Dish wise, it has more elements than the CM4251, so definately more potential for gain. Make sure you bend them back smooth.

Thanks 300ohm and those are not mine that is up but from the person i got mine from. I think he is going to take the one down in the spring and bend the elements back smooth. I'll take some pics and measurements when i start putting mine together. It makes me wonder why Channel Master chose not to use as many elements but also you can see the CM is built really good and strong.

wow that second photo


http://webpages.charter.net/rscarbro/Fincop7bb.jpg


It looks like they have that parabolic on a rotor...

The rotor is fixed to the side of the tower at the base and at the top of the mast that the parabolic is attached to (the top of the mast is supported with a bearing support that allows the vertical support mast to turn yet be laterally braced) .... hmmmm... food for thought...

DOH!!! the wife will have a fit if she sees something like a CM4251 being mounted on the side of my tower.

Here is a few more pics. The tower with the p7 mounted to the side is a 100 ft tower and the p7 at the top of the tower is on a 60 ft tower.

Notice on this p7, it has more than just the clips on the elements, to brace it up more.

http://webpages.charter.net/rscarbro/fincop7d.jpg

Here is the tower up at the top and i think the other antennas are for amateur radio.

http://webpages.charter.net/rscarbro/FincoP-7a.jpg

Here is the other p7 on the 60 ft tower and looks like a small vhf antenna below it.

http://webpages.charter.net/rscarbro/FincoP-7b.jpg

but also you can see the CM is built really good and strong.
Yeah, but you could strengthen up your P-7.

Putting on the 2 long vertical curved 1" diameter rods like on the CM4251 isnt practical.

But if you got hold of some soft 10 - 14 gauge aluminum wire, you could wrap 2 to 4 more long vertical wires around the horizontal reflectors (similar to the two that already exist) that would significantly beef it up. :p

yes, but will it perform correctly with a not so perfect restoration, especially the parabolic reflector alignment issues?

thanks

Post some more pictures of the damage.

From what I see, it doesnt look like that hard of a repair job. Heck, it still even has some of the original gold anodizing on it, heh.
The dimensions for it from VIP-307:
The monster is here!
Here come the specs!
It IS 7 feet in diameter!
the dish is 10 inches deep
the bow ties are 33 inches from the dish back
the dish elements are 5/8 inches thick
there are 15 curved elements
the top and bottom rail and the side upright rails are 1 inch
And it arrived safely!
Any questions, let me know.
============================================================ ==============================================
Here are more dimensions
the uprights are 30 3/8 inches on center
the uprights stand 79 1/2 tall but their curved length is 82 3/4
the curved top piece at the top of the arch is 6 3/4. At the connect point to the uprights they are 3 1/2.
I was able to get a better dish depth measurement. It is actually 14 1/2 inches deep (sorry about the wrong # earlier).
============================================================ ===============================================

You want to get the dish as smooth and even and symmetrical as a real dish. If you have a dent on the back side of the dish, thats not as important as it would be on the front side.

thanks, 300ohm,

ill post more pics this evening,

in the meantime i noticed another oddity, or maybe im missing something here, the uprights are about 30 1/2" apart, making the center at 15 1/4",

but for some reason, with the crossarm of the bow-tie assembly removed from the parabolic, i measured the location where the bracket holds the arm to the parabolic at about 14 1/4", instead of 15 1/4", off center by 1" ! :eek:

is that a manufacturing defect or what....

i tried to resquare the parabolic frame thinking it would correct the center, but its not enough...

in the meantime i noticed another oddity, or maybe im missing something here, the uprights are about 30 1/2" apart, making the center at 15 1/4",

but for some reason, with the crossarm of the bow-tie assembly removed from the parabolic, i measured the location where the bracket holds the arm to the parabolic at about 14 1/4", instead of 15 1/4", off center by 1" !

is that a manufacturing defect or what....

VIP-307 got "the uprights are 30 3/8 inches on center".

