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#11
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loops and 4:1 baluns
Ed Cregger wrote:
I am beginning to suspect that traditionally made baluns are not as exact in practice as they are theoretically. This is not a surprise, really. Few things in electronics are exact as we humans like to assume, as you well know. Good seeing your post, OM. Ed Cregger, N2ECW former NM2K The whole problem is "liking to assume" that things are simpler than they are. When the theory you apply is too simple, guess what -- you'll find that the real thing doesn't behave as your oversimplified "theory" predicts. Theory works just fine, and accurately predicts how a real object will work. Oversimplified "theory" often doesn't work so well. A well made balun or RF transformer behaves reasonably well like an ideal transformer, that is, infinite winding inductance, no coupling capacitance, zero leakage inductance, no loss, and so forth, but only under quite a narrow range of circumstances. Those circumstances include being terminated with a fairly narrow range of impedances and over a limited frequency range. Usually, one side is designed to be terminated with 50 ohms, purely resistive. That means the other side of a 4:1 balun has to be terminated with something fairly close to 200 or 12.5 ohms (depending on how it's designed), also resistive, in order for it to work as intended. If the impedance differs very much at all from that value, you'll find that the transformation ratio is no longer 4:1, and that the balun will add a series and/or shunt impedance to the circuit. This can be accounted for by theory, but only with great difficulty since it requires careful characterization of the core and windings. People tend to design, and often test, a 4:1 balun in a 50 ohm environment, then attach it to a multiband antenna that has rather extreme (but entirely predictable) impedance variations. Then they're surprised because the impedance seen looking into the balun isn't 4 times or 1/4 times the antenna impedance, but is something wildly different. They shouldn't be. A 4:1 balun or transformer that effects a nice 4:1 impedance transformation when presented with a very wide range of termination impedances simply doesn't exist. Any "theory" that predicts it is oversimplified and invalid. Roy Lewallen, W7EL |
#12
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loops and 4:1 baluns
"Roy Lewallen" wrote in message treetonline... Ed Cregger wrote: I am beginning to suspect that traditionally made baluns are not as exact in practice as they are theoretically. This is not a surprise, really. Few things in electronics are exact as we humans like to assume, as you well know. Good seeing your post, OM. Ed Cregger, N2ECW former NM2K The whole problem is "liking to assume" that things are simpler than they are. When the theory you apply is too simple, guess what -- you'll find that the real thing doesn't behave as your oversimplified "theory" predicts. Theory works just fine, and accurately predicts how a real object will work. Oversimplified "theory" often doesn't work so well. A well made balun or RF transformer behaves reasonably well like an ideal transformer, that is, infinite winding inductance, no coupling capacitance, zero leakage inductance, no loss, and so forth, but only under quite a narrow range of circumstances. Those circumstances include being terminated with a fairly narrow range of impedances and over a limited frequency range. Usually, one side is designed to be terminated with 50 ohms, purely resistive. That means the other side of a 4:1 balun has to be terminated with something fairly close to 200 or 12.5 ohms (depending on how it's designed), also resistive, in order for it to work as intended. If the impedance differs very much at all from that value, you'll find that the transformation ratio is no longer 4:1, and that the balun will add a series and/or shunt impedance to the circuit. This can be accounted for by theory, but only with great difficulty since it requires careful characterization of the core and windings. People tend to design, and often test, a 4:1 balun in a 50 ohm environment, then attach it to a multiband antenna that has rather extreme (but entirely predictable) impedance variations. Then they're surprised because the impedance seen looking into the balun isn't 4 times or 1/4 times the antenna impedance, but is something wildly different. They shouldn't be. A 4:1 balun or transformer that effects a nice 4:1 impedance transformation when presented with a very wide range of termination impedances simply doesn't exist. Any "theory" that predicts it is oversimplified and invalid. Roy Lewallen, W7EL ------------ Roy, that is precisely what I said, but rather imprecisely. Ed, N2ECW |
#13
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loops and 4:1 baluns
Ed Cregger wrote:
Such things as variances in construction materials from one batch to another and the variations that one human will introduce to construction versus another human also induce characteristics that do not always jibe with theory. IMO, the major problem with baluns is that they are designed for specific impedances and most often used with unknown impedances. -- 73, Cecil http://www.w5dxp.com |
#14
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loops and 4:1 baluns
