Re: VSWR doesn't matter?

Richard Fry wrote:
"Dan Bloomquist" wrote

This is not correct. It has nothing to do with the output impedance of the drivers. Even a 50 ohm Thevenin source will reflect _all_ the power back to the load as long as it is 'matched'.

_______________

Following is a quotation from a paper titled "A Study of RF Intermodulation Between FM Broadcast Transmitters Sharing Filterplexed or Co-Located Antenna Systems," by G. N. Mendenhall, P.E., who then was the VP of Engineering for Broadcast Electronics, Inc, and now is VP of Product Engineering for Harris Corporation, Broadcast Division. Broadcast Electronics and Harris manufacture a full range of high power broadcast transmitters. Mendenhall is highly regarded in the broadcast industry.

In the context of the quote, the "interfering signal" means a signal coupled into the transmitter PA output circuits that was generated external to that transmitter. But the interfering signal also could be a reflection of the output signal of that transmitter from a mismatched load. Therefore the information in the quote addresses the subject of these posts.

QUOTE
"Output Return Loss" is a measure of the amount.of interfering signal that
is coupled into the output circuit versus the amount that is reflected back from
the output circuit without interacting with the nonlinear device.

To understand this concept more clearly, we must remember that although the
output circuit of the transmitter is designed to work into a fifty ohm load, the
output source impedance of the transmitter is not fifty ohms. If the source
impedance were equal to the fifty ohm transmission line impedance, half
of the transmitter's output power would be dissipated in its internal output
source impedance.

The transmitter's output source impedance must be low compared to the load
impedance in order to achieve good efficiency. The transmitter therefore looks
like a voltage source driving a fifty ohm resistive load. While the transmission
line is correctly terminated looking toward the antenna (high return loss), the
transmission line is greatly mismatched looking toward the output circuit of the
transmitter (low return loss). This means that power coming out of the transmitter
is (almost) completely absorbed by the load while interfering signals fed into
the transmitter are almost completely reflected by the output circuit.
END QUOTE

If the terminating impedance for reflected/reverse power on a transmission line looking back into the PA matched the impedance of that transmission line, it is rather unclear why the PA would reflect _all_ that power back toward the load, rather than dissipate it in that termination.

Hi Richard,
He says it in the last sentence. But here is an example. Take a 50 ohm thevinin source. Power off, it looks like 50 ohms back into it. Take a second thevinin source to represent a reflection and drive 5 volts into the first source. Now set your first source 180 degrees to the reflection and drive forward 5 volts.

(s)-----/\/\/\--------(c)-----/\/\/\--------(r)

(s)source (c)connection (r)reflection. With (s) 180 degrees out of phase from (r), (r) will see a short at (c). It is because of the power generated at the source that the impedance into it can look purely reactive. And, you can use 5 ohms with 1 volt at the source, (c) will still look like a short to (r). The source resistance doesn't matter as long as a 'match' is made.

And for the same reason, why the 50 ohm line doesn't look like 50 ohms is because of reflected power. Drive an open quarter wave line and it looks like a short because the reflected voltage is 180 degrees out from the source.

RF

Best, Dan. (DB :)

.

Relevant Pages

  • Re: VSWR doesnt matter?
    ... Even a 50 ohm Thevenin source will reflect _all_ the power back to the load as long as it is 'matched'. ... In the context of the quote, the "interfering signal" means a signal coupled into the transmitter PA output circuits that was generated external to that transmitter. ... But the interfering signal also could be a reflection of the output signal of that transmitter from a mismatched load. ... the output circuit without interacting with the nonlinear device. ...
    (rec.radio.amateur.antenna)
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  • Re: Efficiency and maximum power transfer
    ... Let us regard a transmitter as an ideal RF generator with a resistance ... It is well known that, for maximum power transfer, the load ... To achieve a higher efficiency, the load resistance should be made ... If the equivalent source impedance is not important, ...
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