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K3 - SWR Indication

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Geoffrey Downs-2

May 03, 2009; 11:12pm

K3 - SWR Indication

In the past week I've been tuning and pruning an inverted V dipole I've put
up for 40m, fed with coax. The job isn't finished yet but I'm noticing
significant differences between SWR readings on my K3 and an external
LP100A.

For example: At 7.000 MHz the K3 currently shows 1.2:1 and the LP100A
2.00:1. At 7.200 MHz the K3 shows 1.0:1 and the LP100A 1.33:1. Both agree
that it's 1.0:1 at 7.145 MHz. My MFJ 259B shows similar readings to the
LP100A.

The differences seem more than one would expect from stray impedance in the
KAT3 as referred to when this subject was brought up on the reflector in
January this year. I've not made direct comparisons like this before so I
don't know if it's a recent phenomenon with my K3 (S/n 266) or has always
been so.

The readings were taken with PWR at 15w and the LP100A coupler connected to
ANT1 with a back-to-back PL259 adapter. ATU was of course in bypass mode.
Firmware 3.11.

What do others see?

73 to all

Geoff
G3UCK

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Steve Ellington

May 04, 2009; 2:35am

Re: K3 - SWR Indication

The method of comparison is flawed. It appears that your coax does not
exactly match the impedance of your dipole (It rarely does). Take any SWR
meter and insert it at different locations along the length of the feeder
and you will get different readings. The only place it is anywhere close to
correct is right at the antenna or at the end of the feedline if it happens
to be exactly 1/2 wavelength (rare). Even though your K3's meter and your
external meters are close together, there is still probably a measurable
electrical distance between them so there's no way they will read the same.
I've seen it a hundred times. You are dealing with a reactive circuit here.
The only way to compare meters is to put them in a resistive circuit, ie.
dummy load. All 3 meters should read 1:1 into a 50 ohm dummy. Connect 2
dummies in series for 100 ohms and all meters should read 2:1. That's how
the ARRL test them.
Trimming that dipole to force your meter to indicate 1:1 does not mean that
there is no swr on your coax. You may just be moving the low spot around
until it lands where your meter happens to be connected. Changing your
feeder length would accomplish the same thing. If you do manage to get a 1:1
reading, try changing the length of your coax. If you really do have a 1:1
swr, the length of the coax won't matter and all your meters "should" read
the same. There are just too many factors that affect the impedance of that
dipole. The height above ground can change the swr from 1:1 to 3:1 very
easily. Try the dummy load trick first and see what you get. I bet they a 3
read 1:1. Just remember, that swr meter in the K3 does not necessarily
indicate the match between your dipole and your coax. That's why turners are
so handy.
Sorry for the ramblings. Good luck!
Steve Ellington N4LQ
[hidden email]
----- Original Message -----
From: "Geoffrey Downs" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, May 03, 2009 7:12 PM
Subject: [Elecraft] K3 - SWR Indication

> In the past week I've been tuning and pruning an inverted V dipole I've
> put
> up for 40m, fed with coax. The job isn't finished yet but I'm noticing
> significant differences between SWR readings on my K3 and an external
> LP100A.
>
> For example: At 7.000 MHz the K3 currently shows 1.2:1 and the LP100A
> 2.00:1. At 7.200 MHz the K3 shows 1.0:1 and the LP100A 1.33:1. Both agree
> that it's 1.0:1 at 7.145 MHz. My MFJ 259B shows similar readings to the
> LP100A.
>
> The differences seem more than one would expect from stray impedance in
> the
> KAT3 as referred to when this subject was brought up on the reflector in
> January this year. I've not made direct comparisons like this before so I
> don't know if it's a recent phenomenon with my K3 (S/n 266) or has always
> been so.
>
> The readings were taken with PWR at 15w and the LP100A coupler connected
> to
> ANT1 with a back-to-back PL259 adapter. ATU was of course in bypass mode.
> Firmware 3.11.
>
> What do others see?
>
> 73 to all
>
> Geoff
> G3UCK
>
> ______________________________________________________________
> Elecraft mailing list
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>

... [show rest of quote]

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Phil Salas

May 04, 2009; 11:43am

Re: K3 - SWR Indication

"Take any SWR meter and insert it at different locations along the length of
the feeder
and you will get different readings."

