiny dish type solar cigarette lighters ?

Are you kidding? The infrared radiation those are intended to focus is measured in nanometers; and the spot produced by focusing such radiation is small enough to be considered a "point" for most (non-scientific) purposes. The spot produced by focusing radiation that is measured in cm or meters is not small...at least not smaller than the wavelength of the channel you're watching, which means the spot, or "point", you're dealing with when focusing tv or radio is measured in cm or meters.That's why a dish needs to be at least several times the size of the wavelength you're dealing with, otherwise it's not much (if at all) better than a flat reflector.

To think that a dish can focus radio-signals, tv-signals, or any em-waves (they're all the same thing) down to a tiny point (smaller than the wavelength in question) demonstrates a misunderstanding of how electromagnetic radiation works.

That's why a dish needs to be at least several times the size of the wavelength you're dealing with, otherwise it's not much (if at all) better than a flat reflector.
I wasnt disagreeing. Yeah, for uhf tv parabolas, the dish diameter has to be like more than 5X the wavelength to start to be more effective than a flat reflector or swept forward reflector.

actual measurements for focus gives 39" (from inside of parabolic at center, to the bow-tie reflector).

measurement from inside parabolic at center to bow-tie element is 33".

So your best guess from measuring is, the focal point is in between the bowtie pairs and the front reflector, six inches in front of the bowtie pairs ? Makes sense.

http://img823.imageshack.us/img823/8145/cm4251focalpoint2.jpg

Makes sense? No, not really.

By putting the element in front of the focal point and the reflector behind it, you're putting both in less than optimal positions. Do you see that, or do you disagree afterall that the focal point isn't really a tiny point at those wavelengths? The element should be at the focal point -the point at which the energy's focus is at a size that most closely matches the element length (the optimal position), with the reflector directing at least some of the energy back again. I find it hard to believe the antenna was designed with the element oustide of the focal point, but if so, I'd be modding it.

The CM4251s front end is unlike most uhf parabolic antennas in that it uses 2 stacked driven elements (bowties) instead of just one driven element and a rod reflector.

http://www.digitalhome.ca/forum/picture.php?albumid=301&pictureid=1386

So a portion of the reflected signal from the big dish is caught by both bowties and what misses is caught by the small front reflector, which is then reflected back onto the bowties.

There are a few compromises made in designing it that way, like the small mesh reflector in front making the large dish area effectively smaller.
That antenna was designed when uhf tv channels went up to 83, long before computer modeling. So they probably arrived at what they thought the best compromise was by moving the element boom in and out and testing.

On the P-5 parabola, the focal point was on the short (backwards towards the station pointing) director.
http://img63.imageshack.us/img63/834/p5antennacraft002.jpg

Ah, Now it makes sense. That's kinda weird. It looks like an old radio-shack antenna I used to have...minus the dish.

I'd be curious to know if it wouldn't perform better with a single element right at the focal point...but I imagine they've tried that too.

I'm surprised nobody's tried an off-axis parabolic reflector with no central obstruction for uhf.

Edit: You added the second pic while i was writing (unless I totally missed it the first time)...that's also weird...it looks like the driven element would get signals hitting it both directly and reflected from the dish, which makes me wonder how phasing problems wouldn't arise...though I imagine the reflected signal would be much stronger than the direct signal.

I'm surprised nobody's tried an off-axis parabolic reflector with no central obstruction for uhf.
Those would be difficult to make out of aluminum tubing and keep straight. Its fine for small satellite dishes because they're one solid piece.

I'd be curious to know if it wouldn't perform better with a single element right at the focal point...but I imagine they've tried that too.

Yeah, that needs to be modeled. And considering the resizing of the element needed for the new uhf band, a single folded dipole arrangement like what the p-5 has may prove to be best.
A 9.5 X 9.5 double bay bowtie arrangement would require about a 2ft by 2ft mesh reflector, which really cuts into the effectiveness of the 7 ft dish.

