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How to model a 6:1 ratio balun using NEC2
I am using Nec Win Plus (NEC2) to view the SWR curve of my modeled
antenna. The actual antenna uses a 6:1 ratio balun. Is it incorrect to just change the characteristic impedance of the line from 50 ohms to 300 ohms to see what effect a 6:1 balun would have on the SWR values? I was just assuming that the line impedance was only really needed to calcuate SWR - hopefully I am right. Thanks, Scott, WU2X |
#2
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How to model a 6:1 ratio balun using NEC2
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#3
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How to model a 6:1 ratio balun using NEC2
Changing the modeled line impedance to 300 ohms is correct if you really
have a 6:1 transformer. But you probably don't. The impedance transformation of a typical 6:1 balun is around 6:1 when the antenna impedance is near 50 ohms resistive. However, when it's not, the transformation can be vastly different. And the balun is likely to add substantial series and/or shunt reactance to boot. So there's no way to know what impedance you'll really see looking into the transformer unless you measure one when loaded with various impedances. You should also know that a "voltage balun", which a 6:1 balun typically is, produces equal currents in the dipole halves, and no current on the feedline, only if the antenna is perfectly symmetrical. If the antenna isn't symmetrical (e.g. if it's fed off center), the "balun" will force common mode current to flow along the outside of the coax. This will affect the feedpoint impedance as well. See http://eznec.com/Amateur/Articles/Baluns.pdf for more information about this. Roy Lewallen, W7EL wrote: I am using Nec Win Plus (NEC2) to view the SWR curve of my modeled antenna. The actual antenna uses a 6:1 ratio balun. Is it incorrect to just change the characteristic impedance of the line from 50 ohms to 300 ohms to see what effect a 6:1 balun would have on the SWR values? I was just assuming that the line impedance was only really needed to calcuate SWR - hopefully I am right. Thanks, Scott, WU2X |
#4
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How to model a 6:1 ratio balun using NEC2
Hi Roy,
Yeah, I just started a thread a few days ago regarding current vs. voltage baluns with respect to my OCF Dipole. I started to play with around with modeling feedline radiation following the notes on Cebik's page about it. Doesn't seem like its really possible to accurately predict the SWR/bandwidth when you start trying to model feedline radiation too. Very carefully modeling the very simple OCF Dipole I have assuming no feedline radiation yields a SWR curve over the HF band that is no-where near my real world measured values. The NEC2 computed values are much higher. -Scott, WU2X Roy Lewallen wrote: Changing the modeled line impedance to 300 ohms is correct if you really have a 6:1 transformer. But you probably don't. The impedance transformation of a typical 6:1 balun is around 6:1 when the antenna impedance is near 50 ohms resistive. However, when it's not, the transformation can be vastly different. And the balun is likely to add substantial series and/or shunt reactance to boot. So there's no way to know what impedance you'll really see looking into the transformer unless you measure one when loaded with various impedances. You should also know that a "voltage balun", which a 6:1 balun typically is, produces equal currents in the dipole halves, and no current on the feedline, only if the antenna is perfectly symmetrical. If the antenna isn't symmetrical (e.g. if it's fed off center), the "balun" will force common mode current to flow along the outside of the coax. This will affect the feedpoint impedance as well. See http://eznec.com/Amateur/Articles/Baluns.pdf for more information about this. Roy Lewallen, W7EL wrote: I am using Nec Win Plus (NEC2) to view the SWR curve of my modeled antenna. The actual antenna uses a 6:1 ratio balun. Is it incorrect to just change the characteristic impedance of the line from 50 ohms to 300 ohms to see what effect a 6:1 balun would have on the SWR values? I was just assuming that the line impedance was only really needed to calcuate SWR - hopefully I am right. Thanks, Scott, WU2X |
#6
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How to model a 6:1 ratio balun using NEC2
All feedlines radiate and receive noise and signals. They can't help
it. But on 99% of occasions the magnitude is small enough to be ignored. If it works satisfactory then don't bother fixing it! |
#7
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How to model a 6:1 ratio balun using NEC2
wrote in message
oups.com... I am using Nec Win Plus (NEC2) to view the SWR curve of my modeled antenna. The actual antenna uses a 6:1 ratio balun. Is it incorrect to just change the characteristic impedance of the line from 50 ohms to 300 ohms to see what effect a 6:1 balun would have on the SWR values? I was just assuming that the line impedance was only really needed to calcuate SWR - hopefully I am right. Thanks, Scott, WU2X Inside a W2AU balun (1:1) - in case you were curious. http://www.the-s38-guy.com/Page_78x.html w9gb |
#8
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How to model a 6:1 ratio balun using NEC2
I actually had a Unadilla W2AU fail from that exact wire coming off
mine inside. -Scott, WU2X g. beat wrote: wrote in message oups.com... I am using Nec Win Plus (NEC2) to view the SWR curve of my modeled antenna. The actual antenna uses a 6:1 ratio balun. Is it incorrect to just change the characteristic impedance of the line from 50 ohms to 300 ohms to see what effect a 6:1 balun would have on the SWR values? I was just assuming that the line impedance was only really needed to calcuate SWR - hopefully I am right. Thanks, Scott, WU2X Inside a W2AU balun (1:1) - in case you were curious. http://www.the-s38-guy.com/Page_78x.html w9gb |
#9
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How to model a 6:1 ratio balun using NEC2
g. beat wrote:
Inside a W2AU balun (1:1) - in case you were curious. http://www.the-s38-guy.com/Page_78x.html w9gb wrote in message I actually had a Unadilla W2AU fail from that exact wire coming off mine inside. - Scott, WU2X ... as they say " now you can rebuild him .. better faster, stronger than before ! http://www.the-s38-guy.com/Page_83x.html http://www.the-s38-guy.com/Page_81x.html So when you see one in the junk pile OR at the $ 1 table at a hamfest - grab it ! http://www.the-s38-guy.com/Page_76x.html Greg, w9gb |
#10
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How to model a 6:1 ratio balun using NEC2
I have rebuilt a W2AU balun for much the same reasons. Like the referenced article, I removed the dual use of the electrical connection for support of the dipole wires. An addition, I changed the coax connector for an N-type for a water resistant cable connection. Having said that, I don't use it much, it is a "voltage balun" and most of my applications are better suited to a "current balun". Owen -- |
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