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"Bal uhn" or "bayl uhn"?
On 8/5/2015 5:43 AM, Jeff wrote:
If the dipole is truly balanced (and the coax is perpendicular to the antenna), how does current get induced into the coax? My understanding is that perpendicular wires have little current induction due to this orientation. Current does not need to be "induced" in the shield outside. There is a voltage on the shield at the antenna feed point and a path for the current on the shield outside. Current flows. Why would the current prefer the antenna element over the shield outside? Yes, the antenna has not changed and it is still "ballanced", but the antenna is not the only load seen by the feed line. The current in the two antenna elements are not equal because some of the current flows on the shield having nothing to do with induction from the antenna. Well NEC and reality do not agree with you!! If the feeder sees an impedance that is identical to its own characteristic impedance that all of the power will go into that load, that is the situation with a 50 ohm resistor and the radiation resistance of a dipole is no different. The feeder is not clairvoyant so that it knows that it is a dipole is attached, and a perfectly matched dipole will not reflect any power back down onto the coax outer, the only load seen will be the dipole's radiation resistance not some mythical addition load in parallel. In a perfectly matched situation the current in the dipole element will be equal, it is only imbalance that will cause currents other than those caused by induction to flow on the outer. This is clearly demonstrable in NEC. NEC also shows that the currents induced on the outer by radiation are much larger than any currents caused by small imbalances in the dipole elements when resonant. Those induced currents are there even if a balun or choke is used. Jeff, I am tired of discussing this. Any time you need to invoke "clairvoyance" I think you have missed the mark. The simple matter is that a dipole is *not* like a resistor load. The resistor load is a closed path and does not accumulate charge (except for non-idealities which we are purposely ignoring), so the current in one side is the same as the current out the other side. A dipole does *not* have this limitation and the currents into the two sides does not have to be the same. There are many ways to use simulation and only one will be correct. If you simulate an antenna with a balanced feed you will not get the results for an unbalanced feed. I found such simulations on the Internet and posted the links somewhere, not sure which thread or if it was even this group. I was discussing this elsewhere with people who deal with EMC and RF for a living and they agree with me. If you have done such simulations, please show us results. Meanwhile, I'll try to dig up the info I found on the Internet. -- Rick |
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