I would think a single element without a reflector would be susceptible to multipath interference - getting the signal on the front side and the reflected off the parabola on the backside.

A smaller antenna like a clearstream2 probably would work well, it has a wide beam path to pick up the reflected signals from the parabola, pretty good f/b ratio and about 10db of gain already.

Yeah, with reflector the C2 is only 20"H X 12" W.
But with parabolas, Ive noticed a higher gain driven element doesnt always translate into overall higher gain. I stuck a SBL loop with reflector (like the X560) in front of a P-5 model instead of its folded dipole element, (about 7 dbi vs 3.5 dbi) and there really wasnt any gain increase to speak of.

300ohm


Using the formula for focus = D^2/16*d = 85^2/16*11 = 41"

actual measurements for focus gives 39" (from inside of parabolic at center, to the bow-tie reflector).

measurement from inside parabolic at center to bow-tie element is 33".


let me clarify,


i should have stated,

-measure from parabolic to bowtie reflector = 39"
-measure from parabolic to bowtie element = 33"

-CALCULATED FOCUS = 40-41"


thus accordingly, the focus would be at or beyond (in front of) the bowtie reflector.....


this to me makes absolutely no sense, and in addition does not correspond to Dragoslavs theory, see my earlier post #138

So, the situation is like this ?

http://img30.imageshack.us/img30/7712/hkaye2.jpg

300ohm,


correct !


BTW, i have emailed CM, and Rocketroberts regarding the focus and the offset center in the side to side direction

300ohm, im modifying the some dimensions with your CM4251 model.

why did you attach the phase lines with a short wire in the x-direction perpendicular to the bowtie....on my antenna the phase wire comes straight in to the bowtie (rivet) and in the same plane as the bowtie?


you:

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antenna:

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I did that to avoid crossing over wires, its only .31 of an inch. I wanted to put the wires where they were riveted onto the solid bowties.

Regarding the mast offset, look at this picture here :

http://www.rocketroberts.com/cm4251/images/austin4251-3.jpg

ok thanks


on the pic it looks like there is more bar to the left than to the right, but not sure if that isnt just the skewed agle of the pic


Channel master and rocket roberts have emailed me saying they were manufactured with an offset to allow for a centered mast, and they dont recommend modifying it....they claim it doesnt affect performance much:eek:

saying they were manufactured with an offset to allow for a centered mast
Correct, thats what I meant to show with the above picture.

If you got more to the left than the right, you may have the dish upside down, heh.

im rerunning the model with variations to the dimensions, but the sweep (470-704 Mhz) is taking forever??? (so far more than 3 hrs...)

changes to file:

http://www.wuala.com/stanicet/Documents/cm4251_CoarseModelrev2_offset.nec

-parabolic with aperture= 85" (could be changed to 84" to compensate for outside of 1" support)

-depth=11.5" (to centerline of parabolic bar - note: VIP measures 10", and 14" depths, but not sure what is the reference point :eek:)

-focus= 40"

-space between parabolic bars - all at 5.32" (could be changed to 5.25" - note: last space to support bar is supposed to be 6.5")

-distance from parabolic to bowtie= 33"

-distance from bowtie to flat reflector=5.25" (this gives total distance = 33 + 5.25 = 38.25")

-offset (sideways from center) bowtie assembly by -1.5"

- changed spacing of horizontal bars in flat reflector (3 different spacings used)

-changed flat reflector bars diameter

-changed phase line diameter

-put phase lines in flat plane vertically

-put phase line separation at feedpoint to 3"

-increased bowtie plate height to 3.87" (you need to verify wire pattern used!)

-agt=0 at 585 (adjusted src wire diameter)

-autosegment at 698 = 11 (you need to adjust segmentation - I had problems!)

Unfortunately the SWR really hurts the gain on this antenna.

The offset by 1.5" has NO effect on the data.

Also tried different bowtie assembly distances but NO big changes - will need to run optimizer maybe


http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2717

http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2718

thanks