[TowerTalk] SWR is what SWR meter measures

[Ian White GM3SEK]

Jim Lux wrote:
><monster snip>
>> >
>> > 73 Tom
>> >
>>
>> I agree with Tom about reflected power not being dissipated in the
>> transmitters final. For all practical purposes, it all gets returned to
>> the antenna and radiated except for the small amount of loss the
>> transmission line causes from the mismatch.
>
>For a tube amp, sure.  And for a solid state amp with a tuner on the output.
>But not for a broadband SS amp, in general.  While they're not 50 ohm
>resistive, they're also probably not too far away, so the mismatch at the TX
>output isn't going to be all that huge.  If there's not a big mismatch, most
>of the reflected power winds up "inside" the TX, and gets dissipated (or fed
>back into the power supply, if you've got an amplifier that can do that).
>

Regardless of the type of transmitter, the concept of "reflected power" 
and [it] "winds up 'inside' the TX" is a highly unprofitable way to look 
at this problem.

A much easier way to think of it is:
1. Measure the impedance at the bottom of the transmission line. This is 
the load impedance that the transmitter sees at its output terminals.

2. Imagine this load replaced by a lumped resistor and 
inductor/capacitor to give the same (R +/-jX). In the steady state, the 
transmitter cannot tell the difference, and behaves exactly the same 
into this lumped load.

3. The power delivered by the TX into this load is the power delivered 
into the R resistor. In principle you can measure it by absolute thermal 
methods.

4. You can now forget about the transmitter. You now have a pure 
antenna/transmission-line problem.

5. All the power delivered by the transmitter winds up either being 
radiated, or dissipated in the transmission line or resistive antenna 
losses.

22. Don't even THINK about "net" power being the difference between 
"forward power" and "reflected power".  You don't need to go there... so 
don't.

Here's another angle:

I hope we can all agree that the load impedance as measured at the 
bottom of the transmission line is purely a property of the antenna and 
the line. SWR is something you can either measure directly on the 
transmission line, or more commonly calculate as a mathematical function 
of (R +/-jX) and Z0. These real-life, measurable quantities are by 
definition not affected by the way the line is energized - they don't 
change when you connect a different type of  transmitter or test 
equipment.

The problem with using those real-life measurements to calculate the 
ratio of "forward power to reflected power" is that it suddenly requires 
us to know things about the transmitter! We're now saying that we need 
to know the type of transmitter, how it is tuned, driven and loaded, etc 
etc. But that flatly contradicts what we said in the previous paragraph 
- namely that we can know everything about the transmission line and 
antenna, without needing to know *anything* about the transmitter.

That seems like sufficient logical proof that "forward power and 
reflected power" is faulty as a concept. Just because you can calculate 
these quantities, that doesn't mean they have any physical meaning at 
all.

If any more proof were needed that the error lies in the concept itself, 
witness how many repeated rounds of argument on the subject have 
stubbornly failed to resolve... and I don't think they ever will,  or 
ever can.



-- 
73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek