This is at least one wrong assumption.3. At the transmitter, where it encounters a large impedance mismatch, the power is totally reflected back to the load.
Any reflections in the steady-state can also be equally expressed as a change in input impedance [from the matched case]. Personally, I find the notion that "Reflections cause damage" is generally a hand-waving explanation, as it doesn't adequately describe the actual problems. As far as I know, there are only two impedance extremes which cause failure modes:"You blew up your rig because you were transmitting into an open circuit. The rig is designed to transmit into a 50 ohm load. An open circuit presents a huge impedance mismatch to the output stage so it overheated and failed. It had nothing to do with reflected power..."
Yes, total power provided to a system is a combination of the emitted waves and the incident. I think in your situation, the presence of the reflected waves cause the amplifier to emit significantly less power.If this is true, which I think it is, power could bounce back and forth between tuner and mismatch but you would think if the transmitter keeps pumping in the same amount of power you'd soon have lightning bolts come out of the cable. Instead, the transmitter can't put out as much power. I think of it as a water pipe with air in it or something else, that takes up space and you won't be able to push as much water through.
Great explanation. The radio I was talking about is an original President Lincoln, which is from a very long time ago so I will give it a pass on the "poorly designed" but otherwise agree with you. In my case the transistor must have not been able to handle the voltage.Any reflections in the steady-state can also be equally expressed as a change in input impedance [from the matched case]. Personally, I find the notion that "Reflections cause damage" is generally a hand-waving explanation, as it doesn't adequately describe the actual problems. As far as I know, there are only two impedance extremes which cause failure modes:
- Reflection in-phase to the transmitting, which is equivalent to an open-circuit load impedance. The failure mode here is the voltage at the output of the amplifier is double what it would be from the matched case, which can obviously cause some components to fail. IMO this only occurs for poorly-designed amplifiers -- most modern amplifiers are designed with components having the appropriate voltage rating so that this doesn't happen.
- Reflection 180 degrees out-of-phase to the transmitting, which is equivalent to a short-circuit load impedance. The failure mode here is what you would expect: the front-end transistor (and associated biasing circuitry) isn't designed to handle a shorted output, and so it burns out. Again, I think this only happens due to poor design; it's my understanding that the transistor can be designed with an appropriate biasing network such that the transistor supplies a finite and tolerable short-circuit current.
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Yes, total power provided to a system is a combination of the emitted waves and the incident. I think in your situation, the presence of the reflected waves cause the amplifier to emit significantly less power.
That's very interesting, a little bit over my head, I'll have to ponder thatAfter reading the original Uniden schematic that was transferred to the President Lincoln Brand around 1988, I now agree with the old fart who said it has nothing to do with reflected waves and has everything to do with mismatched impedance. In the classical sense when the signal rise time is much slower or longer than the wave propagation delay from the final stage to the open antenna port, we can use lumped RLC elements and Kirchoff Laws to explain everything. In reality the waves move back and forth so fast that you cannot see the reflections with a slow risetime in the VHF band .
A conventional 50 ohm generator just outputs twice the voltage with no load.
The unlisted Uniden transistors in the last few stages of bottom left corner drive a high Q low pass filter which is damped by the antenna load. It is driven from a CE common emitter with step-up coils in between . The High output power is converted from a low impedance CE amp that drives step-up transformer coils to raise the impedance. This would fail from over current if the coils saturate or the voltage amplifies much more than 2x Vcc.
Given that Uniden knows how to design transistors with a loaded Q LPF, I expect they warned users in the manual to never operate without an antenna. .
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So, if the termination is an ideal inductive or capacitive load?But one more question @PlanarMetamaterials, what would happen if there is 100% reflection but on the Smith chart it is at the very top or bottom, so right between open and closed?
The two are obviously related, so to say that one has "nothing to do with" the other is a bit misleading. Both descriptions are valid and useful in different contexts.I now agree with the old fart who said it has nothing to do with reflected waves and has everything to do with mismatched impedance.
So, if the termination is an ideal inductive or capacitive load?
I don't think the impedance description is very helpful here; I'd revert to the incident and reflected wave viewpoint. The reflections will be equal magnitude and +/-90 degrees phase-shifted from the incident waves. This will cause a sqrt(2) rise in both voltage and current magnitudes (math). So, these loads could be problematic -- but realistically, should be within engineering safety factors!
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The two are obviously related, so to say that one has "nothing to do with" the other is a bit misleading. Both descriptions are valid and useful in different contexts.
Yes. As I wrote above, the description by incident/reflected wave and the description by impedance at the transmitter are both valid and equivalent, and lead to the exact same results.I can see though as one gets closer and closer to the active component(s) at some point it doesn't sound right to call it a reflection anymore even though technically it is at least I think.
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