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Any low-loss UHF baluns available?

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Old 01-30-2006, 09:52 AM   #1
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Default Any low-loss UHF baluns available?

I've been fiddling with my CM4228 installation to try to figure out why it has *lower* gain than my old CM3018. My expectation was to have higher gain from this antenna. I currently am investigating three theories about its poor performance:

1) Because of the attic installation, the CM3018 does better since it is a single element positioned between joists, minimizing signal blockage. The CM4228 may experience additional loss due to signals of different strengths being received at different elements. If this is my main problem, then I'll have to live with it, since I like the fact that the CM4228 can be rotated.

2) Perhaps the balun sucks on the CM4228. That was an easy thing to test: I swapped it with the old (10 years) balun from the CM3018 and the signal level at my television went from 72 to 93! So, that made me wonder "How much loss is in one of these baluns?" A web search uncovered this bit of alarming balun measured data: http://www.kyes.com/antenna/balun.html . Unfortunately, he doesn't have any measurements for any Channel Master baluns. In any case, it does not appear that the balun shipped with the CM4228 is not particularly good, since the old one works better (at least for some frequencies). As a matter of fact, I am now able to get DTV from one of the Baltimore stations for the first time ever by using the old balun.

3) Finally, there is reason to believe that the feed lines on the CM4228 are poorly designed: http://www.hdtvprimer.com/ANTENNAS/cm4228.html . But to even think of evaluating this point, I would still need a good pair of matched baluns, which I don't currently have. Perhaps the AntennasDirect DB8 has better baluns included, but there is no way to tell. I may just purchase one and compare it...

The bottom line here is that the balun performance is the one thing that is not considered in the charts on the HDTV Primer website: http://www.hdtvprimer.com/ANTENNAS/comparing.html . Unfortunately, it is critically important to proper antenna performance. Those curves are somewhat meaningless when you may have several dB of balun loss thrown into the equation...

So my question is this: Is there a low-loss (<1 dB or so) 300-ohm-to-75-ohm balun available for the UHF frequency band? If so, where can I purchase two of them?

Other thoughts?

Reg
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Old 01-30-2006, 10:01 AM   #2
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Your problem as nothiong to do with your bauln: ther answer is simple: the 4228 is much more directional than the 301X that you have mounted in the attic: You can't possibly exploit what the 4228 can do when it is not mounted on the roof: it's like asking a racehorce to pull a plow! Unless you are willing to mount that 4228 out in the clear, away from any close obstructions, it won't perform at 50% of what it's capable.
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Old 01-30-2006, 12:37 PM   #3
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Quote:
Originally Posted by tigerbangs
Your problem as nothiong to do with your bauln:
I think you'll have a hard time backing up that statement. If you have measured data for ChannelMaster baluns, I'd be happy to see it. Otherwise, all anyone can say is that the insertion loss is greater than 0 dB. My test tells me the new balun is worse than the old one when attached to the CM4228. BTW, the only part number I can find on the new balun provided is "Made in China". ;-)
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Originally Posted by tigerbangs
ther answer is simple: the 4228 is much more directional than the 301X that you have mounted in the attic: You can't possibly exploit what the 4228 can do when it is not mounted on the roof: it's like asking a racehorce to pull a plow! Unless you are willing to mount that 4228 out in the clear, away from any close obstructions, it won't perform at 50% of what it's capable.
Agreed. The *theoretical* gain of the CM4228 is 6 dB higher than that of a CM3018 into a 300-ohm load. However, I'm not driving a perfect 300-ohm load, but rather I'm driving a balun with unknown impedance characteristics and insertion loss followed by a CM7777 which has an unknown input impedance. The other big difference is that the corner-reflector yagi-style UHF antennas sum their signals in free space, while the bowtie antennas sum the signals in a combiner circuit. Certainly I believe that the bowtie antennas will be more affected by the vagueries of the attic (or tree limbs, etc.), not because it has additional gain, but because of fact the signals are combined *after* being received. Any amplitude imbalances can thus result in a much lower signal once combined.

