TxMon and K3 (tuner bypassed) SWR missmatch

[ve3dvy]

 I recently installed the TX monitor in my P3 and have a discrepancy
with the SWR reading between it and the K3

with a dummy load connected the P3 and K3 read 1.12 and 1.0  
respectively   a level of error that is to me quite acceptable.

however  when connected to an antenna  I will see the SWR on the K3
often close to 1:1   when the P3 will show up to a 1:2.   as the SWR
climbs above 1:2 on the P3  the K3 reading will track it fairly
closely but about a level of 1 below.  example
K3 1:1  P3 1:2,
K3 1:1.5  P3 1:2.5
K3 1:2   P3 1:3   and so on

Note that this is done with the KAT3  bypassed of course.

it made me wonder which is correct.   so I put in a meter.   old but
faithfull KW electronics meter seems to track much closer to the P3's
reading  which makes sense since the chances that my dipole would have
a wide are at a solid 1:1 match before climbing as I move away from
resonance  makes me wonder if the K3's SWR readings are out of
calibration and if this can be adjusted.     my concern is that I may
be excepting the SWR from the KAT3  when it may be in fact a little
too high for the amps safety.

The power readings from both seems to be quite close.

Any thoughts?





David Moes
VE3SD


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[Don Wilhelm]

David,

You are trying to compare apples and oranges and expecting them to be
the same.

The K3 SWR indication is at the *input* to the KAT3, and the TX Monitor
is indicating the SWR at the *output* of the KAT3.
That is no reason that they should match.

The TX Monitor will always give you the unmatched SWR of your antenna,
while the K3 will always indicate the match obtained after the antenna
is tuned.

If you could place the TX Monitor sensor between the K3 PA stage and the
KAT3, you could expect them to agree, but that connection location is
impossible without seriously modifying your K3.

Even with the KAT3 in bypass, there is still some residual reactance
which will make the readings different.
With the K3S, and its KAT3A, there is a relay that will truly bypass the
L/C portion of the KAT3A and the readings should be closer, but still
not exact.

With wattmeters as they are, expect a 10% difference in readings without
alarm - wattmeters are not usually precision instruments unless you get
into lab instrumentation which costs many, many dollars.

73,
Don W3FPR

On 11/4/2016 6:59 PM, [hidden email] wrote:

>
> I recently installed the TX monitor in my P3 and have a discrepancy with
> the SWR reading between it and the K3
>
> with a dummy load connected the P3 and K3 read 1.12 and 1.0
> respectively   a level of error that is to me quite acceptable.
>
> however  when connected to an antenna  I will see the SWR on the K3
> often close to 1:1   when the P3 will show up to a 1:2.   as the SWR
> climbs above 1:2 on the P3  the K3 reading will track it fairly closely
> but about a level of 1 below.  example
> K3 1:1  P3 1:2,
> K3 1:1.5  P3 1:2.5
> K3 1:2   P3 1:3   and so on
>

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[k6dgw]

On 11/4/2016 3:59 PM, [hidden email] wrote:

>
> I recently installed the TX monitor in my P3 and have a discrepancy with
> the SWR reading between it and the K3
>
> with a dummy load connected the P3 and K3 read 1.12 and 1.0
> respectively   a level of error that is to me quite acceptable.
>
> however  when connected to an antenna  I will see the SWR on the K3
> often close to 1:1   when the P3 will show up to a 1:2.   as the SWR
> climbs above 1:2 on the P3  the K3 reading will track it fairly closely
> but about a level of 1 below.  example
> K3 1:1  P3 1:2,
> K3 1:1.5  P3 1:2.5
> K3 1:2   P3 1:3   and so on
>
> Note that this is done with the KAT3  bypassed of course.
>
> it made me wonder which is correct.   so I put in a meter.   old but
> faithfull KW electronics meter seems to track much closer to the P3's
> reading  which makes sense since the chances that my dipole would have a
> wide are at a solid 1:1 match before climbing as I move away from
> resonance  makes me wonder if the K3's SWR readings are out of
> calibration and if this can be adjusted.     my concern is that I may be
> excepting the SWR from the KAT3  when it may be in fact a little too
> high for the amps safety.
>
> The power readings from both seems to be quite close.
>
> Any thoughts?

