On Sun, 28 Sep 2008 11:48:18 -0700, John Smith
wrote:

Cut power to 25% and the other guy will see (if his s-meter is accurate)
a drop to half the reading ... at 80% power, little difference, other
than if you were just above noise floor in the first place ... however,
"tweaking" an antenna to perfect match/design criteria has always proved
to we worth the effort ... and especially to my ears--but my s-meter
also ...

I think you mean guess-meter:
http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/SMeterBlues.htm
S-meters are notoriously inaccurate.

We have an Icom PCR-1000 remote receiver setup on a local mountain top
run by HRD (Ham Radio Deluxe) software. It's quite handy for tuning
and adjusting antennas and generating a polar plot for HF yagi's and
quad's. There are substantial site errors in doing it this way, but
it's still better than a field stength meter, or the traditional
"how's my signal" report.

I designed the 150w PEP power amplifier section for the Intech M3600
HF marine radio. Using VMOS xsistors, I was able to demonstrate
unconditional stability, at rated power, with any non-resonant load up
to about 8:1 VSWR. Above that the current went up a bit too high for
the 12 ohm load, and Vce climbed for the 400 ohm load. I might have
done better but didn't want to destroy the test radio. The radio was
specified to operate up to a 2:1 VSWR at full power and would reduce
power when the threshold reflected power was detected. Although I
demonstrated that it was possible to operate quite nicely up to 8:1
VSWR, without power reduction, nobody wanted to add that to the data
sheet for fear that someone would actually use it that way.

My guess(tm) is that most modern HF radios can also safely operate
into high VSWR loads, but suspect that manufacturers are also hesitant
to guarantee such operation. Most manufacturers don't even specify a
maximum VSWR and simply reduce power if excessive reverse power
(usually 2:1 at full power) is detected.

Disclaimer: If you trash your radio trying this without monitoring
the Pout, PA temperature, PA xsistor voltage/current, and checking for
oscillations on a spectrum analyzer, don't blame me.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"Jeff Liebermann" wrote in message
...


My guess(tm) is that most modern HF radios can also safely operate
into high VSWR loads, but suspect that manufacturers are also hesitant
to guarantee such operation. Most manufacturers don't even specify a
maximum VSWR and simply reduce power if excessive reverse power
(usually 2:1 at full power) is detected.

That sounds right..

I have two HF radios, (Kenwood TS-120, TS-870) and I often observe the
forward power climbing as I manually tune and reduce the reflected power.
At first glance, this would seem to indicate the rig had previously
throttled back its output in response to the consequences of a mismatch.

I never intentionally tune at anything more than a few watts, so perhaps the
protection circuits operate at less than full power, too. The autotuner in
the TS-870 uses about 10 watts.

Sal M. Onella wrote:
I have two HF radios, (Kenwood TS-120, TS-870) and I often observe the
forward power climbing as I manually tune and reduce the reflected power.

This is normal operation not necessarily associated with
foldback. As the tuner is tuned closer to a Z0-match,
more and more reflected power is redistributed back
toward the antenna. Although not often done, one could
conceivably tune the system by monitoring the forward
power on the antenna side of the tuner and tuning for
a maximum. It's a conservation of energy thing. Maximum
forward power to the antenna = minimum reflected power
toward the source. Who says the tuner has no effect at
the antenna?
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

"Cecil Moore" wrote in message
...

snip

This is normal operation not necessarily associated with
foldback. As the tuner is tuned closer to a Z0-match,
more and more reflected power is redistributed back
toward the antenna.

Ah, yes. I believe I picked up that little fact (here) in the recent past.
Let it get away from me, I did.

"Sal"

On Sun, 28 Sep 2008 22:21:49 -0700, "Sal M. Onella"
wrote:

"Jeff Liebermann" wrote in message
.. .
My guess(tm) is that most modern HF radios can also safely operate
into high VSWR loads, but suspect that manufacturers are also hesitant
to guarantee such operation. Most manufacturers don't even specify a
maximum VSWR and simply reduce power if excessive reverse power
(usually 2:1 at full power) is detected.

I have two HF radios, (Kenwood TS-120, TS-870) and I often observe the
forward power climbing as I manually tune and reduce the reflected power.
At first glance, this would seem to indicate the rig had previously
throttled back its output in response to the consequences of a mismatch.

That's normal. You're not measuring forward power. You're measuring
RF voltage near the radio antenna connector. Once the load (coax +
antenna) moves away from a 50 ohm resistive load, all bets are off as
to what the voltage means. The same for a VSWR meter or watt meter
installed between the radio and the coax cable to the antenna. Once
the load gets away from 50 ohms, the voltage readings are not an
indication of power.

I never intentionally tune at anything more than a few watts, so perhaps the
protection circuits operate at less than full power, too. The autotuner in
the TS-870 uses about 10 watts.

