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#11
July 5th 15, 10:43 PM
posted to rec.radio.amateur.antenna
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An antenna question--43 ft vertical
Brian Reay wrote:
On 05/07/2015 21:17, wrote: Roger Hayter wrote: wrote: snip The impedance of a transmitter output will be nothing like 50 ohms resistive, as this would result in an efficiency well below 50%, with all the normal amplfier losses plus the actual RF power produced being 50% dissipated in the PA. This is why matching in the forward direction coexists with a mjaor mismatch in the reverse direction. This is good because if there is any reflected wave we don't want it to add yet more to the PA dissipation. But it does explain what is happening, and why there are increased losses in the feeder as well as the matching networks. The output impedance of an amateur transmitter IS approximately 50 Ohms as is trivially shown by reading the specifications for the transmitter which was designed and manufactured to match a 50 Ohm load. They are designed to drive into a 50 ohm load, that doesn't mean they have a 50 ohm source impedance. Otherwise efficiency would be rather 'disappointing'. Nope. If they didn't have a 50 Ohm source impedance, the SWR with 50 Ohm coax and a 50 Ohm antenna would be high. It is not. The PA stages are designed to operate safely with a load equivalent to a SWR of (typically) 1.5:1 . Any higher, and it means the load is out of spec, and the PA leaves its safe area of operation (assuming there is no mechanism to reduce the power). This is were the myth of RF 'entering' the PA came from - people thinking that a high SWR meant the reflected RF was getting into the PA and causing damage. In fact, it 'sees' a mismatch and therefore can't enter the PA. You are mixing circuit theory with transmission line theory. Designed to operate with a low load SWR means the output impedance is designed to be about 50 Ohms, i.e. commonaly available coax. -- Jim Pennino |
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