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K3 SWR Accuracy - reprise

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Steve Ellington

Re: K3 SWR Accuracy - reprise

Nope:
The 1/4 wave line transforms the high impedance to a low one and the SWR
meter reads low. It's called a transmission line transformer and is very
common. It's the reason everyone is having trouble understanding why SWR
meters read differently. The ONLY way to compare them is to swap them with
each other. Putting them in series fouls up the readings for both meters.
Steve
N4LQ
[hidden email]
----- Original Message -----
From: "Kok Chen" <[hidden email]>
To: "Elecraft Reflector" <[hidden email]>
Cc: "Steve Ellington" <[hidden email]>
Sent: Wednesday, November 04, 2009 7:55 PM
Subject: Re: [Elecraft] K3 SWR Accuracy - reprise

>
> On Nov 4, 2009, at 4:41 PM, Steve Ellington wrote:
>
>> 4. Example: A full wave dipole center fed with 50 ohm coax. SWR reads
>> infinite at the antenna but with 1/4 wavelenth of coax, SWR reads low!
>
> Nope -- the *impedance* at the end of a 1/4 wave transmission line
> when it is looking at a very large impedance, is close to zero,
> therefore the SWR remains close to infinite. The SWR definitely won't
> read low unless there is something wrong with the instrument.
>
> 73
> Chen, W7AY
>

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Kok Chen

Re: K3 SWR Accuracy - reprise

On Nov 4, 2009, at 5:05 PM, Steve Ellington wrote:

> It's called a transmission line transformer and is very common.

Yes, we all know about them. Just walk 180 degrees on a constant SWR
circle on the Smith Chart, with the transmission line impedance at the
center of the Smith Chart (or use 1/4 wavelength in the Telegrapher's
Equation).

But this is what you'd stated (I am not changing a single word):

> 4. Example: A full wave dipole center fed with 50 ohm coax.

You can use a 600 ohm transmission line to transform a high impedance
to get a reasonably close match to 50 ohms because the impedance at
the center of that dipole is *not* infinite but some large number
(W8JI has good estimates in the Zepp article on his web site).

But you cannot transform anything other than a 50 ohm feed point into
a 50 ohm termination by using a 50 ohm transmission line. (Unless the
line is infinitely lossy.)

It should be obvious from the Smith Chart -- constant SWR circles
won't hit 50+j0 unless the SWR circle itself has 0 radius (i.e., SWR =
1.0:1)

73
Chen, W7AY

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Steve Ellington

Re: K3 SWR Accuracy - reprise

"But you cannot transform anything other than a 50 ohm feed point into
a 50 ohm termination by using a 50 ohm transmission line. (Unless the
line is infinitely lossy.)"
Don't worry....It will be!

Steve
N4LQ
[hidden email]
----- Original Message -----
From: "Kok Chen" <[hidden email]>
To: "Steve Ellington" <[hidden email]>
Cc: "Elecraft Reflector" <[hidden email]>
Sent: Wednesday, November 04, 2009 8:51 PM
Subject: Re: [Elecraft] K3 SWR Accuracy - reprise

>
> On Nov 4, 2009, at 5:05 PM, Steve Ellington wrote:
>
>> It's called a transmission line transformer and is very common.
>
> Yes, we all know about them. Just walk 180 degrees on a constant SWR
> circle on the Smith Chart, with the transmission line impedance at the
> center of the Smith Chart (or use 1/4 wavelength in the Telegrapher's
> Equation).
>
> But this is what you'd stated (I am not changing a single word):
>
>> 4. Example: A full wave dipole center fed with 50 ohm coax.
>
> You can use a 600 ohm transmission line to transform a high impedance
> to get a reasonably close match to 50 ohms because the impedance at
> the center of that dipole is *not* infinite but some large number
> (W8JI has good estimates in the Zepp article on his web site).
>
> But you cannot transform anything other than a 50 ohm feed point into
> a 50 ohm termination by using a 50 ohm transmission line. (Unless the
> line is infinitely lossy.)
>
> It should be obvious from the Smith Chart -- constant SWR circles
> won't hit 50+j0 unless the SWR circle itself has 0 radius (i.e., SWR =
> 1.0:1)
>
> 73
> Chen, W7AY
>
>

Don Wilhelm-4

Re: K3 SWR Accuracy - reprise

In reply to this post by Steve Ellington

I believe Steve is onto something critical to this discussion - here is
my 'take' on the differences -- Remember that we *are* talking about
the resultant SWR indications on a mis-matched line.

The way most "wattmeters" indicate SWR is to detect the forward power
and the reflected power - then an SWR is computed from those values of
associated detector output voltages. The result is an SWR indication
based on the absolute values of those magnitudes. No consideration is
provided for the phase angles (the actual forward and reflected values
are complex numbers). The forward power is indicated accurately (it is
proportional to the square of the forward voltage minus the square of
the reflected voltage), but the computation of actual SWR is more involved.

