Observations
 

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

Observations
 

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

Observations
 

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

Observations
 

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

Observations
 

In article ,
Sal M. Onella wrote:

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.

My own experience with a TS-2000 strongly suggests that this radio
does reduce its transmit power into difficult loads, even at
relatively low output-power settings (e.g. the 10-watt "tune" setting).

I've tried using this radio with an SGC longwire autotuner I picked up
for cheap at a hamfest, and have had great difficulty getting a
successful tuning. I hit the "tune" button, the radio starts
transmitting, and the autotuner simply chatters away indefinitely. I
can see the radio's SWR indicator bouncing all over the place, but it
never settles down and the tuner never "locks".

If I use the same autotuner, and the same antenna setup, with a
Ten-Tec Scout, the tuner will often achieve a successful lock on the
very same frequencies within 3-4 seconds, even though the Ten-Tec's
"tune" power setting is at the lower end of the SGC's tuning-power
specification range.

The TS-2000 has a high-SWR power scaleback. The Ten-Tec does not
(according to the manual it's SWR-protected in a different way). I
infer that the TS-2000 is probably varying its transmit power
constantly, as the SGC tries different matching combinations, and that
the rapid fluctuations in forward power are "confusing" the
autotuner's match-search algorithm.

So, it's entirely possible that Kenwood uses a transmit-power
throttleback algorithm which is sensitive to the output SWR, rather
than to the absolute level of reflected power seen by the finals.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Observations
 

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

Observations
 

Jeff Liebermann wrote:

Half the power, 1/4th the voltage.

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

73, ac6xg

Observations
 

On Mon, 29 Sep 2008 12:22:35 -0700, Jim Kelley
wrote:

Jeff Liebermann wrote:
Half the power, 1/4th the voltage.

You probably mean half the voltage, 1/4 the power.
73, ac6xg

Complex concepts I can easily understand. It's the simple things that
drive me nuts. Thanks. Gone sulking...

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

Observations
 

"Dave Platt" wrote in message
...
In article ,
Sal M. Onella wrote:

snip

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.


snip
So, it's entirely possible that Kenwood uses a transmit-power
throttleback algorithm which is sensitive to the output SWR, rather
than to the absolute level of reflected power seen by the finals.

Could be. I have not analyzed the innards of the beast, but the -870
detects and displays SWR at all power levels -- so it _knows_. That's one
of the bar-graph metering choices and if there's more than a watt or two
out, it will report SWR, good or bad.

Observations
 

"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"