Thank you for posting this. This is the clear and simple explanation of
all of this I have sought from someone who has considered all of the
elements of the discussion and who is speaking from the practical
application world of daily Amateur Radio operation.
Is this summary correct?
The TX ships out 100w, it travels out to the antenna (hopefully barring any
significant mismatches or losses along the way), on occasion an antenna
mismatch occurs (perhaps at the band edges of the antenna) and some power
is reflected back where it is resolved before reaching the TX by a manual
tuner or autotuner which then reflects it back to the antenna.
The result is that a miniscule amount of the 100w of power is lost in the
system and the rest is radiated by the antenna in the desired direction.
And no one at the receiving station is any the wiser that any reflection
has occured.
Question: If I run mostly 75 ohm low loss 3/4 inch CATV cable from my
shack to my antennas (about 200 feet when all is said and done) but use
flexible 50 ohm cable at the end points, have I injected loss and
mismatches that cost me the loss saved by the use of the CATV cable? Is
the use of 50-75 ohm baluns then justified? Is 200 feet not worth the
hassle of using the CATV? (It's free!)
My operation is mostly 160-10m w/occasional 2 and 6m. SSB, Amtor and
Pactor. I will be running 100w most of the time and full legal as
necessary.
73, DavidC AA1FA
> ->Consider that if these reflections are hapenning at the speed of light
> >(small multiples of).
>
> Nope! Always the speed of light in the coax or twin lead reduced by
> the velocity factor of the particular line, typically, 60 to 80% of
> llight speed.
>
> >The echo or delay will be unnoticeable on SSB or CW
> >especially when dealing with atmospheric conditions that will certainly
> >introduce more noticable distortions.
>
> Of course not, the echos on the transmission line are in microseconds,
> millionsth of one second. Absolutely NOTHING in an HF amateurs
> radio can respond to microsecond time duration signals, in particular
> your headphones, speaker, printer, etc., etc.!! Atmospherics are
> large fractions of a second, or seconds in duration.
>
> >Fast scan Television is another story because the modulation technique
is
> >highly sensitive to distortions of the phase relationships. Ghosting
> >results.
>
> Yes, when the Fast Scan is up at UHF and microwave frequencies,
> a-la commercial television! But meaningless at HF SSTV applications!
>
> >For SSB & CW, the distorted waveshape that results from reflections
will
> >still energize the antenna and produce the electromagnetic fields.
>
> So what??! Is there anyone on the planet who can hear it?? Of course
> not. Does it matter to the transmitter, the rcvr, the universe? If
you
> think so, please explain.
>
> Reflections, microseconds, 2 or 3 at most, at "dit" at 30 wpm, 43,000
> microseconds. Can you hear a dit at 30 wpm; think you could hear
> the microsecond bounce??
>
> >So the questions remaining are; how much more loss is incurred in all
that
> >makes up the transmission line including tuner, balun and connectors
> > and how much power is the transmitter able to make?
>
> I have absolutely no idea what this double talk, nonsense statement
> is trying to say! Do you have any idea what you are trying to say,
> or ask?
>
> >These are valid points because if the transmitter is unable to make
> >full power, it won't be there to lose.
>
> If the transmitter sees the correct impedance, resistance and
> reactance, it will develop full power. An antenna tuner,
> transmatch, line impedance matcher, wharever you
> choose to call it, will insure the transmitter WILL develop
> full power, and it will go to the antenna and be radiated!!
> Absolutely!
>
> > And even though out of phase relationships of the incident to reflected
> >waves can't cancel out the power, they will cause higher peak values of
> >both voltage and current that can increase losses.
>
> Yes, that is what we call vswr, and the losses are well known--
> tiny!
>
>
> Jim, for heavens sake, quit trying to restate in non-correct words
> what you have been told now for days!! If you understand what you
> have been told, you now can determine EXACTLY what your
> suffered losses will be, OK? Incredibly low, and meaningless!!
>
> Do you want to know why the Big Guns design for low vswr??
> It is to save the time they would lose in a contest situation to
> adjust the tuner when doing rapid QSY -- that is the reason!
> And they do a lot to keep a low vswr over a small percentage
> of a particular CW segment, or SSB segment of the band;
> certainly not the entire given band. Why, to save the time
> otherwise spent adjusting the antenna impedance matching
> device in the rig to antenna transmission system.
>
> If you really need it, I have a video, actually a video copy of a movie
> that was produced at Bell Telephone Laboratories decades ago
> that explains standing waves, power transport and all about
> transmission lines. However, if you would take the time to
> read the ARRL Radio Amateur's Handbook or better, the
> ARRL Antenna Book, in particular the chapters on connecting
> the transmitter to the antenna and the stuff about about
> transmission lines and tuners, you would LEARN a bunch!
>
> The BTL video uses a mechanical device, dashpots, and
> even line impedance matchers in a amazingly beautiful
> demonstration of all this.
>
> 73, Jim, KH7M
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