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Originally Posted by Mike. Watterson
Yes there is. And the longer the feeder is (in relation to wavelength) the worse the problem is. It's no fun having 200W radiated by a feeder cable mostly inside the building, even though the transmitter SWR is "good".
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You may be confusing impedance matching with common-mode current.
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No, its just the feeder + aerial matched to the radio. It's terrible and can result in pickup or radiation by the feeder. You lose a lot of receive or transmit efficiency.
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Yes, I think you are confusing impedance matching with common-mode current.
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A common mode choke will help reduce feed cable radiation or pickup. You need it at both ends!
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No, you don't always need it at both ends. Sometimes both ends can help.
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It also can make it harder to match at the Radio.
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Only when the antenna happens to be a harder match than antenna+feeder_outer. Normally a good balun makes matching easier.
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But the stupid feeder only is a transmission line (a feeder) if it has the Characteristic impedance at BOTH ends. Otherwise it becomes a radiator, an aerial NOT a transmission line.
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No! This is a complete untrue! I thought you said you were professionally involved with antennas? Why propagate such myths? You really need to do some reading and get your facts straight, especially as you presume to teach others. People trying to learn about antennas should ignore almost everything in post 83.
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If the feeder is long and badly matched to aerial, then matching at the radio simply turns the feeder into an aerial with a capacity hat.
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And post 84 too.
Let us be clear, a feeder is a feeder whether or not it is matched. Whether a feeder is also a radiator is not a matter of matching, but a matter of balance: is there a current imbalance, also seen as common-mode current or feeder outer current (in the case of coax). Sadly, many radio amateurs and CBers (and, it seems, some professionals) get this all mixed up. Because common-mode current can affect impedance and SWR the myth has developed that impedance mismatch can cause common-mode current. This is simply untrue.
It is sad that here as well as other websites almost all threads about antennas and transmission lines quickly get polluted by popular myths confidently asserted. Hopefully on here there will be enough people with genuine subject knowledge to correct the errors.