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Old 2009-01-22, 02:53 PM   #16
k6sti
 
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What an interesting outfit. I'll bet they make great stuff. But it's all custom, made to order, and it looks like prices begin at $30 Canadian. That's a lot for a balun. I don't know the typical return loss of a UHF-TV balun. A return loss of 16 dB is equivalent to an SWR of 1.38, and yields a mismatch loss of 0.11 dB.

I know an RF engineer who has developed a very low-loss balun for 88-108 MHz. He offers plans for free and he will sell you a partial kit of parts to make your own. I'm going to see if I can interest him in making a low-loss balun for UHF TV.

Here's his FM balun:

http://homepage.mac.com/kenwetzel/Low_Loss_Balun.htm

Note that this is a current balun, which has some advantages over the common voltage balun. Google "current balun" for the full story.

Brian
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Old 2009-01-22, 03:02 PM   #17
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If you're interested primarily in UHF, then a very simple coax-loop 4:1 balun costs only a buck or two to construct, and will show about 0.5dB insertion loss across much of the the UHF television band.

That kind of proposition is hard to beat!

Speaking of which.. can any of you hams enlighten me further on the gamma match feed for a dipole? To me, it looks very much like a 1/2 wavelength loop balun in disguise.

Cheers
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Old 2009-01-22, 03:38 PM   #18
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I know an engineer in Sweden who uses a half-wave balun for his UHF-TV antennas. Although it is exactly a half-wavelength long at only one frequency, he said he was surprised how well it worked over the whole band. I think he made some measurements and I'll see if I can get more info. It surely is the easiest and cheapest way to go if it works decently. It is not nearly as small as a ferrite balun, but usually that's not a concern.

Think of a gamma match as tapping down on a dipole until you find the impedance you want. It is a matching device and does not perform a balancing function. But if you shunt-excite one half of a dipole with current, the current will flow into the other half as well. If the feedline uses a perfect current balun, the excitation current has nowhere else to go.

Brian
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Old 2009-01-22, 04:27 PM   #19
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Quote:
looks like prices begin at $30 Canadian. That's a lot for a balun.
At that price range, just get a CM0064 or CM0264 300ohm in 75ohm out preamp and kill a couple of birds with one throw.
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Old 2009-01-22, 04:39 PM   #20
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Quote:
Originally Posted by 300ohm View Post
At that price range, just get a CM0064 or CM0264 300ohm in 75ohm out preamp and kill a couple of birds with one throw.
Good point! Sort of an amplified balun. I'm using the old model 0064C (or something like that). I found it in an orchard. A few years later I found the proper power supply / injector at a garage sale. I just held on to them until recently, when I coupled them with my Radio Shack UHF zigzag. I checked the preamp last week. I get 24-30 dB gain over 470-700 MHz (includes an unknown amount of loss for the balun I used to feed the preamp input). It has LOTS of coils inside, with separate steep-skirted passbands for low-VHF/FM, high-VHF, and UHF. It has a removable plug that engages/disengages a tunable FM trap. I discovered that if you remove the plug entirely, the VHF passbands disappear leaving a UHF-only preamp. Since it is only connected to a UHF antenna, I removed the plug to reduce any intermod.

Brian
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Old 2009-01-22, 05:51 PM   #21
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Yeah, I got a CM0264 in June and couldnt be happier with it. It has both a 300ohm uhf input and a 300ohm vhf input and one 75ohm output, just what I need, no more no less. (unless they came out with a model for combining two separate uhf antennas also, ie two 300ohm uhf inputs, one 300ohm vhf input and one 75ohm output)

The CM0064 is probably the same circuit board, just without two separate input terminals. (the price difference between the two models is squat)

I have the power injector mounted outside in a weather proof box to minimize the clutter behind the TV.
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Old 2009-01-22, 06:06 PM   #22
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My power supply / injector is in the garage for the same reason. It drives the preamp with AC power. That really threw me when I first opened the preamp and poked around. Any idea why they did that rather than just feeding it DC? My preamp uses a TRW transistor. It was the only semiconductor I remember seeing inside, although that's a bit hard to believe with all the gain I measured, which seemed a bit excessive to me. But with the FM trap plug removed and the VHF passbands disabled, I see no evidence of intermod. I wasn't sure, but there may have been some with the plug in place.

Brian
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Old 2009-01-22, 09:20 PM   #23
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Quote:
It drives the preamp with AC power. That really threw me when I first opened the preamp and poked around. Any idea why they did that rather than just feeding it DC?
Hmm, that is an older model. My power injector is the CM0747. (it seems CM#45-12796 is the replacement for it, since from what I hear, CM0747 is unavailable)

IIRC, its 18VDC output. When I measured mine, it was somewhere around 20VDC no load output. (I wrote all that stuff down on the back of it for future troubleshooting if need be, which is now bolted to the outside wall, heh)
It definately had four diodes in it (1N004 IIRC). No fancy design, just a standard linear power supply design with a good sized transformer, the four way full bridge, and the filter caps. Piece of cake to build a new one if need be. I replaced the short stock power cord with a long one from an old vacumm cleaner. It only burns 4 watts or less, IIRC.
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Old 2009-01-22, 10:00 PM   #24
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I looked at the label on my power supply earlier today and it said the same 4 watts. At first I thought the preamp handled AC so it could be sent positive or negative voltage, but I don't think it has a bridge rectifier inside and I don't think its local ground is floating. I couldn't figure out what the two large electrolytics were doing in there until I traced a bit of the circuit and saw the power diodes. I think it's 12 VAC or 18 VAC or something that's sent up the coax. I'd like to see the schematic some time. There are just a zillion little copper coils inside, each carefully tuned by spreading a turn here and there. I was very impressed with the steepness of the skirts of the bandpass filters. I wish I had taken a photo of the spectrum analyzer screen. I had it set on 10 dB/div and the slopes still were impressive. They even went to the trouble of implementing separate VHF-low and VHF-high TV passbands. I suspect my early model uses some sort of bipolar transistor and they probably had lots of intermod problems in high-signal areas. I'm sure the current models are lower in noise and handle larger signals.