"Cecil Moore" wrote in message ... Ed Cregger wrote: Such things as variances in construction materials from one batch to another and the variations that one human will introduce to construction versus another human also induce characteristics that do not always jibe with theory. IMO, the major problem with baluns is that they are designed for specific impedances and most often used with unknown impedances. -- 73, Cecil http://www.w5dxp.com ---------- Very true, but often times they work well enough with varying impedances to get us on the air. If it is that or nothing, then I'm all for doing it. Ed, N2ECW |
#15
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loops and 4:1 baluns
Roy Lewallen wrote:
Ed Cregger wrote: I am beginning to suspect that traditionally made baluns are not as exact in practice as they are theoretically. This is not a surprise, really. Few things in electronics are exact as we humans like to assume, as you well know. Good seeing your post, OM. Ed Cregger, N2ECW former NM2K The whole problem is "liking to assume" that things are simpler than they are. When the theory you apply is too simple, guess what -- you'll find that the real thing doesn't behave as your oversimplified "theory" predicts. Theory works just fine, and accurately predicts how a real object will work. Oversimplified "theory" often doesn't work so well. A well made balun or RF transformer behaves reasonably well like an ideal transformer, that is, infinite winding inductance, no coupling capacitance, zero leakage inductance, no loss, and so forth, but only under quite a narrow range of circumstances. Those circumstances include being terminated with a fairly narrow range of impedances and over a limited frequency range. Usually, one side is designed to be terminated with 50 ohms, purely resistive. That means the other side of a 4:1 balun has to be terminated with something fairly close to 200 or 12.5 ohms (depending on how it's designed), also resistive, in order for it to work as intended. If the impedance differs very much at all from that value, you'll find that the transformation ratio is no longer 4:1, and that the balun will add a series and/or shunt impedance to the circuit. This can be accounted for by theory, but only with great difficulty since it requires careful characterization of the core and windings. People tend to design, and often test, a 4:1 balun in a 50 ohm environment, then attach it to a multiband antenna that has rather extreme (but entirely predictable) impedance variations. Then they're surprised because the impedance seen looking into the balun isn't 4 times or 1/4 times the antenna impedance, but is something wildly different. They shouldn't be. A 4:1 balun or transformer that effects a nice 4:1 impedance transformation when presented with a very wide range of termination impedances simply doesn't exist. Any "theory" that predicts it is oversimplified and invalid. Thanks for the information! Ed, good to run into you again! What brought you to Georgia? Ray - I found a Dover edition of Transmission Lines, Antennas and Waveguides. Thank you for the suggestion! One final question about the 4:1 balun: Assuming a single band Delta Loop with a feed point impedance of approx 100 ohms, with or without a 4:1 balun you have approximately a 2:1 SWR - so why use the balun? Thanks and 73, Tad Danley, K3TD |
#16
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loops and 4:1 baluns
"Tad Danley" wrote Ed, good to run into you again! What brought you to Georgia? Thanks and 73, Tad Danley, K3TD ---------- Dupont decided that they needed the wife in Chattanooga, TN. That was slightly over ten years ago. We are living in the Northwest corner of Georgia, just below Chattanooga, TN. It is a very, very nice place to live. What the heck are you doing in Texas? Ed, N2ECW |
#17
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loops and 4:1 baluns
Ed Cregger wrote:
Very true, but often times they work well enough with varying impedances to get us on the air. If it is that or nothing, then I'm all for doing it. When I was in high school, a ham came back to my "t-e-s-t-d-e-w-5-d-x-p" signal while my 40W Globe Scout was driving a 100W light bulb through 3' of wire on a table top. He gave me a 5x5 signal report. No balun required. :-) -- 73, Cecil http://www.w5dxp.com |
#18
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loops and 4:1 baluns
Tad Danley wrote:
One final question about the 4:1 balun: Assuming a single band Delta Loop with a feed point impedance of approx 100 ohms, with or without a 4:1 balun you have approximately a 2:1 SWR - so why use the balun? The resonant feedpoint impedance of a loop is often a little higher than 100 ohms, e.g. 115 ohms according to EZNEC, in which case a 4:1 balun will lower the SWR - sometimes alleviating foldback problems. -- 73, Cecil http://www.w5dxp.com |
#19
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loops and 4:1 baluns
Cecil Moore wrote:
Ed Cregger wrote: Very true, but often times they work well enough with varying impedances to get us on the air. If it is that or nothing, then I'm all for doing it. When I was in high school, a ham came back to my "t-e-s-t-d-e-w-5-d-x-p" signal while my 40W Globe Scout was driving a 100W light bulb through 3' of wire on a table top. He gave me a 5x5 signal report. No balun required. :-) http://palomar-engineers.com/1_1_Bal...alun_kits.html http://palomar-engineers.com/4_1_Bal...alun_kits.html |
#20
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loops and 4:1 baluns
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