On Thu, 18 Dec 2003 07:29:39 +0000, William F. Hagen wrote:

an antenna has an impedence at the frequency it is being used at, and an
impedence at its resonant frequency. If either of these impedences
happen to be 50 ohms and the coax being used is 50 ohms, and the
transciever is working at 50 ohms, then the swr is 1:1, and the swr is
on the transmission line, not on the antenna.

The antenna does not have to have an impedence of 50 ohms at either the
frequency being used at or at its resonant frequency, and these two
freqeuncies could be the same, and the transmission line does not have
to be at 50 ohms, and for that matter neither does the transciever. If
any one of these is mismatched, then the swr is not 1:1.
An impedence transformer at the antenna-transmission line junction will
transform a mismatch so there is no reflection on the transmission line,
amd if this impedence is the same as that of the transmitter, then the
swr is 1:1, if the impedence is not the same, then the swr is not 1:1
unless it is also transformed at the transmitter, and again the swr
would be 1:1 on the transmission line, which is where the swr is, it
is not on the antenna.


I was going to say something similar. a 1:1 SWR means that the Load
matches the Transmission line. It says nothing about the condition of the
antenna. Most antennas have a matching system or "Tunner" built into
them, like a Gama Match for example, to transform their Impedance back to
the standard 50ohm coaxial transmission line.

Ron

When the so-called SWR meter is located immediately at the output the
transmitter, or inside it, as it usually is -

THE METER TELLS YOU NOTHING ABOUT THE SWR ON THE TRANSMISSION LINE TO THE
ANTENNA.

It only indicates WHETHER or NOT the load on the transmitter has a
particular value, usually a nominal 50 ohms. And nothing more.

In all other respects, SWR meters and the corresponding reflected power
meters, are a hoax, a scam, a swindle. It's time they were, or at the very
least their name, consigned to oblivion.


Call out the riot police!
----
Reg, G4FGQ

"Reg Edwards" wrote in message
...

When the so-called SWR meter is located immediately at the output the
transmitter, or inside it, as it usually is -

THE METER TELLS YOU NOTHING ABOUT THE SWR ON THE TRANSMISSION LINE TO THE
ANTENNA.

It only indicates WHETHER or NOT the load on the transmitter has a
particular value, usually a nominal 50 ohms. And nothing more.

In all other respects, SWR meters and the corresponding reflected power
meters, are a hoax, a scam, a swindle. It's time they were, or at the
very
least their name, consigned to oblivion.


Call out the riot police!
----
Reg, G4FGQ


give up reg, while you may be convinced that swr meters don't measure swr...
the name is deeply embedded in the culture and won't go away. and while
maybe they aren't exactly measuring what you would like them to given the
name, they do provide useful indications... and that is the best thing about
them, they provide an indication that can be used for really using your
station. they may not be accurate, they may not be good for detailed system
engineering, they may not measure what you think they should, but they are
useful to the everyday ham... and after all that is what most of us reading
this group are.

Reg, G4FGQ wrote:
"Call out the riot police!"

And a riotous new year to you too!

An SWR meter is a built-in or external circuit which measures the
standing-wave-ratio at the transciever end of the feedline. Actually it
measures the degree of mismatch which causes SWR.

SWR = R/Zo or Zo/R, as required for a quotient1.

At the junction of a uniform feedline and a mismatched antenna load, the
reflection coefficient is a vector ratio between the electric fields
associated with the reflected and incident waves. The magnitude of Rho,
the reflection coefficient, is related to VSWR by:

Rho = VSWR - 1 / VSWR + 1

So, mismatch produces reflection, the magnitude of which depends on how
great the mismatch. The reflected wave traveling back toward the source
of the incident wave makes a standing-wave-pattern. SWR is the ratio of
the maximum of voltage (or current) along the line if the line is long
enough. If not, an SWR may be inferred from the forward and reflected
powers for practical purposes.

The built-in or external meter that indicates severity of mismatch, is
usually labeled "SWR" because the determination was traditionally made
by sensing the sum of forward and reverse fields, either electric or
magnetic, along the transmission line. The ratio of maximum to minimum
is SWR.

The meter could have been called a "mismatch meter", but that won`t
happen. "SWR meter" is ingrained.

"Water Tank" was a code name given the military vehicle by its British
developers during WW-1 to maintain secrecy. After they were fielded,
attempts were made to no avail to call the vehicles "assault carriages"
or "combat cars". "Tank" is the name now and forever. Same with "SWR
meter".

Best regards, Richard Harrison, KB5WZI

Reg Edwards wrote:
THE METER TELLS YOU NOTHING ABOUT THE SWR ON THE TRANSMISSION LINE TO THE
ANTENNA.

Minimum 50 ohm SWR occurs at a current maximum point on my ladder-line.
Given that the current maximum point is purely resistive and knowing
the 50 ohm SWR allows me to calculate the only two possibilities for
the SWR on the ladder-line. Knowing the approximate feedpoint impedance
for my antenna and the exact length of the ladder-line allows me to
select which of the two SWRs is the valid one. In other words, knowing
the 50 ohm SWR allows me to estimate, with good accuracy, the SWR on
the ladder-line. These estimates agree closely with actual ladder-line
SWR measurements. Moral: In a previously characterized antenna system,
the SWR meter tells you virtually everything about the SWR on the
transmission line.
--
73, Cecil http://www.qsl.net/w5dxp



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