For horizontally polarized North American TV, the vertical uprights do little to nothing for gain, they just hold the reflectors in place.

Do they look bent ?

The important thing is to mount element bar in the center of the dish, no matter where it is in relation to the uprights.

he grinded the 60 ft tower into parts and let the huge 15 ft VHF antenna and parabolic UHF antenna with rotar all fall to the ground....

Thats a shame about that tower, Im sure it could have been repaired too. If the trash man hasnt come, I would pick up the pieces and especially the aluminum rods to that huge 15 ft VHF antenna, the pieces are very handy to build other antennas.

this picture shows the offcenter for crossarm (third horizontal bar from bottom with greyish marks and 2 drilled out holes), it is at 13 3/4" from the left side, not 15". should i assume this is to allow for the passing of the mast which is attached at 15" (centered) on the parabolic at the top and bottom....

the verticals are 30" apart at the middle, and 30 1/2" apart at the top and bottom of the parabolic



http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2661

should i assume this is to allow for the passing of the mast which is attached at 15" (centered) on the parabolic at the top and bottom....

Thats a tough question to answer from here, because the center element boom could be on a slight angle mount. The important thing is that the driven element and reflector end up at focal point area of the reflector. I would put it back in the original holes, then check the whole alignment later and bend it if need be.

Checking for alignment later could be done with a string. The focal point is where the element would be the same distance from the dish reflector, at any point on the dish reflector.

Ill have to check my notes in the parabola thread as to whether the focal point is on the bowties or on the small front reflector.

the center mount seems to be in a straight line with the straight bar, but i did notice that when standing the arm on its reflector end, the arm was not perfectly vertical, so i tried making vertical, oops I guess i should have left it alone ;)


i thought i understood the focal point math, but im not sure how, if, a lateral shift affects the focal point....

http://mathworld.wolfram.com/Parabola.html

according to this definition, the focal point is that unique point where there is equal distance between the focal point to any given point on the prabolic and between that point on the parabolic and the directrix line which is a straight line at some distance behind the parabolic...and this for all points along the parabolic




thanks for the tips

The definition is fine, thats why I said use a piece of string to check the focal point. The bar that extends from the dish is meaningless to the focal point. The position of the bowties and the small bowtie reflector are what matter. So the bar may have been angled/offset to compensate for the mast.

i just rechecked the arm, placing the small reflector surface flat on a level surface with the arm sticking upward, and put a level against the arm, and the angle in the arm will actually be in the vertical direction when mounted, weird that it will effectively have a vertical tilt and also the horizontal offset we discussed, relative to the parabolic...:confused:

i know im obsessing now, and i will put it back together the way it was, but am i correct in understanding a focal point, by definition (and logic) MUST be centered in the Y direction and Z direction (using the coordinate convention for nec modeling)....


in the meantime here is the other issues you asked me about (pictures)...

http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2659

http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2651


so the bar onto which the mast attaches has been squished and twisted, i think if i try bending or hammering it it will just break....i can drill 2 new holes for the mast bracket which can then arrive squarely in position...and since the bar is twisted, the holes may need to be moved sideways to compensate, and for alignment with the upper mast bracket....

http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2663


i think here you can see the misalignment of at least one of the parabolic bars (look at the 3rd bar up from the center, bottom of this pic) , particularily at there ends, some tend to curve in more than the otheres, and some of the bar to bar spacings are not consistent with each other...again if i bend i may break, there doesnt seem to be any play. i cannot see exactly where the change in shape happened along the bars, or why, there are no kinks, dents, twists, or slippages????

i know im obsessing now, and i will put it back together the way it was, but am i correct in understanding a focal point, by definition (and logic) MUST be centered in the Y direction and Z direction (using the coordinate convention for nec modeling)....