Well, as an over-thinker who regularly got advice from his Troopers to
"Stop picking fly poop** out of the pepper," I don't think you have a
problem.  I think I understand there's no "antenna tuner" in your
circuit, a little confusing toward the end.

First off, you're not likely to hurt an Elecraft radio with high SWR,
and none of what you mentioned is actually anywhere near "high."
Secondly, none of the "SWR measuring instruments" you or I have are
overly accurate.  I've got 5, of various provenance -- into a Bird
Termaline, they all differ by as much as 0.5 to 0.9 ... and they vary by
frequency.  Sounds to me like you have a well matched antenna.

73,

Fred K6DGW
- Sparks NV DM09dn

- Northern California Contest Club
- CU in the Cal QSO Party 7-8 Oct 2017
- www.cqp.org

**they used a synonym for "fly poop."
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[Vic Rosenthal]

I have always understood that the SWR on a line will be almost the same
wherever it is measured (it will increase slightly as you approach the
load due to losses). It is the impedance of the load as transformed by
the line that changes.

One thing that I've noticed is that SWR meters often differ when the SWR
is low, because the amount of reflected power is very small, and diodes
are often nonlinear dealing with small signals. The difference between
an SWR of 1.1:1 and 1.2:1 is a very tiny amount of reflacted power.

73,
Vic, 4X6GP
Rehovot, Israel
Formerly K2VCO
http://www.qsl.net/k2vco/

On 5 Nov 2016 04:43, Ron D'Eau Claire wrote:

> How much coax is between the K3 and P3 monitor? Only a few feet will create
> a growing error as the SWR increases, since the standing waves on the
> coaxial line produce different readings at different points along the line.
>
> If you want to compare the SWR meters in the K3 and P3, be sure the P3
> sensor is mounted directly on the K3 without a length of coax in between.
> Even so, the K3 SWR sensor will be "looking" at the P3 sensor through the
> KAT3. Although it is bypassed, the older KAT3 ATUs still routed RF through
> them on its way to the K3 rear-panel ANT connector.
>
> Even so, as Fred pointed out, there will be differences since SWR meters are
> no "precision" devices because they don't have to be. The K3 is quite happy
> at load SWRs up to 2 or 2.5:1 and even above that the amplifier will protect
> itself.
>
> 73, Ron AC7AC
>
>
> On 11/4/2016 6:59 PM, [hidden email] wrote:
>>
>> I recently installed the TX monitor in my P3 and have a discrepancy
>> with the SWR reading between it and the K3
>>
>> with a dummy load connected the P3 and K3 read 1.12 and 1.0
>> respectively   a level of error that is to me quite acceptable.
>>
>> however  when connected to an antenna  I will see the SWR on the K3
>> often close to 1:1   when the P3 will show up to a 1:2.   as the SWR
>> climbs above 1:2 on the P3  the K3 reading will track it fairly
>> closely but about a level of 1 below.  example
>> K3 1:1  P3 1:2,
>> K3 1:1.5  P3 1:2.5
>> K3 1:2   P3 1:3   and so on

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[Matt Zilmer-3]

It also depends on the line loss per unit length.  A high-loss feedline
will be more forgiving as measured, but less energy is transferred to
the load.  If a line is very low-loss per length, you can tune out the
reactance or mismatch (by transformation or direct compensation) without
losing much energy.  High loss feedline just burns up the energy in the
feedline and the load can't radiate very much.  In this case, a good SWR
"match" won't help get a lot of signal on the air.  This is one reason
that source-end SWR isn't a good measure of radiation efficiency (not
considering the actual antenna).  I can get 1:1 on a good 50 ohm load,
just like anyone else, and no one can hear my 1kW transmitter.

The SWR varies along the length of all feedlines, but at low SWRs it
doesn't make much difference on energy transferred from source to the
load.  In most cases 2:1 is perfectly acceptable to the transmitter's
protection circuitry.

Vic is also correct about diodes; their I-V curve is nonlinear. Ar low
power levels, this can matter a lot on precision of the SWR measurement.