Brian
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Old 2009-01-22, 10:55 PM   #25
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My CM0264 only has one transistor in it (plus the many coils, caps and resistors for the matching cirucuits), the BFG198 NPN 8Ghz Wideband transistor. If you download the datasheet from Phillips, it has the typical circuit.

http://www.datasheetcatalog.com/data...1/BFG198.shtml
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Old 2009-01-23, 01:56 PM   #26
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Quote:
Originally Posted by 300ohm View Post
At that price range, just get a CM0064 or CM0264 300ohm in 75ohm out preamp and kill a couple of birds with one throw.
The C-M SPARTAN Preamps with 300-ohm input STILL have a ferrite balun...built-in.
See photo here:
http://www.avsforum.com/avs-vb/attac...3&d=1153268779

C-M Spartan Preamps for either 75-ohm or 300-ohm input specify UHF NF=2.2 dB.
C-M Titan Preamps with 75-ohm input specify UHF NF=2.0 dB (CM7775/7777)
and 2.2 dB (CM7778).

HOWEVER, C-M never stipulated the "standard test conditions". Given the UHF
loss typically found in even GOOD baluns, I think the NF specs are for a
test configuration where the NF is ACTUALLY referenced to the direct 75-ohm
RF input--thereby ignoring the balun loss--whether internal or external.
Furthermore, the extra two tenths of a dB specified for CM7777 are likely
due to either a more stringent parts selections process....or marketing hype.....
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Old 2009-01-23, 02:06 PM   #27
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Quote:
Originally Posted by mlord View Post
If you're interested primarily in UHF, then a very simple coax-loop 4:1 balun costs only a buck or two to construct, and will show about 0.5dB insertion loss across much of the the UHF television band.

That kind of proposition is hard to beat!

Speaking of which.. can any of you hams enlighten me further on the gamma match feed for a dipole? To me, it looks very much like a 1/2 wavelength loop balun in disguise.

Cheers
Do you have any info on how well a "quarter-wave" balun works across either Hi-VHF or
even wider UHF band (where "quarter-wave" is a meaningless concept)????

Although inherently narrow-band antenna matching techniques are very useful for
narrow-band HAM radio applications (e.g. beta/hairpin, gamma, T-match, et.al.),
TV needs wideband solutions....but there's lots of wiggle room, given ferrite balun loss.

Last edited by holl_ands; 2009-01-23 at 02:30 PM.
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Old 2009-01-23, 04:48 PM   #28
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Great picture. The transistor is on the backside.
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Old 2009-01-24, 12:36 AM   #29
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Quote:
Originally Posted by holl_ands
Do you have any info on how well a "quarter-wave" balun works across either Hi-VHF or
even wider UHF band (where "quarter-wave" is a meaningless concept)????
No hard data, but the half-wave loop baluns are thought to have +/-10% bandwidth, which at UHF frequencies is enough for most of the TV band.

From my own experiments, with relatively strong (local) TV stations, my UHF loop baluns work fine for most VHF-hi stations, and vice-versa. Some notes on that are scattered about through this thread.

Cheers
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Old 2009-01-24, 01:35 PM   #30
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I used a circuit simulator to model a halfwave balun. T1 is a transmission line whose length is an electrical halfwave.



Here are the model results for a halfwave of 75-ohm coax. The Y-axis is mismatch loss in dB and the X-axis is frequency in MHz.

I decided to build a balun to validate the model. I used 93-ohm coax because it should have only 0.24 dB loss at the band edges and I just happen to have some around. In fact, I located an 8.5" piece, which is what I calculate the length should be for 585 MHz, the arithmetic center of the 470-700 MHz band.

I cut 1/4" of vinyl off the ends of the cable, mashed the shields together, and soldered them. I stripped 1/8" of dielectric off the center conductors and soldered a half-watt resistor that measured (before heating it!) within an ohm of 300 across the tips. The tips touch the resistor body, with 3/8" of excess resistor lead length hanging in space (I may want to use this selected part for another project). I soldered a few inches of 75-ohm RG-6 to the balun using the shortest leads I could manage. I plugged the male F-connector on other end into my power splitter.

Minimum return loss for the balun occurred at 555 MHz so I centered my 200-MHz spectrum analyzer passband there. The power splitter return loss (open-circuit reference only--too hard to short the female F-connector on the power splitter) was 32 dB at 455 MHz, 33 dB at 555 MHz, and 29 dB at 655 MHz. The balun return loss was 15, 19, and 12 dB for the same frequencies. This is equivalent to SWRs of 1.43, 1.25, and 1.67, and mismatch losses of 0.14, 0.06, and 0.28 dB. Although a bit lopsided because I didn't quite center the response, this isn't all that far from the 0.17 dB that the circuit model predicts 200 MHz away from bandcenter for 93-ohm cable. I don't have an independent way of assessing what the 300-ohm load looks like over this frequency range.

I think this is a nifty balun for UHF-TV, particularly if you can get your hands on a few inches of 93-ohm coax (RG-62).

Brian
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