When the impedances are close to the design point (balance point) of the
meter, the error is small, but the error grows as the actual impedance
departs from that design impedance.

Those meters that properly detect the phase as well as the magnitude of
the forward and reflected powers can indicate that the SWR does not
change as the meter position is moved along the line - but most do not
have phase detection capability nor complex number computation
capability, so for those meters, the SWR indicated will change with the
meter position along the feedline.

In other words, use a good VNA and you should see a constant SWR along
the line, but common wattmeters are not VNAs, so some error in SWR
indication is to be expected when the impedance is removed from the
design point.

Even the well-respected Tandem Match computes the SWR as Vf+Vr/Vf-Vr,
which is the correct formula, but the detector reports only the
magnitudes of Vf and Vr and does not consider the phase angle, so it is
not entirely correct either - it will be entirely correct when the SWR =
1.0.

73,
Don W3FPR

Steve Ellington wrote:

> Nope:
> The 1/4 wave line transforms the high impedance to a low one and the SWR
> meter reads low. It's called a transmission line transformer and is very
> common. It's the reason everyone is having trouble understanding why SWR
> meters read differently. The ONLY way to compare them is to swap them with
> each other. Putting them in series fouls up the readings for both meters.
> Steve
> N4LQ
> [hidden email]
> ----- Original Message -----
> From: "Kok Chen" <[hidden email]>
> To: "Elecraft Reflector" <[hidden email]>
> Cc: "Steve Ellington" <[hidden email]>
> Sent: Wednesday, November 04, 2009 7:55 PM
> Subject: Re: [Elecraft] K3 SWR Accuracy - reprise
>
>
>
>> On Nov 4, 2009, at 4:41 PM, Steve Ellington wrote:
>>
>>
>>> 4. Example: A full wave dipole center fed with 50 ohm coax. SWR reads
>>> infinite at the antenna but with 1/4 wavelenth of coax, SWR reads low!
>>>
>> Nope -- the *impedance* at the end of a 1/4 wave transmission line
>> when it is looking at a very large impedance, is close to zero,
>> therefore the SWR remains close to infinite. The SWR definitely won't
>> read low unless there is something wrong with the instrument.
>>
>> 73
>> Chen, W7AY
>>
>>
>

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AC7AC

Re: K3 SWR Accuracy - reprise

In reply to this post by ab2tc

CONTENTS DELETED

The author has deleted this message.

Stewart Baker

Re: K3 SWR Accuracy - reprise

In reply to this post by n7ws

Wes,
I was being a bit tongue in cheek using the word immaterial.

Of course the input SWR has to be somewhere in the ball park for any
ATU to tune.

Without provoking any further discussion on the subject (which has already
gone on for longer than I expected), all I wanted when starting out was some
correlation (within say 20%) of SWR reading between my LP-100 and K3.

A number of reasons that have been put forward for the differences that I have seen.
I will (when I have time) repeat the measurements to try and identify where the
discrepancies are occurring.

I thank everyone who has contributed to this thread for their comments.

73
Stewart G3RXQ
On Wed, 4 Nov 2009 11:21:41 -0800 (PST), Wes Stewart wrote:
> --- On Wed, 11/4/09, Stewart <[hidden email]> wrote:
>>
>> Of course, as others have pointed out the value of SWR measured is
>> immaterial, as the KAT3 will try and adjust to 1:1 when it is selected.
>>
>>
> I don't think I would go so far as to say SWR is immaterial.

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David Woolley (E.L)

Re: K3 SWR Accuracy - reprise

In reply to this post by Phil Salas

Phil & Debbie Salas wrote:

> The impedance changes, but not the SWR. Looking at this on a Smith
> Chart, you can see that you just rotate around a constant SWR circle.

You don't even have to introduce Smith Charts. The basic definition of
SWR involves distributed measurements along the feeder, That means that
a different SWR cannot exist at different points on the cable, because
it cannot be measured, by its proper definition, at a single point.
Unfortunately a lot of mystique has arisen over "SWR"s as a figure of
merit, when it was originally just a pragmatic measurement. Modern SWR
meters actually have to do excess calculation in order to get the "SWR"
from equally good measurements (i.e. |reflected voltage / forward
voltage|) of the goodness of match to the nominal impedance.

The original definition of SWR is (RMS) voltage at a voltage maximum on
the cable over (RMS) voltage at a voltage minimum (and can only be
directly measured for a cable that is at least a quarter wave long).

SWR meters don't actually measure the SWR on the line, but rather the
SWR that would appear on an ideal 50 ohm line on the input side of the
meter. A PA will be killed by excess voltage or current, not by "SWR",
so the same "SWR" may or may not be dangerous, depending on the phase.

[Cutoff point for thread kills was Digest 67:10. At that time the
fundamental definition of SWR hadn't be raised.)
--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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