The bottom line is that those gain curves, for whatever reason, do not apply in my situation...to the tune of >6dB error. I'm sure part of it is due to the attic installation. However, I am beginning to believe that part of it is also due to flaws in the design of the balun as well as the network that sums the signals from the two halves of the antenna.

Antennas are cheap, and I'm sure I can find a new home for a CM4228 if needed, so I'm up for further experimentation. I may purchase an AntennasDirect 91XG, even though it won't be overly steerable, just to see what might be possible from my attic. My bet is that it will significantly outperform the CM4228, but who knows. In the meantime, if anyone knows where I can buy a good-quality, low-loss UHF balun, I would certainly appreciate it. I've looked, but have failed to locate anything.

Thoughts?

Reg
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Old 01-30-2006, 01:07 PM   #4
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Damn, Man...you don't listen either: Put it on the roof, fer chrissakes! Stop looking up your own butt and solve the problem the RIGHT WAY!
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Old 01-30-2006, 01:23 PM   #5
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Nor do you. I've detailed the reasons why it won't go on my roof. If you have a problem with me trying to improve my attic installation, I'm sorry, but that's your issue, not mine.

The issue of poor-quality baluns (if real, and I think it is) affects every installation, regardless of whether the antenna is on the roof or in the attic. Does it really make sense to install an antenna that is *twice*, or even *four times* as big as is needed just because antenna manufacturers won't include a decent balun? No.

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Old 01-30-2006, 02:04 PM   #6
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I'm sure that the impedance/mismatch of these cheap, mass produced baluns across the UHF band varies quite a bit. You may just have found one that worked a bit better at the frequency of the Balto station that you now receive.

The VSWR of these antennas w/baluns and pre-amps across the UHF bandwidth varies all over the place. When you put your antenna in the attic the VSWR of the entire antenna system is very different than if you had it high and away from obstructions. That's going to change again, dramatically, when you have 6"+ of snow on your roof.

I don't believe the CM4228 design is the problem. I'm sure the UHF loop antenna would probably be more immune to multiple reflections in an attic than the 4228 and it's use of multiple elements and crossover feed network. If the one antenna works better for you in an attic then use it. But to conclude that bad baluns and a poor design of the 4228 antenna is your problem is incorrect.
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Old 01-30-2006, 03:04 PM   #7
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Agreed. I don't want to imply to anyone that the CM4228 is in any way "bad". After all, I'm getting HDTV in my attic from four different stations that are 50 miles away and from several other stations that are closer than that. That's from an antenna which I bought for $50, shipped.

I agree that the antenna/combiner/balun/coax/amplifier mismatch problem is a very complicated issue which is made even moreso by the likely near-field issues relate to my attic (like the gutter). Still, not all balun loss is due to mismatch effects. Many of these things apparently have pure resistive losses at UHF of around 2 dB or so.

I guess I was thinking that a balun designed for *just* the 2:1 frequency range of UHF could be made cheaply that would have significantly lower loss than one that also included VHF. I imagine that such a thing is what is built into the 91XG. At least that's what I hope. But such a balun does not make sense for a CM4228, since it actually is a decent antenna at some VHF frequencies. I can even see channel 4 (fuzzy analog) from 50 miles away using this antenna. So I understand why a broadband balun would be included with the antenna.

Unfortunately, I've looked, but I haven't succeeded in finding a low-loss UHF one and was hopeful that someone here would know of something. I trolled around the Dxer sites, but many of them use the antennas with the custom-designed built in baluns. Perhaps that says it all...

I DO think that there may be more loss in the combiner network than is modeled by the hdtvprimer site, simply due to the fact that it "floats" in mid air and may be much closer to the antenna elements than is modelled, though I haven't checked that. Perhaps a good project for a rainy day...

I have the 91XG on order and will report its performance relative to the CM4228 here once I get it set up. I won't be able to rotate that one very far, but if it works well, I may not need to. We shall see.

Thanks!

