Reg Edwards wrote:
Jim,

Perhaps there's some misunderstanding about location of the meter and
what it is measuring. Let's try to clear it up.

Would you please do us both a favour by answering the following simple
question?

There is a 50 ohm line feeding a 100 ohm antenna.

There is an SWR meter located at the line-antenna junction.

What does this mean?

The meter has a reading. Does the reading apply to SWR of the
antenna, or does it apply to the SWR along the feedline?

The reading is the SWR at that point.

Antenna or Feedline?
----
Reg.

An SWR meter reads the SWR of the thing connected to its output port
with respect to the reference impedance the meter was designed for.

The SWR meter reads the SWR *AT THE POINT OF CONNECTION* of the
connected system.

Not the middle of the system, not the other end (if it has one)
of the system, but the input point.

If you measure a SWR (50 Ohms reference) of 2:1 for a black box,
what is in the box?

A. A 25 Ohm resistor.
B. A 100 Ohm resistor.
C. A cable spool of coax with some impedance at the end of it.
D. Could be any of the above.

In general there is no guarantee that the SWR at any point of a
transmission line will be equal to the SWR at any other point on a
transmission line other than for special cases.

What seems to have you terribly confused is that all the transmission
lines, tuners, antennas, connectors and whatevers become a *SYSTEM*
and the SWR at the input connector to the *SYSTEM* is not guaranteed
to be the same as the SWR at some arbitrary point inside the system.

When you measure the SWR of a line with a load on the end, you are
measuring the SWR of the entire system relative to your reference,
not the load.

--
Jim Pennino

Remove .spam.sux to reply.

Cec, you have YOUR explanation and I have MY explanation.

Which is the most simple?

There is a bridge.

When the variable arm, the load, is 50 ohms the bridge is balanced and
the meter indicates SWR = 1:1

When the variable arm is either 0 ohms or infinite ohms, the meter
indicates SWR = infinity :1

What can be more simple than that? How it works can be visualised.

But the meter is ambiguous. It cannot distinguish between loads of 0
ohms and infinite ohms. Additional information is required.

This serious ambiguity also applies to your weird contraption. ;o)
----
Regards, Reg.

Cec, I notice that you and others have begun to use my description of
"indicate" rather than "measure".
----
Reg.

Jim, I'm sorry you are unable to answer the simple question "Feedline
or Antenna?".
----
Reg.

Richard,

If's and But's are not required.

The antenna is just an arbitrary load.

Does the meter reading indicate SWR on the feedline (which is what is
usually required), or does it not?

This is not a "catch question". It is not a troll.

"Antenna or Feedline?" please.

KISS
----
Reg.

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

"Richard Fry" wrote in message
...
"Reg Edwards" wrote:
There is a 50 ohm line feeding a 100 ohm antenna.
There is an SWR meter located at the line-antenna junction.
The meter has a reading. Does the reading apply to SWR of the
antenna, or does it apply to the SWR along the feedline?
______________

It applies to the match of the RF network that follows the SWR meter
to the
impedance for which the SWR meter was calibrated.

And if in your example the SWR meter has been calibrated for 50
ohms, and is
moved to the input end of that line+antenna RF network, it will also
have a
reading -- which will be the same as when it was inserted at the
antenna-line junction, less the round-trip RF attenuation of the
transmission line (assuming that the transmission line is 50 +/- j0
ohms
throughout its length).

In fact it is a common practice to optimise the transmission
line/antenna
match of commercial FM and TV broadcast antenna systems by use of a
variable
transformer inserted at the antenna input, whose adjustment is made
by
reference to the far-end reflection seen at the sending end of the
transmission line, using a high-directivity reflectometer, or SWR
meter.

The same physics applies to ham antenna systems and methods/means of
measurement.

RF

Visit http://rfry.org for FM transmission system papers.

"Reg Edwards"

"Antenna or Feedline?" please.
KISS
___________

Antenna.

And a big smooch to you, too.

RF

Reg Edwards wrote:
Cec, you have YOUR explanation and I have MY explanation.

Mine is a lot simpler. The Heath HM-15 has two pickup elements.
If you install a Z0 resistor load at one end it "picks up" the forward
wave. If you install a Z0 resistor load at the other end it "picks up"
the reflected wave. The two pickup voltages are rectified and compared
through a calibration procedure.

The parts that came with the HM-15 kit in the 50s-60s included two
72 ohm resistors. RG-ll was very popular at the time. If one wanted
a 72 ohm SWR meter, one installed the 72 ohm resistors. If one wanted
a 50 ohm SWR meter, one installed the 50 ohm resistors. A switch could
be installed that switched between 50 ohms and 72 ohms calibration.

This serious ambiguity also applies to your weird contraption. ;o)

Actually, the Heathkit design concept is easier to understand than
is the bridge explanation or the toroid-pickup/phasor-addition
explanation. The first SWR meter I built in the 50s, used two
lengths of insulated wire shoved under the braid of the coax.
It worked but, at the time, I had no idea why it worked. Heath's
little slotted line pickup device was pretty slick. I sometimes
see them for sale at hamfests.
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:

Cec, I notice that you and others have begun to use my description of
"indicate" rather than "measure".

What a meter movement "measures" is current. What a meter "indicates"
can be anything in the world depending upon the calibration scale.
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
"Antenna or Feedline?" please.

If the meter is calibrated for the feedline Z0, it will read
the SWR on the feedline.

If the meter is calibrated for the antenna Z0, it will read
the SWR on the antenna.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 27 Sep 2005 15:48:35 +0000 (UTC), "Reg Edwards"
wrote:
But the meter is ambiguous. It cannot distinguish between loads of 0
ohms and infinite ohms. Additional information is required.

Hi Reggie,

Without recourse to that "additional information," explain how you
achieve the unambiguous by your method of probing lines (be they
parallel, coaxial, or waveguide).

In other words, your objection is a non sequitur, it is meaningless
because you need the same additional information and you cannot
demonstrate any measurable difference between the manifold methods of
coming to the same determination.

Of course, if you throw a spanner in the other guy's gear-box, you
might win the race.

73's
Richard Clark, KB7QHC