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Antennas 101
Following below are some thoughts of mine in response to an email sent to
me. There are some good engineers reading this NG, and I am asking for comments on what I wrote -- corrections where seen necessary, and any other thoughts. Thanks. RF + + + + + ----- Original Message ----- The term "accept power" is interesting. To me, it's just a measure of the input impedance of the antenna. If the resistance (radiation plus loss resistance) is zero, you're not going to get it to absorb power no matter what you do. ____________ Yes. And the input conditions depend on the ability of the radiator to generate EM fields. No current can enter and "flow through" a radiator if it doesn't have some place to go. If a radiator is not electrically long enough to allow differential current to exist along its length, it cannot generate EM fields. It is the di/dt along the radiator length that generates those fields. Adding a matching network at the antenna input doesn't change the instrinsic ability of an antenna to radiate. That is determined by the factors described in the paragraph above. A matching network can permit the tx to increase its rated, safe output power while driving that poor antenna, but any extra power available at the antenna input because of that will be subject to the same poor radiation efficiency as if the matcher wasn't used. And much of any added power from the tx may get dissipated in lossy output system components other than the antenna, rather than being radiated. The antenna itself will still have the same directivity/gain that it had before the matching network was added. Improving the ability of a poor antenna to generate EM fields per unit of source power is possible in a limited way only by increasing its electrical length. "Capacity hats" and inductances incorporated into the radiating structure can be used, as examples. This also raises the antenna radiation resistance and reduces the reactance at the antenna input -- making it easier to match into, and reducing system losses. Someone gave an example of a pager antenna or something that had a rated gain that was below isotropic. It'd be interesting to see where the losses were. This is covered in my comments above, I believe. // |
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