: Building a UHF Parabolic Reflector Like A CM4251?
2007-12-11, 02:15 PM
The parabolic was hugely successful for UHF when Channel Master had its CM4251 on the market, but their costs to fabricate it were too high and when they came up with the CM4228 it was close enough in performance and a lot less of a hassle to mount so they discontinued the big dish. AntennaCraft offered a competitor but it was said to have been poorly fabricated and the tolerances were too loose to guarantee consistent performance. You can find lots of the CM4251s still up and running in the U.S. but they're old now and not worth the trouble of buying.
Nowadays Wade Antenna continues to market giant parabolics for CATV head ends (in single or double mounts) with the selling point not being their very high absolute gain but rather their phenomenal dBv output, allowing much better signal distribution options with little or no amplification needed until much further down the distribution chain. http://www.digitalhome.ca/forum/showthread.php?t=41075
The big single-dish CATV parabolic from Wade is minimum about $3,000 but you also really need to have their engineers consult on your head end tower's ability to hold one of those, amongst other things like the electronics and signal properties.
2008-03-14, 06:24 PM
Does this new antenna have a better gain over the UHF parabolic antennas?
2008-03-14, 06:29 PM
There aren't any commercially manufactured consumer parabolic UHF antennas out there anymore apart from used ones, but given that the CM4228 8-Bay Reflector is close or equal in performance to the last of the great consumer parabolics, the CM4251, and that the DBGH smokes the CM4228, I think its safe to say that the answer to your question is yes.
If you're talking about Wade parabolics for CATV Head Ends, that's a completely different league! :)
2008-06-06, 04:50 AM
I have been a huge fan of satellites for a long time and I am now looking into OTA UHF TV. I have a few dishes in my back yard, 3 are the regular 18 inch Bell style ones, but I have on seven and a half foot C band dish. I don't use it anymore, and it is facing West with a clear view of everything.
I was wondering, if I were to put a UHF antenna at the focus point of the dish, where the LNB used to be, and then put a preamp, ran a cable to my house and boosted it again would this work to get me a decent UHF signal?
I am very new to the game and have read up as much as I could on the subject, I found a forum on another site about someone asking if he could use a 18 inch dish to do this and people told him that at most he would get 10DB gain, but I was wondering if this alternate setup would serve a purpose.
Hmm it is almost 4 AM so I hope I am making sense and not making too many typos.
I am in montreal, about 45.35 -73.33
What do you guys think?
Is it worth my time.
*ps my idea for a preamp and booster could be completely wrong I am just trying to make sense of all the things I have read tonight.
2008-06-06, 03:17 PM
Head ends for CATV have been using large parabolic antennas for UHF like the ones from Wade for many decades, and the gear required to handle the energy they develop is high end and very expensive for a typical consumer.
That's because electrically you can expect no real increase in gain over a typical UHF antenna like a CM4228 but you will have a very high voltage induced in the reception dipole. This could be risky for any typical consumer-grade downstream gear.
On the other hand, the much smaller CM4251 parabolic was a great antenna that worked well in consumer use but it was basically obsoleted by the CM4228 (same or similar performance as the parabolic but with a much sturdier design that was easier to manufacture). The AntennaCraft antennas shown in HDTV101's post were taken off the market long ago because they were not as powerful as the CM4251 and they were flimsy too.
If you go ahead and build a UHF parabolic it will be very interesting to see your results. :)
2008-08-28, 05:39 PM
Heres a gain chart of the 7 foot CM4251 uhf parabola antenna, probably THE best consumer parabola ever made, for comparison. Keep in mind their results were not computer modeled, but tested over their test range. As such, I think their dBd gains are a little over optimistic. From Ken Nist's page "Comparing some commercially available antennas" :
Why computer simulations? (non-essential reading)
A few years ago QST, which is the principal publication of the HAM radio community, announced that they would no longer accept advertising for antennas if the ads contained gain figures that were measured experimentally. Henceforth any such gain figures would have to be the result of computer simulations. There were two big problems with the experimental data:
1. The experimental antenna is affected by its surroundings. Computers can do true “free space” modeling.
2. The process of choosing the surroundings encouraged overly favorable choices. Most of us would call it cheating, but they justified it to themselves by the belief that their competitors were doing it.
Also, note the odd scaling between 590 to 730 mhz and the relatively poor performance at channel 23.
but their numbers don't add up.
Yeah, I find it odd too. Maybe its a typo and should read 2.2 dB ?
2008-11-18, 12:44 AM
I recently acquired a Dish Network parabolic reflector. ( the newer large eliptical style model ) I put a "classic" Radio Shack single bay bowtie in front of it at its focal point, using a gooseneck from an old lamp. The focal points on those dishes are off-center. So far the results havent been that good.
Parabolic reflectors need to be about 5 to 10 wavelengths wide before they start really being effective.
2008-11-18, 11:30 AM
Parabolic reflectors need to be about 5 to 10 wavelengths wide before they start really being effective.Like one of these Wade CATV Head End monsters:
And that's just the single head version... :D
2008-11-19, 02:47 AM
we've got alot of the old channel masters parabolas around here and i thought about trying to get one down but it seems they are way more optimized for high uhf, hence not worth trying to secure a 7foot wide 'wind sail' when i want channels 17-30
from what i remember the guys on that forum saying, those wade dishes might be 4 figures cost wise
2008-11-19, 02:59 AM
we've got alot of the old channel masters parabolas around hereYep, the venerable CM4251 that ruled the UHF roost until Channel Master realized that by putting 2 CM4221s side by side with crossbars and feeds (what became the CM4228) they could get basically the same performance with much lower manufacturing costs. Its amazing how many of the CM4251s are still up and functional after 30+ years!
