Home |
Search |
Today's Posts |
#8
![]() |
|||
|
|||
![]()
On Tue, 5 Sep 2006 06:48:57 -0500, "Richard Fry"
wrote: "Owen Duffy" wrote The mechanism is not "that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating". Take an example of the 50 ohms load discussed, and an electrical half wave of 70 ohm line connected to a transmitter designed for a 50 ohm load. The transmitter behaves exactly as if that line were 50 ohms. Though there is a reflected travelling wave on the line, it does not travel back to the PA anode where it is absorbed and converted to heat. ____________ Really, the mechanism is there -- only the unique circumstance you describe protects the PA from seeing reflected power in such cases. Other line lengths in this scenario could stress PA components beyond their ratings. Richard, There is not such a mechanism in the general case, the example I gave shows that the reflected wave does not necessarily travel back to the source where it is absorbed. If you re-read my words "Next thing, you will be thinking that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating." Remembering the context was a wave on the dipole reflected from the o/c end, and the word "must" was used to mean "necessarily". Sure,transmission lines with VSWR may transform impedance, have higher losses (if they are long enough), operate at higher voltages in places (if they are long enough), operate with higher currents in places (if they are long enough). Nothing I have said is in conflict with that or suggests otherwise. Transmitters operated at other than their rated load impedance may operate at higher voltages, higher currents, different power etc and may damage components. Nothing I have said is in conflict with that or suggests otherwise. But, the mechanism is not that the reflected wave *necessarily* travels all the way back to the PA anode by virtue of some kind of momentum (as sometimes expressed by some amateurs). In the case raised by the OP, the reflected wave on the dipole and the forward wave resolve (as in resolution of phasors) to an impedance of 50+j0 (OP's hypothetical example), and the constraints / conditions at the feedline / feedpoint junction are fully satisfied with no reflected wave on the feedline. (I used the term resolve, Cecil must call it destructive interference.) The reflected wave on the dipole does not have a momentum that *must* carry it to the PA anode to be absorbed there. Owen -- |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Workman BS-1 Dipole Antenna = Easy Mod to make it a Mini-Windom Antenna ! | Shortwave | |||
Antenna Suggestions and Lightning Protection | Shortwave | |||
Imax ground plane question | CB | |||
Introduction to "AM" Medium Wave DXing - by the Ontario DX Association (ODXC) | Shortwave | |||
Smith Chart Quiz | Antenna |