[Amps] SWR and amplifiers

[sm0aom at telia.com]

The SWR meter should measure voltage reflection coefficient or "gamma".

These are vector quantities having magnitude and phase which can both add and subtract, and in a lossless line
they are constant over the overall length of the line.

SWR is formally defined as 1+|gamma|/1-|gamma|, and power neither forward nor reflected enter the definition.

It is somewhat unfortunate that we have instruments that are calibrated in terms of power and SWR, because the concept of mismatches are much easier handled when thinking in terms of reflection coefficients.

When the forward wave passes the coupler on its first trip to the load, is then partially reflected back at the load and registered as reflected power, and finally again re-reflected at the conjugate match at the output network. It is again added in phase to the forward power on its next trip towards the load and so on...

This causes the forward reading to be added to the reflected power, so the indicated forward power can exceed the available source power. You of course need an adjustable output network that can be adjusted to present a conjugate match.
Otherwise, the mismatch loss at the junction between the output network and feedline will reduce the actual source power.

If the output stage is broad-band tuned and biased in Class A, it will absorb some or all of the reflected wave which may increase heat losses. In this case, the reflected wave is not re-reflected and will not show up as an increase of the forward reading. 

Otherwise, the major cause for losses due to SWR in the feedline is the increase in I2R losses at the current nodes, where a much higher current flows.

A properly made directional coupler based SWR meter should show the same reading regardless of where it is inserted into a low-loss feedline, as it should indicate the load reflection coefficient. If the reading varies much, something is amiss with the directional coupler.

73/
Karl-Arne
SM0AOM

----Ursprungligt meddelande----
Från : donroden at hiwaay.net
Datum : 2017-05-25 - 11:24 (CEDT)
Till : amps at contesting.com
Ämne : Re: [Amps] SWR and amplifiers

A broke analog clock is right twice a day.
For it to be right, you just have to know "when" to read it.

If you could insert that same SWR meter at various lengths along your  
transmission line you might see 200 watts or ZERO watts reflected.

Did your amplifier load or tune change due to this different reading ?  No.

Did your amplifier change power output due to this different reading ?  No.

So what does your swr meter "really" measure ?

We know it measures "something".......... BUT WHAT ???



Power ( watts ) = voltage x current
( or does that apply  in a transmission line ? Hummmmmmmmm   )




Don W4DNR

DonR
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