Reg
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Old 01-30-2006, 03:52 PM   #8
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that link http://www.kyes.com/antenna/balun.html tell you how to make a low loss balun out of 10 worth of coax
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Make Your Own. If you are interested in a narrow range of frequencies, such as one low band TV channel, FM broadcast, high VHF (Band III), a chunk of UHF only, or lets say about 20% bandwidth, you can make your own low loss balun out of a 1/2 wave length piece of co-ax. Cut a 1/2 wave piece of co-ax as shown. To make it for your frequency, simply find the wave length, and correct for the lower speed of light in co-ax cable. To match 300 ohm to 75 ohm, use 75 ohm cable (e.g RG 59) instead of 50 ohm cable as called for in this link. This is about 66% (solid) or about 78% (foam) shorter in co-ax than in free space.
- why not try that - The length of the "loop" would be between 10" and 7" - you could try various 3 or 4 lengths to see which works best for less than $1 total -(use the 2 best ones) -- and then report back here -
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Old 01-30-2006, 04:40 PM   #9
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Thanks, I saw that but didn't pursue it since he said it would give about a 20% bandwidth. I was hoping for something to cover all of UHF. Maybe I'll model it to see what the match and loss look like for such a structure and see what *exactly* he means by 20%. I'm expecting that this will have gigantic suck-outs within UHF...

Thanks again,

Reg
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Old 01-31-2006, 07:57 AM   #10
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Well, I modeled the balun as it was described. To my surprise, it looks quite decent for UHF application. I used an ideal transmission line, which should be a fairly reasonable assumption, and got an insertion loss of less than 0.5 dB across the entire UHF band, from channel 14 to 69 and return loss better than 10 dB. If the range were limited to channels 14 to 51, then the max loss would be even lower. Here are the results:

http://www.highdefforum.com/attachme...ntid=639&stc=1

(Sorry, I'm not sure how to make the picture show in the post...)

I'll let you know how this works if I put one of these together with my CM4228...

Thanks!

Reg
Attached Images
File Type: png CoaxBalun.png (36.7 KB, 249 views)
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Old 01-31-2006, 04:04 PM   #11
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Reg,
I am interested in this balun issue as well. Being basicly a tinkerer rather than a TV watcher, my suggestion was to use a preamp with 300 ohm inputs and dispense with the baluns altogether. Twinlead has lower loss than even the best coax anyway. I was talked out of it on this forum and since I could not afford to do the system twice, I took their advice. Please look at the thread OVA preamp advise ,dated 12/10/05. Please keep me informed of your findings.
John
PS: I made a Lexan mounting block to keep the baluns 1/2"+ from any metal and made sure the leads were 1/2"+ apart at all points.
PPS: At least you can experiment without climbing a roof and tripod in the snow...
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Old 02-02-2006, 09:46 AM   #12
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Quote:
Originally Posted by RegGuheert

Still, not all balun loss is due to mismatch effects. Many of these things apparently have pure resistive losses at UHF of around 2 dB or so.

Reg
Just received an e-mail from CM stating that the insertion loss of the their baluns is 1 dB max across the UHF band.
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Old 02-03-2006, 07:36 PM   #13
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So, 1db... Is there a second balun in the preamp? Is the preamp actually a 300 ohm device with a balun inside at its input? If so, you have two 1db losses, one at each end of the connecting coax and a 300 ohm preamp would elimanate both of these.
Do you agree that if all unshielded 300 ohm devices need to be 1/2" or more (some say 4") from any metal, then clipping the balun to the boom with the provided metal clip is a no-no?
John
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Old 02-03-2006, 08:56 PM   #14
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I recently compared the gain on the cm4228 with my winegard hd8200p. I wanted to rest my mind and give it a shot.The winegard outperformed the cm4228 on uhf in my case. The winegard also tamed the multipath. I was wondering also why the cm4228 performed as it did.

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Old 02-04-2006, 08:06 AM   #15
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I find the RS baluns to be of high quality and better than the lowes or HD baluns based on construction.
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