Yep. That's the way it should be done. The auto-tuners intentionally
reduce power to prevent damage if they cross through some odd
resonance that causes the voltage across some internal component to go
sky high. Half the power, 1/4th the voltage. It could zap the tuner,
but also zap the power amplfier if tuned at full power.

That begs the question "What happens to a transmitter if one
intentionally runs it with a high VSWR?" I don't know the full
answer. Conventional wisdom is that the xmitter finals overheat. I've
never seen much of that except with a tube power amplifier. With the
power amps I worked on, the voltage across the transistor would either
increase dramatically, or the current through the transistor would
increase dramatically. Excess voltage would zap the junction. Excess
current might cause overheating, but more commonly, would simply turn
the xsistor wire bonds into fuses or melt the junction even before the
heat sink could extract the heat.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:
That's normal. You're not measuring forward power. You're measuring
RF voltage near the radio antenna connector. Once the load (coax +
antenna) moves away from a 50 ohm resistive load, all bets are off as
to what the voltage means. The same for a VSWR meter or watt meter
installed between the radio and the coax cable to the antenna. Once
the load gets away from 50 ohms, the voltage readings are not an
indication of power.

It's not an indication of *NET* power but it is certainly an
indication of forward power (or reverse power). If it weren't
anyone could sue Bird for false advertising.

That begs the question "What happens to a transmitter if one
intentionally runs it with a high VSWR?" I don't know the full
answer. Conventional wisdom is that the xmitter finals overheat. I've
never seen much of that except with a tube power amplifier.

A high VSWR can cause over-voltage or over-current conditions
depending upon phase. Over-current conditions can indeed cause
the finals to overheat. Over-voltage conditions can cause punch-
through of the finals. The key to understanding the effect of
SWR on the finals is in understanding constructive and destructive
interference.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

On Mon, 29 Sep 2008 11:47:25 -0500, Cecil Moore
wrote:

Jeff Liebermann wrote:
That's normal. You're not measuring forward power. You're measuring
RF voltage near the radio antenna connector. Once the load (coax +
antenna) moves away from a 50 ohm resistive load, all bets are off as
to what the voltage means. The same for a VSWR meter or watt meter
installed between the radio and the coax cable to the antenna. Once
the load gets away from 50 ohms, the voltage readings are not an
indication of power.

It's not an indication of *NET* power but it is certainly an
indication of forward power (or reverse power). If it weren't
anyone could sue Bird for false advertising.

Bird is very specific that their 50 ohm wattmeters are only useful in
a 50 ohm system. Here's one of your articles explaining how it works
in a 75 ohm system:
http://groups.google.com/group/rec.radio.amateur.antenna/msg/717e8daabed08fd9
Incidentally, I've been running cheap 75 ohm coax in 50 ohm systems
for years without any problem. Max VSWR= 1.5:1
http://www.qsl.net/n9zia/wireless/75_ohm_hardline.html
It's especially useful at 2.4Ghz, where the rather high coax cable
losses almost totally eliminates any detectable reflected power.

That begs the question "What happens to a transmitter if one
intentionally runs it with a high VSWR?" I don't know the full
answer. Conventional wisdom is that the xmitter finals overheat. I've
never seen much of that except with a tube power amplifier.

A high VSWR can cause over-voltage or over-current conditions
depending upon phase. Over-current conditions can indeed cause
the finals to overheat. Over-voltage conditions can cause punch-
through of the finals. The key to understanding the effect of
SWR on the finals is in understanding constructive and destructive
interference.

Agreed. I didn't want to go into the mechanism for where the excess
voltage or current came from, just the risks involved in tuning at
full power. You can run a radio at full power with a high VSWR load,
but it's likely that either the PA voltage or the current are going to
approach the point of no return. Reducing power is much safer than
finding the limit the hard way.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:
Bird is very specific that their 50 ohm wattmeters are only useful in
a 50 ohm system.

What you seem to be missing is that it takes only a few
inches of 50 ohm coax on each side of the directional
wattmeter to force a "50 ohm system". 50 ohm coax *IS*
a 50 ohm system, i.e. the load doesn't matter.

All this is easily proven by wading through the math.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

"Cecil Moore" wrote in message
...
Jeff Liebermann wrote:
Bird is very specific that their 50 ohm wattmeters are only useful in
a 50 ohm system.

What you seem to be missing is that it takes only a few
inches of 50 ohm coax on each side of the directional
wattmeter to force a "50 ohm system". 50 ohm coax *IS*
a 50 ohm system, i.e. the load doesn't matter.

All this is easily proven by wading through the math.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

The accuracy of a Bird for measuring power goes out the window for an SWR
other than 1:1. Are you saying that if you put an unterminated 3 inch piece
of coax on the Bird you have a 50 ohm system? That sounds like semantics
rather than Math. In fact you would have a 50 ohm system if you left the
antenna port disconnected, Then you could say you had an unterminated 50
ohm system. Real handy huh?

Jeff Liebermann wrote:

Half the power, 1/4th the voltage.

You probably mean half the voltage, 1/4 the power.

73, ac6xg