One thing I just realized about the value of these antennas is the fact that there is no BALUN signal loss. According to some articles that I read they state that you can lose more than half of the signal with a bad balun!
So my hypothesis now is that it may be better to get a preamp with 300 ohm inputs rather than a 75 ohm input because this will eliminate the need for a signal stealing balun.
For this reason my SS-2000 works better than anticipated.
I have been wondering how to best tweak antenna setups to minmize all losses and maximize signal. My assumption is that an antenna is being used in conjuntion with a preamp for GTA viewers trying to get Buffalo stations.
I guess that you need the balun if you are not using a preamp.
I noticed that there was a preamp but no balun. The preamp was connected to the 300 ohm leads!
After doing some reading I learned something that appears to be common sense but I have not seen it discussed here.
If you have an antenna that has 300 ohm output then use a 300 ohm input preamp.
This means that if you have a CM 4221 you should not be using a CM 7777. With a CM 4221 or 4228 you should be using a Channel Master CM 0264 or Channel Master CM 064 preamp!
Match the antenna output and preamp input and eliminate the balun!
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).
You use a 1/2 Wave balun to get 300 Ohm unbalanced to 75 Ohm Balanced. I am looking to get 150 Ohm unbalanced to 75 OHm balance. So is it a simple matter to replace the 300 Ohm Resistor which a 150 Ohm restor ?
It shouldn't matter which, as both are 75ohms. Where it matters is when actually cutting to length for a real (non-model) build, where one has to take the VF into account.
Excellent work Brian! If this holds up to scrutiny (it ought to), then those little loop-baluns I've been using are probably better than I thought they might be! (despite the sloppy construction of some of them).
I didn't mention one other thing. What you really want is a current balun, one that forces equal currents in the balanced load or feedline. A halfwave coax balun and a ferrite balun are voltage baluns that force equal voltages. This is OK as long as the surge impedances of the two conductors of the balanced side are equal. Usually they will be. But if you're feeding twin-lead, for example, and one side is closer to metal than the other, the impedances will differ. In this case a voltage balun will actually cause line imbalance by equalizing the voltages with respect to ground, which the unbalanced impedances promptly turn into unbalanced line currents. It's the currents that count since that's what causes radiation in the transmit case or signal pickup in the receive case.
The unbalanced currents will induce a current on the outside of the coax feedline past the voltage balun. If you can kill that current then the parallel line currents will rebalance themselves. Perhaps the best way to do that at UHF is with a sleeve balun. This is a quarterwave cylinder placed over the feedline. It is open at the feedpoint and shorted to the feedline shield at the far end. It acts like a high impedance to current on the outside of the coax. You can make one with solid metal or with flexible copper braid stretched over the coax vinyl. Google "sleeve balun" for more info. Ideally, you should follow your 4:1 voltage balun with a current balun to do the whole job.
I neglected to mention that the halfwave coax balun model used a lossless transmission line. I checked typical loss values for small coax at UHF-TV frequencies and with the short lengths required, the losses are negligible (a few hundredths of a dB). If you want to use 75-ohm coax, a friend tells me that Belden 1855A is very nice. It is subminiature and bends easily. Its minimum bend radius spec is 1.5". The specs for the Belden RG-59 and RG-6 I checked were 2.5" and 3" respectively. I didn't check RG-62, but I probably violated its minimum bend spec when I made the test balun. I'm not sure what can happen if you don't meet the spec, but years ago I heard horror stories about the center conductor slowly migrating through certain kinds of dielectric at bends, eventually shorting to the shield.
I think the main concern is that the foil sheathing will crack when the bend radius limit is exceeded. Not a problem with most RG59, though! (no foil in that..)
I found a better load resistor, 1/8 or 1/10 watt, that measured about 320 ohms and should have lower shunt capacitance than the half-watt I was using before. Using the new resistor the balun center frequency rose to 610 MHz and showed a huge return loss dip (more than the splitter shows with a 75-ohm load, so there's some reactance compensation going on). Mismatch losses 100 MHz away were 0.09 and 0.14 dB. The model says they should be 0.17 dB. 115 MHz away, corresponding to the width of the UHF-TV band (counting 698 MHz as 700 MHz), I measured mismatch losses of 0.11 and 0.22 dB. The model says those numbers should be 0.24 dB. This time I averaged both an open and a short as the reference for the return loss measurements. I'm not sure why the physical balun is now beating the model with the new load resistor, but the numbers aren't that far off in absolute terms. In any event, the calculated and measured losses are small, much lower than those of any ferrite balun.
I'm not sure what can happen if you don't meet the spec, but years ago I heard horror stories about the center conductor slowly migrating through certain kinds of dielectric at bends, eventually shorting to the shield.
Thats probably using high power transmitters. The voltages and currents in TV receiving antennas are miniscule. But its still a good idea not to force the coax into tight bends, expansion and contraction in it due to temperature changes could be hard on it over time.
From a Belden technical paper describing type 9913F7 cable:
Besides the dissipation factor, old-style chemically-foamed cable typically suffers from another problem: center conductor migration. Because the elasticity of the conductor material and the polyethylene is quite different, over time the conductor tends to creep or cold flow through the polyethylene. Conductor migration is accelerated when the cable is installed with a tight bend radius, continuously flexed, or exposed to elevated temperatures. This migration would radically alter the impedance at that point, causing serious VSWR, and dramatically reduce the voltage-breakdown ratings. The poor mechanical performance can be attributed to the materials used which were typically low or medium density polyethylene.