Yes, if its a perfect dish, like I think the CM4251 is. (other dishes like the Dish Network's satellite dishes arent, so the LNB mount is offset)

so the bar onto which the mast attaches has been squished and twisted, i think if i try bending or hammering it it will just break....i can drill 2 new holes for the mast bracket which can then arrive squarely in position...and since the bar is twisted, the holes may need to be moved sideways to compensate, and for alignment with the upper mast bracket....
Thats the top bar, correct ? That may be easier to fix if you took it off, by drilling out the pop rivets. Then you could hammer it into correct shape against a soft surface. Then re pop rivet it or bolt it back on. With aluminum, you want to work carefully and have some softer support behind it.
i think here you can see the misalignment of at least one of the parabolic bars (look at the 3rd bar up from the center, bottom of this pic) , particularily at there ends, some tend to curve in more than the otheres, and some of the bar to bar spacings are not consistent with each other...again if i bend i may break, there doesnt seem to be any play. i cannot see exactly where the change in shape happened along the bars, or why, there are no kinks, dents, twists, or slippages????

I would lay the dish onto a wooden plywood board and then tap the very end of the bent reflector with a wooden or rubber mallet until its into correct shape, ie following the dish pattern of the others. With aluminum, you want a relatively soft material like wood to support and tap against.
I wouldnt worry about the difference in vertical spacings between the reflectors as long as they are about +/- 1" of each other.

Keep in mind, youll probably never get it perfect (and it may not have been perfect originally), but you want to get it close to an even dish.

It doesnt look that bad.

so far, i have done the following;

-rivet fastened a broken off end of one parabolic bar,
-clutching the parabolic bars connector sleeves i bent back into alignment a few of the worse bars
-using stainless hardware and sleevs, i have reattached the feed cross arm assembly
-ive removed that rubberized mastic from the cable connections on the phase lines
-i worked a bit on the kinked support bar


now, im not sure which end was up or down, but based on the stain markings of the hardware i think i have it right, and the kinked bar appears to be the bottom end, also given the angle of the cross arm assembly, i will have a slight tilt of the feed upwards relative to the parabolic, and i think still that sideways offset, I think...:confused:

is undoing that kinked support bar really worth it, im not sure i can get the completely flatened section out of the bar to make it round again????

is undoing that kinked support bar really worth it, im not sure i can get the completely flatened section out of the bar to make it round again????
Maybe not. Its mounted on the back of the vertical support bars and the rest of the reflectors are mounted on the front of it. So its probably not that big of a deal, except for structural support. Most of that bar looks fine anyway.

so the bar onto which the mast attaches has been squished and twisted
Is that particular bar 5/8" or 1" in diameter ?

Looking at the picture, its my gut feeling that if that bar (and its bottom counterpart) was placed on the inside of the dish, instead of the outside as it is now, there would be a little more gain. (but maybe only around .5 dbi) Of course, that would mean different placement of the mast holes in the center bar.

One way to strengthen it if need be, would be to place a split tube of aluminum, ID same as the OD of the bar, over it and pop rivet that in place.

Also, for a bit more lower end gain on the CM4251, look at post #97 of this thread.

the bars are 1" OD


heres what I did, using spare parts, i had the exact same size, gauge, color aluminum from the boom of another VHF antenna,

i bent it using a makeshift jig, of course i kinked it along the way a bit, but its a vast improvemnt over the original, i think - boy the way i did that, it was VERY hard to bend, if you go to hard for too long a time, bang, it kinks...

anyway i think im off by max 1/8" at the max curvature, when compared to the other of the 2 bars

heres a pic of the new bar, i think this is the bottom end, the only clue i have to go on is that the mark of the danger label is still left on the underside of the feed cross arm assembly, and I researched old pics of this model, and it appears like the label was placed on the underside of the cross arm in manufacturing....

http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2671

other repairs:

http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2669

http://www.digitalhome.ca/forum/picture.php?albumid=355&pictureid=2670




thanks 300 ohm for the info, ill have a look into the gain hacks....