> >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.
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!!
This is why open wire line, even with very high standing
wave ratios and extremely high return loses, "return losses"
that would imply only a few percent of power is transferred
often operate with very high efficiency.
Now if the source is a generator or transmitter through a
very large attenuator pad of an impedance equal to the
impedance you are considering "return loss" as being
referenced, you could consider the "return loss" as a direct
reduction of power. Of course that loss would have to be
added to the increase in loss in the transmission line
caused by the termination mismatch.
For amateurs, in order to not be confused by models of
"reflected power" and "return loss", it's best to just
consider the additional loss from the increase in SWR. We
can only do that if the line is near 1/4 wl long or longer.
When the line is short, losses can actually DECREASE with
increased return loss if the current integrated over the
length of the line decreases!!
So if your line is fairly long compared to 1/4 wl and if you
have SWR, the best advice would be to ignore everything else
and look at a table showing the increase in line loss caused
by an increase in SWR. If the transmitter does not fold
back, that is pretty much the only loss you will have and it
can be pretty small.
When MFJ wanted to add "return loss" to the MFJ analyzers I
objected, because return loss is very misleading. It really
is just another way of expressing SWR (another SWR model),
and doesn't mean that amount of power is "lost" or
unavailable.
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. 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
frequency. That problem will just as well manifest itself
with a PERFECT SWR on the operating frequency when the load
presents a critical impedance on any frequency passed by the
output filters of the PA. An example of this is a solid
state mobile amplifier driving a mobile antenna on 160 or 80
meters. The antenna changes impedance so rapidly with
frequency the amplifier can be considered to be operating
into a nearly pure
reactance, and that reactance can change phase at the output
of the PA so much that the negative feedback circuits that
normally reduce gain now increase gain. The result is an
oscillation that is out-of-band. It has nothing to do with
SWR or return loss on the operating frequency, but rather
mistermination of the system out of band whether
transmitting or not!
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
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