73,

matt W6NIA


On 11/4/2016 9:10 PM, Vic Rosenthal 4X6GP wrote:

> I have always understood that the SWR on a line will be almost the
> same wherever it is measured (it will increase slightly as you
> approach the load due to losses). It is the impedance of the load as
> transformed by the line that changes.
>
> One thing that I've noticed is that SWR meters often differ when the
> SWR is low, because the amount of reflected power is very small, and
> diodes are often nonlinear dealing with small signals. The difference
> between an SWR of 1.1:1 and 1.2:1 is a very tiny amount of reflacted
> power.
>
> 73,
> Vic, 4X6GP
> Rehovot, Israel
> Formerly K2VCO
> http://www.qsl.net/k2vco/
>
> On 5 Nov 2016 04:43, Ron D'Eau Claire wrote:
>> How much coax is between the K3 and P3 monitor? Only a few feet will
>> create
>> a growing error as the SWR increases, since the standing waves on the
>> coaxial line produce different readings at different points along the
>> line.
>>
>> If you want to compare the SWR meters in the K3 and P3, be sure the P3
>> sensor is mounted directly on the K3 without a length of coax in
>> between.
>> Even so, the K3 SWR sensor will be "looking" at the P3 sensor through
>> the
>> KAT3. Although it is bypassed, the older KAT3 ATUs still routed RF
>> through
>> them on its way to the K3 rear-panel ANT connector.
>>
>> Even so, as Fred pointed out, there will be differences since SWR
>> meters are
>> no "precision" devices because they don't have to be. The K3 is quite
>> happy
>> at load SWRs up to 2 or 2.5:1 and even above that the amplifier will
>> protect
>> itself.
>>
>> 73, Ron AC7AC
>>
>>
>> On 11/4/2016 6:59 PM, [hidden email] wrote:
>>>
>>> I recently installed the TX monitor in my P3 and have a discrepancy
>>> with the SWR reading between it and the K3
>>>
>>> with a dummy load connected the P3 and K3 read 1.12 and 1.0
>>> respectively   a level of error that is to me quite acceptable.
>>>
>>> however  when connected to an antenna  I will see the SWR on the K3
>>> often close to 1:1   when the P3 will show up to a 1:2.   as the SWR
>>> climbs above 1:2 on the P3  the K3 reading will track it fairly
>>> closely but about a level of 1 below.  example
>>> K3 1:1  P3 1:2,
>>> K3 1:1.5  P3 1:2.5
>>> K3 1:2   P3 1:3   and so on
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

--
Always store beer in a dark place.  - R. Heinlein

Matt Zilmer, W6NIA
[Shiraz]

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[Alan Bloom]

The SWR is the same anywhere on the feedline, assuming the loss is low.
If changing the feedline length a few feet causes a significant change
in SWR reading, then either the directional coupler in the SWR meter
isn't doing a good job, or (more likely) there is feedline radiation.
In the latter case, the feedline is effectively part of the antenna;
that's why changing the length changes the SWR.

Feedline radiation is more common than most people think.  See the
article in November QST.

Alan N1AL


On 11/04/2016 10:52 PM, Matt Zilmer wrote:

