Re: Just how good (or not) is error correction on DAB

On 10 May, 16:34, Richard L <usenet@xxxxxxxxxxxxx> wrote:
In message <Kwr0i.5166$r4.4...@xxxxxxxxxxxxxxxxxxxx>
Richard Evans <R.P.Evans.NoS...@xxxxxxxxxxxx> wrote:



Richard L wrote:
In message <XIo0i.11678$d9.6...@xxxxxxxxxxxxxxxxxxxx>
Richard Evans <R.P.Evans.NoS...@xxxxxxxxxxxx> wrote:

Boltar wrote:
mention this, since running high-power transmitters with the load
disconnected used to be a good way of destroying them, until
How come? If you run an audio amp with the speakers disconnected it
does it no harm at all since no current flows in the output stage. Why
are transmitters different?

B2003

An audio amp used a different type of circuit than an RF amp.

To be honest I never completely understood why an RF amp can be damaged
if an aerial is not connected. I have some idea, but I'm not sure of all
the details.

The difference is that the RF output is matched to a transmission line
of a defined impedance, which should be terminated by a load of the
same impedance in order to achieve optimum delivery of power.
Yes I understand that part.

The part I'm not 100% sure about is why, when there is no load, the amp.
doesn't simply draw less current from the supply, in the same way as an
audio amp.

A hi-fi audio amp isn't matched to the load -- essentially, it
presents to your loudspeaker a source impedance of zero ohms. So it
doesn't make much difference to it whether the load is 8 ohms (a
loudspeaker) or an open circuit. But it's necessary to have a short,
fat connection to the loudspeaker, since any resistance or reactance
or interference pickup in your cable will in effect be appended to
your loudspeaker, degrading its performance. If you want lossless
transmission over a substantial distance, you can use an matched
transmission line instead. But this won't work properly unless it is
correctly matched, whether at audio frequencies or RF. If you mismatch
it by having no load at the end, power will be reflected back along
the cable or waveguide -- as waves in a swimming pool or canal lock
are returned when they hit the side wall. There is no matched absorber
there to mop them up.

Presumably this must be to do with the type of circuit used. Obviously
changing the load changes the properties of the circuit. The part I'm
not certain about is exactly how those changes mean that the circuit
continues to draw power, and hence burns out.

Power will be dissipated across the source impedance of the
transmitter. You will also get potentially destructive overvoltages
caused by the standing waves which will arise in the cable/waveguide.
Do you remember SWR meters from your CB days?

That explains more of a what than a why to be honest. Why does power
get dissapated across anything if theres no antenna attached?

B2003


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