At 03:18 PM 5/12/2005, Gene Bigham wrote:
>I hate to sound stupid, but at that risk here goes.
>
>When I was in school for psychology one course on IQ and testing said, "IQ
>is what an IQ test measures." I have transliterated that into, "SWR is
>what an SWR meter measures." There are some pretty basic designs to
>measure SWR using diodes to detect voltages. There are also some more
>sophisticated designs and instruments to measure SWR which yield
>(relative) impedance.
>
>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,
some of the 100W is actually reflected back to the antenna, and eventually
some part of that will get radiated (or at least 90% will, 10% gets
reflected back towards the transmitter a second time.)
Now, all that power bouncing back and forth is subject to loss in the coax.
This is what happens when you use a tuner at the shack end. Your tuner
just reflects back the incident power that was already reflected from the
antenna. (the net of all the back and forth is called reactive or
circulating power).
The real problem is that 100W incident on the transmitter's output. It's
got to go somewhere, and that somewhere is heat in the transmitter. At
3:1, about a quarter of the power is reflected back. Get up towards a 5.8:1
VSWR and half the power is reflected back.
If your transmitter has a really burly thermal design, then dissipating
that much power might not be an issue. Take a typical 100W SSB amplifier
(like in my FT-757). It's only rated at 25W continuous duty, which tells
you something about the amount of heat they expect to dissipate in the
finals (about 25W... run of the mill amplifiers run around 50% efficiency).
There's also a voltage and current limit issue with the reflected power
(infinite SWR means that the peak voltage and current is potentially
doubled, and component breakdown might be an issue).
I think the issue for most solid state amp foldback circuits is not so much
whether it folds back at 3:1 or 2.5:1 or 5:1. The idea is to keep it from
burning up with 1000:1, which doubles the power dissipation. They have to
pick somewhere. They could just as easily do the limiting with a thermal
sensor (which actually would make more sense.. If I'm operating in 140F, I
shouldn't be dissipating as much heat, if I don't want to burn the
amplifier up, but you'd still have to deal with the doubled voltage problem.
Then you have the instability problem:
With some amplifier designs (those with poor reverse isolation), they may
become unstable operated into a mismatched load. Say your amplifier has
10dB foward gain (typical for bipolar HF amps). It might have only -13 dB
of reverse isolation (that is, if you put a 100 Watts into the output of
the amp, 5 watts will appear at the input). Hook up a load that reflects a
significant amount of power back (RL worse than -3 dB, in this case) and
the amplifier will oscillate.
This is especially a problem at harmonics. Some tuned amplifiers have very
poor reverse isolation at a harmonic of the input frequency (TWTs, for
instance, have about the same gain forward and backward at the second
harmonic of their design frequency). Your load may be perfectly matched at
the design frequency, but totally reflective at the harmonic. All amps
generate some harmonics, and if the fwd/reverse margin is too small, and
the reflection happens to be just the right phase, the amplifier will
oscillate at the harmonic. Or, at some totally different frequency (the
neutralization or parasitic oscillation problem)
Yesiree... designing reliable, stable, efficient broadband amps IS a
challenge.
>I suspect the perception is negligible and that any efforts at reducing
>SWR lower than 2:1 are futile, a waste of time, and fruitless other than
>for personal satisfaction. Is this a correct observation or largely in error?
Very small effect for the listener. Huge effect for the sender.
>Let the flames begin.
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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