> It also depends on the line loss per unit length.  A high-loss feedline
> will be more forgiving as measured, but less energy is transferred to
> the load.  If a line is very low-loss per length, you can tune out the
> reactance or mismatch (by transformation or direct compensation) without
> losing much energy.  High loss feedline just burns up the energy in the
> feedline and the load can't radiate very much.  In this case, a good SWR
> "match" won't help get a lot of signal on the air.  This is one reason
> that source-end SWR isn't a good measure of radiation efficiency (not
> considering the actual antenna).  I can get 1:1 on a good 50 ohm load,
> just like anyone else, and no one can hear my 1kW transmitter.
>
> The SWR varies along the length of all feedlines, but at low SWRs it
> doesn't make much difference on energy transferred from source to the
> load.  In most cases 2:1 is perfectly acceptable to the transmitter's
> protection circuitry.
>
> Vic is also correct about diodes; their I-V curve is nonlinear. Ar low
> power levels, this can matter a lot on precision of the SWR measurement.
>
> 73,
>
> matt W6NIA
>
>
> On 11/4/2016 9:10 PM, Vic Rosenthal 4X6GP wrote:
>> I have always understood that the SWR on a line will be almost the
>> same wherever it is measured (it will increase slightly as you
>> approach the load due to losses). It is the impedance of the load as
>> transformed by the line that changes.
>>
>> One thing that I've noticed is that SWR meters often differ when the
>> SWR is low, because the amount of reflected power is very small, and
>> diodes are often nonlinear dealing with small signals. The difference
>> between an SWR of 1.1:1 and 1.2:1 is a very tiny amount of reflacted
>> power.
>>
>> 73,
>> Vic, 4X6GP
>> Rehovot, Israel
>> Formerly K2VCO
>> http://www.qsl.net/k2vco/
>>
>> On 5 Nov 2016 04:43, Ron D'Eau Claire wrote:
>>> How much coax is between the K3 and P3 monitor? Only a few feet will
>>> create
>>> a growing error as the SWR increases, since the standing waves on the
>>> coaxial line produce different readings at different points along the
>>> line.
>>>
>>> If you want to compare the SWR meters in the K3 and P3, be sure the P3
>>> sensor is mounted directly on the K3 without a length of coax in
>>> between.
>>> Even so, the K3 SWR sensor will be "looking" at the P3 sensor through
>>> the
>>> KAT3. Although it is bypassed, the older KAT3 ATUs still routed RF
>>> through
>>> them on its way to the K3 rear-panel ANT connector.
>>>
>>> Even so, as Fred pointed out, there will be differences since SWR
>>> meters are
>>> no "precision" devices because they don't have to be. The K3 is quite
>>> happy
>>> at load SWRs up to 2 or 2.5:1 and even above that the amplifier will
>>> protect
>>> itself.
>>>
>>> 73, Ron AC7AC
>>>
>>>
>>> On 11/4/2016 6:59 PM, [hidden email] wrote:
>>>>
>>>> I recently installed the TX monitor in my P3 and have a discrepancy
>>>> with the SWR reading between it and the K3
>>>>
>>>> with a dummy load connected the P3 and K3 read 1.12 and 1.0
>>>> respectively   a level of error that is to me quite acceptable.
>>>>
>>>> however  when connected to an antenna  I will see the SWR on the K3
>>>> often close to 1:1   when the P3 will show up to a 1:2.   as the SWR
>>>> climbs above 1:2 on the P3  the K3 reading will track it fairly
>>>> closely but about a level of 1 below.  example
>>>> K3 1:1  P3 1:2,
>>>> K3 1:1.5  P3 1:2.5
>>>> K3 1:2   P3 1:3   and so on
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
>

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[ve3dvy]

 Thanks  for replies


I will borrow a Bird meter to see what it say  but I think in this
case the error is in the K3 and not the P3.      as one mentioned a
reading of 1:1,1 vers 1:1,2 is neglegable  but this is a error by a
whole number   1:1 vs 1:2

My concern is that if its really a 1:3.5  and it thinks its a 1:2.5  
is the K3 still doing its job for protection  or is this protection
sensed elswhere.

The coax between the K3 and the sensor is about a foot  so very close.
 but as some said it should not be a big difference.
and I understand that the K3 is probably one of the most tolerant rigs
out there for SWR.    My concern is when I am using an antenna say a
doublet  where the tuner is not able to get it to a perfect match.  
say only to a 2:1 where if the K3s SWR internally is misreading it and
its really a 3:1  that is getting  a little out of its safe zone and
it should start to roll the power back   or will it?  if its reading
lower than what it really is it may not roll the power back as early
as it should.



