| Index | Thread | Archive | Jan-2001 | Archive | Send |
 |
  |
|
   |
|
  |
|
Nice discussion on application. FYI, the materials used in Force-12 baluns is as follows: The B1 uses type 73 beads over RG-303; the B1-C uses type 77 beads over RG-393, and the B1-5 uses RG-303 on a type 61 toroid core. Sincerely, Ken Hirschberg, K6HPX John Petrich wrote: > Hi Greg, > > Thanks for the reply and comments about baluns. It is good to share > ideas with people who make observations and have ideas. > > Yes, I am familiar with the W1JR balun and have used it in some > applications. It was good 30 years ago, it is still good today. The only > reason that it is not as popular as it once was, is that the bead baluns are > easier to construct and harder to goof up on. There may be a small > advantage in terms of bandwidth for the bead baluns. In some applications, > bandwidth is very important. In other applications, bandwidth is really not > important at all. > > I know what you mean about unrecognized balun heating. So many baluns > are located up at the antenna feedpoint and the heating is only discovered > after the balun has failed. Antennas can be properly constructed yet it is > of major importance to pair the balun, the antenna and the band of operation > correctly to avoid balun heating and unwanted feedline radiation. Feedline > radiation isn't always a problem. Wanted feedline radiation can make for a > useful antenna i.e. G5RV. > > Balun heating is the result of common mode currents flowing on the > outside of the coax shield. These currents are then dissipated in the real > component of the complex common mode impedance characteristic for that > balun. There is no other source for heating for the ferrite beads. This > heating problem occurs just the same way and for the same reason with all > ferrite baluns, whether they are constructed with ferrite toroids or ferrite > beads. The phenomenon is the same. It is interesting, if you carefully > examine an overheating bead balun, the beads closest to the high impedance > connections are the warmest. The beads closest to the low impedance > connections are the coolest. It is as if each little bead functions as an > individual little attenuator element. The entire stack of ferrites does not > act like a resistor. The power from the common mode current is not > dissipated uniformly as it would along a purely resistive element. > > There are two independent factors that contribute to common mode current > flow and the resultant risk of balun heating: > > 1) INSUFFICIENT COMMON MODE IMPEDANCE TO CHOKE OFF COMMON MODE CURRENT > FLOW: Anytime, repeat "anytime", one of these 800 ohm common mode > impedance bead baluns is connected across a high impedance load, such as a > 80 meter doublet excited on 40 meters, there is the risk of severe balun > overheating. The same goes for trying to operate a old style tribander on > 17 or 24 meters with a ferrite balun. Low power operation won't heat the > balun, BUT, the common mode current is still flowing, and the system could > be operating at a disadvantage. This limitation from the balun's common > mode impedance in a high impedance environment is BY FAR THE MOST > SIGNIFICANT FACTOR that contributes to bead balun overheating. High power > makes the heating problem easier to recognize. Low power doesn't cause as > much heating but the system may not be functioning in an ideal manner. But, > "everything works." A better solution for a balun in a high impedance > enviroment is to use one of those coiled coax or "Badger", baluns. This > particular style of balun is capable of exhibiting extremely high common > mode impedance values if properly constructed and tested for the frequency > of use. Just like an old antenna tuner of years gone by. > > 2) FERRITE MIX: Yes, ferrite mix can make a difference, but don't get > overly excited on this one. Any importance that ferrite mix has on balun > heating is not because one mix is "better" than another, or one mix is > "worse" than another. The reason that ferrite mix can contribute to balun > overheating problems is because of #1 above- Insufficient Common Mode > Impedance. The Force-12 balun, I'm guessing, acts like a string of #43 mix > ferrite beads. The Maxwell, W2DU, bead balun uses a string of #77 mix > ferrite beads. The Force -12 balun has a good peak common mode impedance > from 40 meters to 10 meters. The Maxwell bead balun has a useful peak > common mode impedance from 160 through 15 meters. There is substantial > overlap for both and both are good. The Maxwell balun might not have enough > common mode impedance on 10 meters and overheat in some 10 meter > applications. The Force 12 balun might not have enough common mode > impedance for a 160 meter installation and overheat in some applications on > that band. I haven't actually tested each balun side by side in the antenna > situations I have referred to but I am extrapolating from their common mode > impedance curves. > > The key to reducing balun overheating probably lies with pairing up the > antenna (and it's feed point impedance), and band of operation, with a balun > having sufficient common mode > impedance to choke off common mode current flow. The standard of comparison > between "current mode" baluns is their measured common mode impedance at the > frequency of use. Some "current mode" baluns have low common mode impedance > compared to other baluns. I have only tested the Force-12 and Maxwell > baluns and they exhibit common mode impedances of about 800 ohms. > Unfortunately, the various manufacturers never publish the common mode > impedance characteristics of their baluns. I think that it is very very > very hard to get common mode impedance values greater than 800 to 1000 ohms > using low Q type #43 and #77 ferrites. Maybe I don't know enough, so take > that statement > with a grain of salt. One can get relatively high common mode impedance by > coiling coax on a higher Q #61 ferrite toroid. The air coiled coax, > "Badger, balun or an old fashioned > antenna tuner will give the highest common mode impedance values that I know > of. > > Let me know your thoughts, Greg. > > John Petrich, W7HQJ > > -------------------------------------------- > Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. > Force 12 Web site: http://www.qth.com/force12 > > Submissions: send to Force12Talk@qth.com > To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com > Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk > For problems with the list, contact n4zr@qth.com -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact n4zr@qth.com |