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Old January 18th 04, 08:45 PM
Thierry
 
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Default SWR read to TX or rather to the antenna ?

Hi,
It seems well-known that the SWR value that you read on an external meter or
an antenna tuner is NOT the value measured between itself and the antenna.
The SWR value "picked-up" and that you can change is the one measured
between this meter or tuner and the transceiver (or the linear amplifier but
we consider it as a black box if you use it properly).

But recently I experimented the burning of my PL due to a high SWR, thus
high current, and moisture and the end of the line (really, see image of
PL's here http://www.astrosurf.com/lombry/qsl-...sion-line2.htm, end of
page ).

The distance between my RTX and SWR reader is short, about 1 m. But there
are about 15m between the SWR and the feed point of my antenna where I had
the problem.

So I would like to know if an SWR meter does only read the value upstream or
if the properties of the feedline to the antenna does not influence is
reading.
At first sight it does as during this problem I had a SWR 4:1 on the
external SWR-meter. This value didn't come from the RTX side which built-in
SWR displayed a VSWR 1:1. It really came from the other side, to the
antenna, thus contrary to what I think and what is usually stated about the
functioning of this device...

Can someone explain me this ?

Thanks in advance

Thierry
ON4SKY





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Old January 18th 04, 09:54 PM
Ralph Mowery
 
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Default

It seems well-known that the SWR value that you read on an external meter
or
an antenna tuner is NOT the value measured between itself and the antenna.
The SWR value "picked-up" and that you can change is the one measured
between this meter or tuner and the transceiver (or the linear amplifier

but
we consider it as a black box if you use it properly).

But recently I experimented the burning of my PL due to a high SWR, thus
high current, and moisture and the end of the line (really, see image of
PL's here http://www.astrosurf.com/lombry/qsl-...sion-line2.htm, end

of
page ).

The distance between my RTX and SWR reader is short, about 1 m. But there
are about 15m between the SWR and the feed point of my antenna where I had
the problem.

So I would like to know if an SWR meter does only read the value upstream

or
if the properties of the feedline to the antenna does not influence is
reading.


The SWR meter indicates the value from the meter to whatever is after it.
If you insert a device that has a high SWR (because it becomes defective or
for some other reason ) between the transmitter and SWR meter ( say a switch
or filter), the meter can show a low value if the antenna is ok, but the
transmitter will see a high SWR and may burn out the finals.


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Old January 19th 04, 12:38 AM
JGBOYLES
 
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Default

The SWR meter indicates the value from the meter to whatever is after it.
If you insert a device that has a high SWR (because it becomes defective or
for some other reason ) between the transmitter and SWR meter ( say a switch
or filter), the meter can show a low value if the antenna is ok, but the

transmitter will see a high SWR and may burn out the finals.

He did not say he inserted anything, just that moisture at the end of his
transmission line caused a fault. If this was the case, an SWR bridge at the
output of his Power Amps should have alerted him to a problem that would
potentially destroyed them.
If you want to use an SWR bridge as a protective device, or indicator, it
must be placed in the proper position in the antenna system. Modern
transceivers have circuits that detect mismatches and reduce power to minimize
failed output devices.


73 Gary N4AST
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Old January 19th 04, 04:45 PM
JDer8745
 
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Default

Someone sed:

"...the meter can show a low value if the antenna is ok, but the transmitter
will see a high SWR and may burn out the finals."

================

It's funny that this old wive's tale about the high SWR "burning out the
finals" still exists. I guess it depends on what the meaning of "high" is.

73, Jack, K9CUN
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Old January 19th 04, 07:29 PM
Cecil Moore
 
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Default

JDer8745 wrote:
It's funny that this old wive's tale about the high SWR "burning out the
finals" still exists. I guess it depends on what the meaning of "high" is.


