[TowerTalk] SWR is what SWR meter measures

[Jim Lux]

----- Original Message -----
From: "Tom Rauch" <w8ji at contesting.com>
To: "Gene Bigham" <jbigham2 at kc.rr.com>; <TowerTalk at contesting.com>; "Jim
Lux" <jimlux at earthlink.net>
Sent: Thursday, May 12, 2005 5:12 PM
Subject: Re: [TowerTalk] SWR is what SWR meter measures


> > >But can I ask, "What is the difference in perception at
> the receive end of
> > >a HF wireless signal transmitted with an antenna measured
> that has a 2:1
> > >(impedance either high or low) SWR versus an antenna that
> has a measured
> > >SWR of 1:1.2 or so - given you are working with the same
> antenna AND given
> > >your modern rig does not power reduce prior to 2:1 SWR?"
> >
> > With a 2:1 mismatch at the far end the return loss is 10
> dB. This is
> > another way of saying that for every 1000 watts you send
> to the antenna,
> > 100 watts is reflected back towards the transmitter.  The
> radiated power is
> > reduced from 1000W to 900W (about 0.5 dB decrease)
> >
> > With 1.2:1, the return loss is 26.4 dB: only 2.2 Watts
> gets reflected back.
> > (0.01 dB decrease).
> >
> > In reality, it's not quite that bad, because most
> transmitters don't
> > present a perfect match to the reflected wave anyway.  So
> in the 2:1 case,
>
> <snip>
>
> Actually not.
>
> Reflected power and return loss is a model that represents
> mismatch. The reflected power can only be taken as a "loss"
> when the source appears to be a "fixed" dissipative
> resistance that looks like 50 ohms, like a signal generator
> through a large attenuator pad would appear.
>
> Real transmitters almost never behave that way, and if the
> transmitter has a tuner it absolutely doesn't behave that
> way. Most SSB transmitters actually favor a load impedance
> higher than 50 ohms, that's to increase linearity.

Which is what the whole last part of my post was all about..

>
> In any event, it is always considered that the reflected
> power makes only one trip, and that 100% of the reflected
> power returns to the antenna from the transmitter.
>
> The end effect of all this is the VAR (volt-amperes
> reactive) power in the transmission line increases. As far
> as we are concerned the only thing that happens is
> transmission line loss increases slightly. Since HF losses
> are predominantly resistive losses in the conductors, unless
> the line is a significant fraction of a wavelength long
> losses can actually DECREASE when the mismatch is in a
> direction that reduces current!!

Hmmm... I haven't run the numbers, but I would think that this is only the
case for lines <1/2 wavelength, and maybe <1/4 wavelength.  Certainly, if
it's more than a wavelength, you'd have run through all the possible phases.

On 160m, I can see having a small fraction of a wavelength. But on 10, 15,
or 20m, that would be more unlikely.

Good exercise for a student or exam question.  you could probably come up
with an analytical expression to describe where the loss would be lower in a
mismatch.


>
> The suggestion that HF PA's become "unstable" because of
> mismatch is also generally untrue in the case of any PA with
> an adjustable tank.

But most SSPAs are broadband and have no tuned tank.  Tube amps with an
adjustable high Q tank and output circuit, of course, will effectively serve
as a conjugate match to whatever mismatch presents at the line.

Conditionally stable amplifiers are probably more the rule than the
exception with microwave amplifiers. (except for the little MMIC amps).
Certainly, we spend a fair amount of time designing and testing for such
things at work.


 There are some solid state PA's that
> suffer instability with certain loads, but that generally
> has little to do with load impedance at the operating

>
> That is actually a MAJOR design headache solid state
> amplifier designers (or any amp with heavy feedback from
> output to input) must face.
>
> Don't get caught up in return loss or reflected power unless
> you understand it and how systems behave totally, otherwise
> it will really mislead you!!!!
>
> 73 Tom
>