David Moes
VE3SD



On Friday 04/11/2016 at 6:59 pm, [hidden email] wrote:

>
> I recently installed the TX monitor in my P3 and have a discrepancy
> with the SWR reading between it and the K3
>
> with a dummy load connected the P3 and K3 read 1.12 and 1.0
> respectively   a level of error that is to me quite acceptable.
>
> however  when connected to an antenna  I will see the SWR on the K3
> often close to 1:1   when the P3 will show up to a 1:2.   as the SWR
> climbs above 1:2 on the P3  the K3 reading will track it fairly
> closely but about a level of 1 below.  example
> K3 1:1  P3 1:2,
> K3 1:1.5  P3 1:2.5
> K3 1:2   P3 1:3   and so on
>
> Note that this is done with the KAT3  bypassed of course.
>
> it made me wonder which is correct.   so I put in a meter.   old but
> faithfull KW electronics meter seems to track much closer to the P3's
> reading  which makes sense since the chances that my dipole would have
> a wide are at a solid 1:1 match before climbing as I move away from
> resonance  makes me wonder if the K3's SWR readings are out of
> calibration and if this can be adjusted.     my concern is that I may
> be excepting the SWR from the KAT3  when it may be in fact a little
> too high for the amps safety.
>
> The power readings from both seems to be quite close.
>
> Any thoughts?
>
>
>
>
>
> David Moes
> VE3SD
>
>
> ______________________________________________________________
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[Richard Ferch-2]

You are talking about SWR as if it were something real and physical that
could damage the transmitter's finals.  It's not - it's just a mathematical
construct that makes it easier to calculate various other quantities (such
as loss in the transmission line). The real physical quantities that can
damage the finals are currents and voltages.

The SWR reading on an SWR meter is back-calculated from currents and
voltages measured, for example, in an SWR bridge circuit. An analog SWR
meter is just a milliammeter with an unusual scale - the scale does the
calculation for you (like a nomograph). A digital meter does the
calculation using a formula.

The protective circuitry does not measure SWR directly - it measures
currents and voltages, and it can measure them close to the finals, not at
the end of some indeterminate length of transmission line. There is no need
for the protective circuitry to calculate an SWR from those measurements -
all it needs to know is the measured voltage at which it needs to act to
limit the voltages and currents inside the finals to safe levels.The
calculated SWR that is displayed on the front panel is there to keep the
operator happy, not because the circuitry itself needs to know it.

The SWR at which the foldback occurs in the finals actually varies with
output power. At very low output power, the SWR can be very high without
causing damage, because the currents and voltages are still low, so the
foldback circuit does not need to operate just because the SWR is high. It
only needs to operate when the voltages or currents start to threaten the
finals.

This being the case, any errors in the SWR calculation based on the
measured voltages and currents at various locations are basically
irrelevant to the issue of protection of the finals, regardless of their
origin.

73,
Rich VE3KI
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[Jim Brown-10]

Yes, and that is exactly the right way to understand it -- when an
antenna is unbalanced, regardless of the reason for the imbalance, the
feedline becomes part of the antenna unless it is choked. Indeed, that's
the major reason to use a good choke -- to prevent noise picked up on
the outside of the coax from coupling to the antenna and from there to
the radio (by coming down the inside of the coax). The choke also
prevents RF current from our transmitter from flowing down the outside
of the coax to radiate into home entertainment equipment, or even onto
the power wiring.

Many hams think that using 2-wire line eliminates the possibility of
imbalance, and even call it "balanced line" and feed it with a
"balanced" tuner. Nothing could be further from the truth -- many
(most?) antennas, especially wire antennas, are unbalanced by their
surroundings, and that imbalance causes current on the two wires to be
unequal. The difference between the two currents is a "common mode"
current, it radiates just like the current on the outside of coax, and
by reciprocity, receives noise and couples it to the radio as a
differential signal, just like with coax.

A major shortcoming of antennas fed with open wire line is that it is
not practical to choke them to kill that common mode current.

Another important point. The SWR on a line is determined entirely by the
match between the line and the load -- when we're transmitting, that's
the antenna, and when we're receiving, it's the receiver. And yes, it's
same everywhere along the line as long as the line is uniform -- that
is, the same impedance, and with no stubs or matching networks.

73, Jim K9YC

On Fri,11/4/2016 11:03 PM, Alan Bloom wrote:
> The SWR is the same anywhere on the feedline, assuming the loss is
> low. If changing the feedline length a few feet causes a significant
> change in SWR reading, then either the directional coupler in the SWR
> meter isn't doing a good job, or (more likely) there is feedline
> radiation. In the latter case, the feedline is effectively part of the
> antenna; that's why changing the length changes the SWR.