If high SWR and the attendant mismatch cannot burn out a final,
I wonder why most commercial finals are protected from a high SWR
by foldback circuitry to avoid damage to the finals?
--
73, Cecil http://www.qsl.net/w5dxp



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Old January 19th 04, 09:21 PM
Reg Edwards
 
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Default

I wonder why most commercial finals are protected from a high SWR
by foldback circuitry to avoid damage to the finals?
--
73, Cecil

==========================

They don't. There isn't an SWR meter on the station.

They are really protecting the finals against excessive deflection of the
TLI. ( Transmitter Loading Indicator.)

But it sure makes a good point of conversation amongst the old wives.
----
Reg, G4FGQ


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Old January 19th 04, 11:38 PM
Thierry
 
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Default


"JGBOYLES" wrote in message
...
The SWR meter indicates the value from the meter to whatever is after it.
If you insert a device that has a high SWR (because it becomes defective

or
for some other reason ) between the transmitter and SWR meter ( say a

switch
or filter), the meter can show a low value if the antenna is ok, but the

transmitter will see a high SWR and may burn out the finals.

He did not say he inserted anything, just that moisture at the end of his
transmission line caused a fault. If this was the case, an SWR bridge at

the
output of his Power Amps should have alerted him to a problem that would
potentially destroyed them.
If you want to use an SWR bridge as a protective device, or indicator,

it
must be placed in the proper position in the antenna system. Modern
transceivers have circuits that detect mismatches and reduce power to

minimize
failed output devices.


Indeed. A good suggestion I could install (in case of)
Thierry



73 Gary N4AST



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Old January 19th 04, 11:40 PM
Thierry
 
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Default


"Cecil Moore" wrote in message
...
JDer8745 wrote:
It's funny that this old wive's tale about the high SWR "burning out the
finals" still exists. I guess it depends on what the meaning of "high"

is.

If high SWR and the attendant mismatch cannot burn out a final,
I wonder why most commercial finals are protected from a high SWR
by foldback circuitry to avoid damage to the finals?


Indeed.
I always hear and read that a high SWR can destroy the PA final of TX,
cranck coils, etc.

Thierry

--
73, Cecil http://www.qsl.net/w5dxp



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Old January 19th 04, 11:45 PM
Thierry
 
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Default


"JGBOYLES" wrote in message
...
The SWR meter indicates the value from the meter to whatever is after it.
If you insert a device that has a high SWR (because it becomes defective

or
for some other reason ) between the transmitter and SWR meter ( say a

switch
or filter), the meter can show a low value if the antenna is ok, but the

transmitter will see a high SWR and may burn out the finals.

He did not say he inserted anything, just that moisture at the end of his
transmission line caused a fault. If this was the case, an SWR bridge at

the

The install was : RTX +50 cm RG58 + AMP + 1m Aircom (like RG213) + SWR +15m
Aircom + G5RV

the built-in SWR of my RTX TS-570D read SWR 1:1. If my external SWR
displayed an SWR 4 that means that it read the info AFTER it, what looks
in contradiction with what ARRL website states in his technical columns...
Who's right ?

Thierry
ON4SKY
http://www.astrosurf.com/lombry/




output of his Power Amps should have alerted him to a problem that would
potentially destroyed them.
If you want to use an SWR bridge as a protective device, or indicator,

it
must be placed in the proper position in the antenna system. Modern
transceivers have circuits that detect mismatches and reduce power to

minimize
failed output devices.


73 Gary N4AST



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Old January 20th 04, 12:05 AM
Cecil Moore
 
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Default

Reg Edwards wrote:

I wonder why most commercial finals are protected from a high SWR
by foldback circuitry to avoid damage to the finals?


They don't. There isn't an SWR meter on the station.


Reg, I'm talking about commercially manufactured ham radio
finals. Virtually every single one of them has protection
circuitry and virtually every one of them has a built-in
SWR metering system.
--
73, Cecil http://www.qsl.net/w5dxp



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-----== Over 100,000 Newsgroups - 19 Different Servers! =-----


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