Some cable made by Times Microwave allows a very small bend radius. For example, the minimum installation bend radius spec for their 75-ohm, quarter-inch LMR-240-75 is 0.75". This is much lower than the Belden specs I quoted. With this stuff you could make really tiny halfwave voltage baluns and coiled-coax current baluns. The latter is an alternative to a sleeve balun. Common coax is too inflexible to make a coiled-coax balun at UHF-TV frequencies, but it should be possible with the Times Microwave cable. They are much simpler than sleeve baluns. An example for 88-108 MHz is here:
The load impedance doesn't affect the balancing function. It just affects SWR and the resulting mismatch loss in the normal way. All a real balun or a simulation of it does is force equal voltages with respect to ground at the balanced terminals. In so doing it changes the impedance by a factor of four. Any impedance variation at the balanced terminals is reflected back to the unbalanced terminal, reduced by a factor of four in absolute terms and not reduced in relative terms. Voltage baluns are equivalent to an RF transformer with a 1:2 turns ratio and a grounded center tap on the secondary. Adding a current balun is equivalent to removing the center tap.
Thats an older paper I guess. The RG6 Ive been using has the foamy core and Im not sure if its technically polyethylene anymore. But that may make it migrate even more, heh.
There was a line of braided-core RG6 a long time ago that allowed tight bend radii, but I don't remember the brand and I had doubts about its longevity. Times Microwave is using a solid core.
question #1 Can a 4:1 current balun be exchanged with a common 4:1 voltage balun (would that not work with typical antennas like DB4, SBGH, folder dipole?)
question #2 I can't find the schematics on the web that would be matching the wiring I see in two common 4:1 baluns I openeds, and I paid attention to the core winding directions too. I actually destroyed it on purpose in the process of making sure I took the wiring right. Meanwhile, in the wiring I see, there is a 4th coil which is surprisingly shorted. I would appreciate if someone open a balun and confirm/link a schematic. If you can also explain how the magnetic fields flow in the usual binocular ferrite core in them, that may help a bit too.
question #3 Now, I'm still trying to find a 1:1 balun for a DIY project about making a clearstream c2 clone(2 parallel 160 ohms loop antennas is 80 ohms, requiring a 1:1 balun to 75ohms coax). For this balun, Would I need a current or a voltage one?
question #4 It looks like I won't find a 1:1 balun in common stores. For the moment I would have to recuperate the the binocular ferrite core of my destroyed balun and make a 1:1 balun myself once I get the plans... Schematic anyone?
I have looked at all links and pdf, twice now (I know how to use google too ). There is no association of any given circuit to the common 4:1 voltage balun we see. Meanwhile they don't answer my questions regarding whether or not the current and voltage balun are interchangeable, and which 1:1 balun I should use for a double loop antenna. At this point I would like a human answer, rather than digging endlessly tons of doc. That's why I came to the forum last
it worked but was lossy, meaning reception was worse than with a CM balun.
I used good quality RG6 and hooked things up exactly as per the instructions.
There's evidently some trick I'm missing
Wow, I didn't know that Tin Lee resold anything... maybe its a misunderstanding because the loss they quote is not the loss posted on the web site for their own balun model.
maybe it's the fact that I wanted high VHF as well as UHF that caused the problem.
The pdf quote definitely has their part number on the top and then the channel master specs on the bottom
I recently enquire with Tinlee about their channel specific low-loss balun, in my case it was for VHF 13. The reply was it is better to use a generic balun because it has a lower loss than a channel-specific balun.
However they mentionned that the advantage of a channel-specific balun is less interference from other frequencies
the price quoted was $75CA, which is quite more costly than a generic balun which is usually $1 to $5
I modeled a 4:1 balun using a halfwave of 75-ohm coax with the load centertapped and grounded. The load represents, for example, a folded dipole with its center electrically connected to the antenna boom and coax shield. Yagi antennas may use this configuration.
Mismatch loss at the band edges is twice as great as when the load is floating. I want to check the details of how the circuit analysis program models the transmission line, but if this result holds it provides a powerful incentive to insulate a driven element.
For reference, this is the floating-load model. Both models use a lossless transmission line. At UHF-TV frequencies the few inches of coax required for a halfwave balun should add negligible loss.
the one that really bummed me out was this: http://www.dlsus.com/catalog/product_info.php?products_id=458
>> Specifications
>> -Bandwidth: 5-900 MHz
>> -Insertion Loss: 0.7 dB
>> -Return Loss: 16 dB
>> -Impedance: Input 300 Ohm, Output 75 Ohm
>> -Balance Ratio: 30 dB
I emailed to get confirmation (again 2007) and couldn't get a straight answer
Sketchy parts descriptions masquerading as real specs are worse than no specs at all....
What are the conditions for 0.7 dB insertion loss?
Minimum they ever measured for the best hand picked part at the best frequency???
Guaranteed max??? 99.99 percentile??? And my favorite: "typical".
Minicircuits has an extensive line of 4:1 Baluns. Fol. is typical 1.24 dB loss at 700 MHz: http://www.minicircuits.com/pdfs/TCM4-19.pdf
As America Ferrera once said, "Real specs have loss curves."
Since their on-line search engine is very difficult to use, suggest ordering their catalog.
And here's an article re an even lower insertion loss Anaren Multilayer 4:1 Balun: http://www.mwrf.com/Articles/Print.cfm?ArticleID=15084
See Fig 4 for VHF/UHF band loss. Call for more info, I didn't find it on their website.
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