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[Jim Brown-10]

On Sat,11/5/2016 12:57 PM, Ron D'Eau Claire wrote:

> ...The SWR on a line is determined entirely by the match between the line
> and the load -- when we're transmitting, that's the antenna, and when we're
> receiving, it's the receiver. And yes, it's same everywhere along the line
> as long as the line is uniform -- that is, the same impedance, and with no
> stubs or matching networks.
>
> 73, Jim K9YC
>
> And the losses are low. Losses go up with increased SWR on a given
> transmission line and losses reduce the SWR measured at the load.

Not correct. And how many of us measure SWR at the load?  It's up in the
air!

73, Jim

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[Wes Stewart-2]

Line loss decreases the SWR measured at the source, not the load.

In this case, thinking in terms of return loss makes this thought experiment
easier.  The line loss adds to the return loss of the load, increasing the input
return loss by a factor of 2 X line loss (dB).  Higher return loss = better match.

While on this subject, let me introduce you to Dan, AC6LA's fabulous software
and tutorials.  Before the death of my wife changed some priorities, I did a lot
of beta testing for Dan after he was gentleman enough to ask permission to
include ladderline data that I developed in his software.

For a discussion of increased loss due to SWR this is a must read:
http://ac6la.com/swrloss.html

If you want to use a wonderful tool to help understand some of this, this
(free!) one can't be beat: http://ac6la.com/tldetails1.html *

His Zplots tool is also useful, and free, but does require that you have some
version of MS Excel.  If you use EZNEC, then I've found AutoEZ to be indispensable.

Wes  N7WS

* If you download and play with this tool, here are some starting values that
are illustrative.

Set
Type = Belden 9913.
Freq = 125 MHz
R = 50
X = 0
R and X = At Load
Length = 100 feet

Results:
At the bottom of the frame click the "Return Loss"
Loss (total) = 1.5 dB (this is the matched line loss)
If you look at the "At Input" and "At Load" columns you can see that:
RL (true) is 61.238 dB and 58.238 dB respectively.  The input RL is exactly 3 dB
greater than the load RL.  (2 X the line loss)

The true values are different from the 50 Ohm values.  This is because the line
Zo isn't 50 Ohm as seen in the "True Zo" box. The "Plot |Zo|" and "Plot VF"
buttons will open another screen where these values can be seen to vary with
respect to frequency.

Return to the first screen and click Show SWR.  Change R to 1000. Note that the
At Load SWR (50) = 20, exactly as expected. The input SWR is ~4.5.  showing that
a moderately lossy line can have a profound effect with highly mismatched loads.
In the loss column the total is 6.599, ~5.1 dB greater than the matched loss.
But selecting Show Return Loss again shows that the input is still 3 dB higher
than at the load, they're just much smaller values.

If you can measure input SWR and want to know load SWR and you've characterized,
or believe the manufacture's specifications for your matched line loss, you can
convert the input SWR to RL, subtract 2 X matched line loss and derive the load
RL.  Convert this back to SWR.  If you don't want to do the math, use Dan's tool
and trial-and-error.




On 11/5/2016 12:57 PM, Ron D'Eau Claire wrote:
> And the losses are low. Losses go up with increased SWR on a given
> transmission line and losses reduce the SWR measured at the load.
>
> (I neglected to make that point in my last post and couple of sharp-eyed
> readers caught it.)
>
> 73, Ron AC7AC
>
>

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[k6dgw]

On 11/6/2016 11:06 AM, Ron D'Eau Claire wrote:
> Sri Jim: You must have misunderstood. You defined the rig as the 'load' so I
> used your definition. Perhaps that's the problem.

I thought that too.  Jim was here for dinner last night but I forgot to
ask him.

> You are right; the antenna is usually up in the air, and some hams climb
> towers to make the adjustments at the antenna in order to ensure a good
> impedance match with the transmission line to minimize transmission line
> losses.

