PDA

View Full Version : DIY 6' parabolic, cloned!



Jim Thomas
11-15-2017, 05:21 PM
I built a clone of the Channel Master 4250, the 6' brother of the 7' model, the 4251. Every knows the CM parabolics were discontinued around 2000 and IF anyone has one which they have refurbished and have put up for sale, it will cost you your life savings or you go to the bank for a loan to buy it. There are some ridiculous prices on fleaBay or CraigsList for refurbished 4251's. And if you decide to go the professional CATV parabolic route, you can go to Wade Electronics and purchase one of their models. You will probably break the bank buying one of theirs also.

Just for kicks, I wanted to see if I could build a similar design parabolic and what kind of performance I would get. I researched this project diligently for about one month, along the way collecting tons of pictures found on the internet. I then spent one week designing this antenna in professional design software, attempting to match the measurements for the 4250. So now - the antenna has been built. It took me the better part of 4 days to build this antenna. And now I really would like to know how it performs. BUT - I have been waiting for any type of DX to show up here in southwest Missouri, quaintly known as the *Tropo Black Hole of the Midwest." Its hard to tell if the parabolic will have any kind of respectable performance during a really good tropo opening, since I'm still waiting. The antenna has been complete for about 30 days.

In the pictures below, I have it mounted on a temporary tilt-over mast, anchored to a concrete slab. I've tested it for one week, facing due south (my best DX direction), with the antenna sitting as seen in the pictures. There is about two feet below the parabolic to the ground. During the one week test, (not counting my locals), the antenna was receiving two stations signals almost 24/7 in sub-decode (no decode), during virtually no DXable weather conditions:

KAFT 9/13 Fayetteville Arkansas @ 106.14 miles;
KFSM 18/5 Fayetteville Arkansas @ 107.78 miles.

These two stations would occasionally get strong enough to fully decode, but most of the time the DX conditions were almost non-existent, yet the signals for those two stations were there almost constantly.


General specs of the parabolic:

6' radius parabola X 11" depth at center;
X3 10' pieces of 1/2" rigid PVC to construct perimeter of parabola (third piece cut to finish length to complete 6' radius);
x2 10' pieces of 1/2" rigid PVC (cut to 6' pieces for vertical support);
1/2" galvanized hardware screen in center as extra duty reflector;
Several pieces of antenna aluminum rod, placed horizontally at 8" spacing;
x2 bay driven antenna array from recycled 4 bay CM UHF antenna, spaced at 32" in front of the rear center of the parabola.

An observation about the PVC support arm for the antenna array - from the U-bolt clamp on the mast, the support arm bows some due to the weight of the antenna array. I haven't figured out yet what to do to *straighten* the PVC curve, without adding any extra weight on the support arm.

Pictures:

Channel Master 4251, the 7' model;
x3 pictures of my DIY 6' parabolic with 2 bay antenna array.

I would like to have comments and opinions about taking on a project like this. This is my third antenna design to complete.

NN2E
11-15-2017, 06:16 PM
That's cool, Jim.

You might consider using some Fiberglas pieces, in place of PVC, where sagging is an issue.

Eventually, my P-7 is gonna have to come down to fix all the damage from the March 1st storm. When I do this, I'll put the tape measure to the antenna and get all of it's dimensions for you. I found the paperwork, from when I modified / restored it but, there are so many numbers scribbled down I'm not sure which ones I finally landed on. :-(

73, Ed NN2E
Owner / Operator - Murphy's Law Test Site & Thunderstorm Proving Grounds

DXseekerMO
11-17-2017, 11:26 AM
"........southwest Missouri, quaintly known as the *Tropo Black Hole of the Midwest."

LOL!!!!

EdmondDxer
11-20-2017, 05:00 PM
If your antenna turns out to be successful you could start your own business. There is a market for a parabolic uhf antenna that isn’t a wallet buster. I would pay $250 for a 4250 style antenna.