Check out this site: http://www.rocketroberts.com/cm4251/cm4251.htm :)
And here's the gain chart for the CM4251, linked from the site above:
2008-11-19, 03:24 PM
Wade's CATV head end parabolic dishes are described in their PDF file here:
I trust I'm not repeating anything here, but ~
I ran across this site that provides lots of pics and - would you believe - a narrative on a new effort to get the definitive consumer parabolic cloned...
I loved to dream about DX'ing with that baby back in the day, and now at least have the opportunity to revisit the 4251 and all the other similar editions that were once available. A great site and a wonderful read!
UPDATE: Well, I guess I did repeat the site that Stampeder mentioned earlier, but -hey- good things do bear repeating! Permit me to recommend however that in the event you haven't visited the site yet - go ahead and take another trip down memory lane.
It's also nice to know that the 4228/A is competitive with the old guy too - thanks Stampeder!
2009-01-26, 05:04 PM
I had this pleasant email conversation with a fellow who still uses his same Finco P-5 UHF Parabolic reflector TV antenna after almost 30 years, including through many Pittsburgh winters and North Carolina hurricanes!
Stumbled onto this site by accident. Anyway, here's a pic of a P-5 that's been serving me well, first for 25+ years atop a 60' tower in Pgh,Pa- now in its first year of service in Hatteras,NC, where I've now retired. Can't put any height to it because of the winds here along with the corrosion factor but she's still pulling in stations inland whose patterns aren't exactly favoring those of us out here 40 miles from the mainland. One reason I put it back up was to get the weather & radar info on one of the local digital channels about 100 mi. inland. Thanks, Dan
(By the way, Youngstown Ohio stations were snow-free and back when there were blackout rules, I used to have the neighborhood over in the 70's to watch the Steeler home games that were unavailable over the air in Pgh.)http://www.user.dccnet.com/jonleblanc/Canada_TV_Stations/finco-p-5.jpgHi Dan, thanks for that! Those P-5s are getting rarer and rarer, especially in any hurricane areas! ;)Thank you for your very nice reply. Your insight concerning our wind strength here is right on the money. I'm very limited here as to what I can do- especially height. The mounting pole is 2-piece so I can lift the P-5 out when calamitous conditions arise. Dan.
2009-01-26, 05:20 PM
Are there any dimensions? Focal length, etc. Sure wood like to model this.
2009-01-26, 05:22 PM
Good idea! I'll have to email him about that but no promises. ;)
2009-01-26, 08:09 PM
That looks like the 5 foot parabolic uhf antenna I used to have. I bought mine from Lafayette Radio, and it was their brand name. Interesting to know it was built by Finco, I assume.
2009-01-26, 08:25 PM
Interesting PDF files:
2009-01-27, 12:46 PM
300 ohm, the curved screen of yours that I have been using in my models. Was it modeled after the Finco? I have just gotten to the point where I want to play with the shape of the parabolic curve. But in 4 inch sections using 2x4 from the Depot. I can work up a basic idea to run through NEC with a five foot reflector, I just hate doing the math to get the correct angle of deflection for each flat section in the reflector screen. If I draw it life size looking at it from the top and use 4 inch mirrors and a light could get me dialed pretty close. But for this monster it looks like distance from elements to reflector is like a foot and a half. It would be interesting to have just the cross section dimensions at the equator of all the commercial dishes, cataloged on a drive. Anyway, once I'm close in Nec, should be fun to play with.
It was interesting to note that as I resized, stretched, cloned, etc. and then checked on the floor with a laser pen/mirror in the real world, elements would be smaller but the screen wanted to focus 100 mm above them somewhere. Once I refocused each four inch section of the screen on to the active elements on paper and then transcribe that to NEC, actual improvements in gain were nominal. But I don’t call myself a stupid old man for nothing. I could be wrong. A reflector distance of 100mm "felt bad" and then try to curve the screen. Now a focal distance of 400 mm might be the ticket. Five foot wide, but a SBGH is 2 1/2 feet tall. 2 1/2X5 foot screen. Actually the screen flipped so it's vertical would be easier to support on a mast.
So near here where I live is the Fisher Plaza, a part of Fisher Broadcast. Rather large building that doesn't block any OTA. Right at the top is mounted what looks like the above. 5' tall 2 1/2' wide reflector, not the whole parabolic, just equatorial cross section, stood on end pointed at their broadcast tower downtown. I am trying to get some photos to post. Be interesting to model that one.
2009-01-27, 01:48 PM
Was it modeled after the Finco?
Nope. If you want a parabola (or patch, plane, cylinder, sphere, box, helix etc) 4nec2 has a 4nec2Build program included that makes it easier.
You want to mount the driven element at the focal point on a parabola. Thats where the concentrated signal is located. Google for a focal point calculator. The focal point of an elipse is the same thing only in 2 dimensions.
Keep in mind, the five foot dish Finco P-5 was designed when uhf channels went up to 83, and its top gain, like the CM4251, was near channel 77 IIRC. For the newer uhf range, an eight to ten foot or more dish would be more appropriate using a folded dipole. On the new uhf range, a DBGH has more gain than a 5 foot parabola.
If sticking a SBGH on a parabola as the driven element, about a 25ft or more dish would be needed, heh.
2009-01-27, 04:07 PM
Here are some Jerrold model numbers of UHF parabolic antennas they sold circa 1960, all with 75 ohm inpedance:
J3065A-72 Channels 14-22
J3065B-72 Channels 23-51
J3065C-72 Channels 52-69
J3065T-72 Channels 70-83
Maybe some documentation exists somewhere for them.