300ohm,

have you modeled the complete 4251 yet...

have you modeled the complete 4251 yet...
Dont have the exact dish dimensions yet, but in a model with an estimated 7' dish, the overall gain pattern is very close to what Channel Master graphed in their tests. (see post #6 in this thread)

300ohm - ...in a model with an estimated 7' dish, the overall gain pattern is very close to what Channel Master graphed in their tests. (see post #6 in this thread)

Your models and analyses of these antennas are appreciated.

Just now noticed your comment in post #6 (http://www.digitalhome.ca/forum/showpost.php?p=970105&postcount=6) regarding the "odd scaling between 590 to 730 mhz".

It appears the data was plotted on standard graph paper and the scales were lettered in by a draftsman (http://en.wikipedia.org/wiki/Technical_drawing), or possibly some of it typed with a typewriter. Sometimes a typewritten number 6 could appear as a 5 after being copied, etc.

If one assumes the horizontal (frequency) scale is linear, then each minor division is an 8 MHz step. That would make what appears in the scale to be 590 MHz to actually be 690 MHz.

Probably the scale (in MHz) goes:

- 450
- 490
- 530
- 570
- 610
- 650
- 690
- 730
- 770
- 810
- 850

Probably the scale (in MHz) goes:
Yeah, I kind of figured that later.

If you come up with a substitute pickup while modeling, let me know. I'll build it and try it this coming weekend.
One very simple way to get about another .5 dBi increase across the board on the P-5 as is, would be to put another identical director and reflector 1 inch below the present ones. :)

P-5 with 2 directors and 2 reflectors at 638mhz, AGT = 1.0, SWR = 1.53 :
http://img137.imageshack.us/img137/392/p53dpattern638mhz2dir2r.jpg

Looks like this weekend is perfect for antenna work!
I will add the pieces and try it out.
Many thanks for the info!

Youll probably only notice the .5 dbi increase on the very weak mid channel stations (not across the board like I previously glanced), but since its such a simple mod, I would do it.

Net Gain P-5 with 2 directors and 2 reflectors as above :
Frequency Net Gain
470 7.4
476 7.6
482 7.91
488 8.32
494 8.84
500 9.46
506 10.18
512 11.24
518 11.88
524 12.69
530 13.51
536 14.22
542 14.79
548 15.17
554 15.38
560 15.44
566 15.44
572 15.51
578 15.69
584 15.78
590 15.74
596 15.61
602 15.5
608 15.75
614 15.87
620 15.85
626 15.78
632 15.7
638 15.68
644 15.81
650 15.93
656 15.98
662 15.98
668 15.95
674 15.9
680 15.85
686 15.82
692 15.85
698 15.89
704 15.9
710 15.84
716 15.95
722 16.34
728 16.65
734 16.9
740 17.12
746 17.33
752 17.52
758 17.71
764 17.85
770 17.9
776 17.85
782 17.64
788 17.36
794 17.12
800 16.75
806 16.12

Wow....thanks 300ohm for your modeling efforts

forget the H1N1 flu
I think I have the more contagious 4nec2 bug : )

I just need to get a big shot of how to use it correctly
and then maybe I can approach the masters level of you guys

Another easy option is 2 directors in front like above, but use a 14" X 4" piece of 1/2" rat wire for the reflector, at the same place as the original reflector.

P-5 at 638mhz, AGT = 1.0, SWR = 1.51 :
http://img18.imageshack.us/img18/3980/p53dpattern638mhz2dirme.jpg

Net Gain for the above:

Frequency Net Gain
470 7.32
476 7.52
482 7.8
488 8.14
494 8.56
500 9.08
506 9.68
512 10.66
518 11.18
524 11.97
530 12.84
536 13.69
542 14.44
548 15.03
554 15.44
560 15.67
566 15.78
572 15.92
578 16.12
584 16.21
590 16.16
596 16
602 15.87
608 16.08
614 16.21
620 16.19
626 16.12
632 16.03
638 16
644 16.11
650 16.24
656 16.28
662 16.26
668 16.22
674 16.15
680 16.05
686 15.98
692 15.97
698 15.95
704 15.9
710 15.8
716 15.87
722 16.21
728 16.49
734 16.71
740 16.93
746 17.13
752 17.33
758 17.54
764 17.71
770 17.8
776 17.78
782 17.61
788 17.37
794 17.17
800 16.86
806 16.3


To get better low channel performance, the folded dipole section would have to be resized and tweaked. That will take some time though.

Ok, got another CM4251 coming soon (if the shipper doesn't squash it). Once I get it I will get dish dimensions and anything else you need.
I didn't get a chance to work outside on the antenna as it was raining and the band was wide open (so I sat in front of the rotor, TV and converter box most of Saturday.
DXing is a sickness.....:p

That would be great. :)

FEDEX tracking says Thursday, the 3rd of December! Hope the claws and beaks don't get to it....

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.

Thats beefier than I thought.

the top and bottom rail and the side upright rails are 1 inch

Whats the distance between the curved top/bottom 1" rail and the next 5/8" horizontal piece at the center ? (it looks to be a different spacing than between the 5/8" ones).

Then, what is the distance between the 5/8" pieces, they look to be the same ?

Also, what is the distance between the 2 vertical 1" side upright rails ?

Measure center of rod to center of rod.

I got some close up detailed pictures from here :
http://www.rocketroberts.com/cm4251/4251sale.htm

The distance from the 5/8 inch elements is 5 7/8 center to center. When I get home tonight I will measure the rest. I had to get it out of the shop because it is so large. Now it's in the..... living room!:eek:

the dish is 10 inches deep
See if you can get that measured exactly. Its pretty critical for finding the focal point.
One way is to take a 7 ft plus straight edge board and lay it across the front middle of the parabola. Then measure from the center of the middle 5/8" rib to the inside edge of the board.

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).
I'm probably not going to bring the antenna to Mississippi this weekend as the weather is supposed to be bad (cold/sleet). If there are any more measurements let me know.
Also, would increasing the bow tie size to work lower channels (no more here over 51) give better gain?

Also, would increasing the bow tie size to work lower channels (no more here over 51) give better gain?
Yes, increasing the bowties 1" to the edge of the reflector gives a good .75 db net gain increase. Look at posts 59 and 61 in this thread. The SWR is less so the net gain goes up. :p

You could cut out the additional sections from some aluminum flashing and pop rivet it on to the original piece, like this.

http://img691.imageshack.us/img691/4643/bowtieaddedsectionnew.jpg

I would only extend the bowtie to the edge of the reflector because extending further would involve more trade offs.

I just purchased a new P-7. No telling when they quit making the Finco's. I think this person had them for 15 to 20 years and he bought them as old stock but new in box. He was keeping a couple spares if anything ever happened to the two he had up.

It will be a couple months before i put mine up as i am going to put another tower up just for the parabolic. If i put another identical director and reflector 1 inch below the present ones, would this be a mod i should do? I guess i could always try it and see what happens and test it.

What about if i closed the back reflector in more, would this be worth it and even that little small reflector at the front. Maybe trying to close the small front reflector in some, i could possibly hurt myself by some of the signal not going into p-7 or in that lil small area. I'll find out the length of the driven element and director also.

This is how they look and both of these are at the same place but these are two different p-7's. I think one is comming down soon for some cleaning up and then going back up.

http://webpages.charter.net/rscarbro/Fincop7aa.jpg

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

I think the P-7 has a 6" director, 6 reflector rods, 13" long, and i think the driven element is 16 1/2" looped.

Nice score, ElvinBishop! Hopefully you can take some measurements before it goes up. :)

I will get all the measurements before it goes up. Some good close up pics and measurements.