Alex brings up a good question. When using an antenna tuner driving
a long feedline, what does my final amplifier see during the transient
response while the energy is sloshing back and forth between the
antenna and the tuner setting up the steady state match conditions?
For a short time (before the reflection from the antenna has time works
its way back to the tuner), the tuner will see the surge impedance of
the transmission line which will be transformed by the matching
network into an impedance that is most likely not 50 ohms. For a very
short time, the finals might see a very bad mismatch.
Hmm, I think I just answered my own question. I guess the relatively
slow rise time of the RF envelope takes care of any problems this might
cause (RF power is increased incremently on a time scale greater than
the settling time of the antenna/matching network).
Gee, there is always more than meets the eye going on with these
sorts of things.
73 de Mike, W4EF................
----- Original Message -----
From: "alex" <alex@sandlabs.com>
To: <W8JI@contesting.com>; <towertalk@contesting.com>
Sent: Wednesday, December 12, 2001 7:17 PM
Subject: [TowerTalk] Perpetum Mobile
>
> OK... here is _minimal_ tech answer... no formulas, no confusion.
>
> SWR is a wave. It does really not exist per say. It is just an envelope
> modulation on the carrier related to group delay, medium velocity and
other
> thousand reasons. Actually SWR is a mathematical SYMBOL for the mismatch
> ratio between a source and load.
>
> SWR caries energy. As any energy it must be transformed in an other form
> otherwise, as
> with any other form of energy, if you remove it's source it will still
> remain alive and well...
> This is not quit what happens, no?
>
> What REALLY happens is that SWR is renewed every time by the bounce back
of
> the antenna side and killed a bit (or DISSIPATED if you will) when is
> bounced of the transmitter output side. Please observe I do not talk about
a
> tuner but the generator (transmitter) output. The net effect on the
antenna
> side is that sometimes when the bouncing coincides in phase with the
> incident wave you will have a SWR GAIN :) i.e. you will radiate a bit more
> :) Isn't this cool ?
>
> On the transmitter side, SWR hits a hard rock. It has no where to go so it
> bounces back BUT a little bit of its energy is released into the rock (the
> transmitter output) and here it gets converted in currents and voltages
that
> ultimately are converted in heat. A small amount of heat. However, heat is
> cumulative. Thermal energy has a long inertia and if not removed faster
than
> the accumulation then you have a thermal runaway or, in nuclear jargon, a
> thermal meltdown :)
>
> Recognizing this fact ( finally) some manufacturers have started to put
fans
> into their
> reactance converters (antenna tuners). Slowly but surely, we realize that
> SWR in last instance, DOES generate heat.
>
> Hope this clarifies the "technical" aspect of this thread that, I hope
will
> end here.
>
> ===================================
> If we can accept the notion that GUNS are
> the killers and not the people, why is it that
> we can not accept the idea that CARS are
> the speeders and not the drivers?
>
> Alex, trying to understand the world around.
>
> ==================================.
>
> ----- Original Message -----
> From: "Tom Rauch" <w8ji@akorn.net>
> To: "alex" <alex@sandlabs.com>
> Sent: Wednesday, December 12, 2001 8:48 AM
> Subject: Re: [TowerTalk] ++ High SWR and Modern Transceivers
>
>
> > > I think this thread should have been called "Perpetum Mobile" :)
> > > While I am a strict adherer to the old saying, "Never smarten a dummy"
> > > I will make an xception now. First, for the record, here is what I
> > > said:
> >
> > If you can write a trechnical response without insults, I will read it.
> > Otherwise get lost.
> > 73, Tom W8JI
> > W8JI@contesting.com
> >
>
>
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