Well, except mine at home.  It's an EF 80-10 wire strung along the top
of the 6' fence.  I can "measure" the SWR anywhere on it I want to since
I'm 6'2" tall. :-)  While its claim to fame is being HOA-proof, its
doing a surprisingly good job on 20 and 15 this AM in SS.  Just S&P as
fresh meat on Sun AM, but almost no IMI's.

If you're willing to cut a 180 deg length of low-loss transmission line
for the band(s) in question, you can measure the characteristics at the
elevated antenna.
>
> But most of us with dipoles, etc., make do with an approximate match based
> on physical measurements. We measure the SWR at the rig to reassure
> ourselves that the SWR at the antenna is reasonable.

It it's at the minimum at the rig, it's at the minimum at the antenna
too, even if the actual value may be different because of transmission
line loss.
>
> And, of course, if the rig is not equipped with an ATU, we are concerned
> that the SWR at the rig is not high enough to be reduce final amplifier
> efficiency and increase heat dissipation. That was the original concern that
> started this thread.

Indeed

73,

Fred K6DGW
- Sparks NV DM09dn

- Northern California Contest Club
- CU in the Cal QSO Party 7-8 Oct 2017
- www.cqp.org


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[Jim Brown-10]

On Sun,11/6/2016 11:06 AM, Ron D'Eau Claire wrote:
> Sri Jim: You must have misunderstood. You defined the rig as the 'load' so I
> used your definition. Perhaps that's the problem.

I think you misunderstood me. )  The rig is the load for the line ONLY
when the line is feeding the RECEIVER.  Loss in the line is almost never
a factor in hearing weak signals. That happens only when the signal is
so low, and the noise received by the antenna is so low, that the signal
is near the level of internal noise. This is usually a factor only for
very weak signal work at VHF and UHF.

> Avoiding that terminology, if there is no reflected power at the rig, the
> SWR will be 1:1. Power can be "lost" in the antenna (hopefully radiated) or
> consumed in the transmission line. But, as long as there is no reflected
> power arriving back at the rig, the SWR at the rig will be 1:1 no matter
> what it is at the antenna.

There is a common misconception that the output impedance of a power
amplifier is 50 ohms resistive. This is NOT generally true. Rather, 50
ohms is the LOAD that the rig is designed to drive. The output impedance
of the power amplifier is generally a lot lower than 50 ohms.  The
"match" that antenna tuners provide is to transform the complex
impedance of the transmission line to that 50 ohm value as seen at the
output terminal of the power amp (whether an external amp or the 100W
amp of a rig running barefoot).  Also, tube power amps include variable
matching networks that perform that impedance transformation for
moderate values of mismatch.

> You are right; the antenna is usually up in the air, and some hams climb
> towers to make the adjustments at the antenna in order to ensure a good
> impedance match with the transmission line to minimize transmission line
> losses.

Hams who have done their homework to learn about modern test equipment
use Vector Network or Vector Impedance analyzers to measure the
impedance in the shack, use the Time Delay Reflectometer function in
those analyzers to find the electrical length of the line, and plug the
data into a program like SimSmith to transform the measurement made in
the shack to the impedance at the antenna.

> But most of us with dipoles, etc., make do with an approximate match based
> on physical measurements. We measure the SWR at the rig to reassure
> ourselves that the SWR at the antenna is reasonable.

The fixation on SWR is unreasonable. The real reason to use an SWR
indicator is to find the approximate resonant frequency of an antenna,
and to determine whether the impedance at the transmitter end of the
line is in the range that the transmitter can provide power to it. Solid
state rigs and power amps have protection circuits that cause them to
shut down if the SWR is higher than about 2:1 -- that's because voltage
and/or current in the transistors get high enough to cause destructive
failure. Also, DISTORTION rises in output devices (both solid state and
hollow state) when they are not driving their intended load. This is
something that my neighbor, fellow contester, and Elecraft engineer K6XX
made certain that I understood when I moved in five miles down the road
from him ten years ago. :)

> And, of course, if the rig is not equipped with an ATU, we are concerned
> that the SWR at the rig is not high enough to be reduce final amplifier
> efficiency and increase heat dissipation. That was the original concern that
> started this thread.

See above.

73, Jim K9YC
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