Other than SWR reduction due to cable loss, the SWR meter should read the
same SWR regardless of where it is on the coax. Picture the constant SWR
circles on a Smith Chart where the SWR stays constant as you change the
length of the transmission line. If you add feedline loss, the SWR circle
will spiral in towards 1:1 as you continue to increase the feedline length.
However, the K3 and an external SWR meter are both electrically so close
that they should read the same (assuming comparable quality and properly
calibrated SWR meters). I have the same issue in that my K3 SWR reading is
always better than the reading I get on my in-line Array Sollutions
PowerMaster. Not a big difference though. My K3 may read 1:1 when the
PowerMaster reads 1.5:1. I haven't really worried about it since I'm
comparing the K3's internal SWR meter to an external $400 meter - and the K3
is happy.

Phil - AD5X

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Steve Ellington

May 04, 2009; 1:40pm

Re: K3 - SWR Indication

Phil
I agree concerning the swr being the same anywhere on the line but that's
not what I said. I said "you will get different readings".
Now I'm sure there's plenty of reasons why we get these different readings
and it's likely due to the meter design but the fact remains.
One thing I've learned over the years. Any statement or comment made about
antennas and feedlines will soon be refuted by another to prove it
incorrect.
Now my Smith Chart tells me it's time to take my morning walk. 73
Steve Ellington
[hidden email]
----- Original Message -----
From: "Phil & Debbie Salas" <[hidden email]>
To: <[hidden email]>
Sent: Monday, May 04, 2009 7:43 AM
Subject: Re: [Elecraft] K3 - SWR Indication

> "Take any SWR meter and insert it at different locations along the length
> of
> the feeder
> and you will get different readings."
>
> Other than SWR reduction due to cable loss, the SWR meter should read the
> same SWR regardless of where it is on the coax. Picture the constant SWR
> circles on a Smith Chart where the SWR stays constant as you change the
> length of the transmission line. If you add feedline loss, the SWR circle
> will spiral in towards 1:1 as you continue to increase the feedline
> length.
> However, the K3 and an external SWR meter are both electrically so close
> that they should read the same (assuming comparable quality and properly
> calibrated SWR meters). I have the same issue in that my K3 SWR reading
> is
> always better than the reading I get on my in-line Array Sollutions
> PowerMaster. Not a big difference though. My K3 may read 1:1 when the
> PowerMaster reads 1.5:1. I haven't really worried about it since I'm
> comparing the K3's internal SWR meter to an external $400 meter - and the
> K3
> is happy.
>
> Phil - AD5X
>
> ______________________________________________________________
> 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

... [show rest of quote]

P.B. Christensen

May 04, 2009; 1:57pm

Re: K3 - SWR Indication

> Other than SWR reduction due to cable loss, the SWR meter should read the
> same SWR regardless of where it is on the coax.

Some reasons for changes in indicated VSWR as measured at different points
along the line:

1) Line loss. As line loss increases, VSWR will show a better reading
closer to the source than at the load, irrespective of the line-to-load
mismatch;
2) The bridge/directional coupler's designed line Z does not equal the
characteristic Z of the line and the load is a complex Z. (Example: Bird 43
with 50-ohm line section being used on a run of 75-ohm line where the line
is terminated into a load with a Z value other than 75 ohms); and
3) Line current imbalance. Normally, current on the outer surface of the
inner conductor should equal the current on the inside surface of the outer
conductor. Current flow on the outside of the outer conductor will cause
line imbalance and incorrect VSWR indications, depending on the magnitude of
current on the outer surface of the outer conductor.

Paul, W9AC

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Geoffrey Downs-2

May 04, 2009; 2:14pm

Re: K3 - SWR Indication

In reply to this post by Steve Ellington

----- Original Message -----

From: "Steve Ellington" <[hidden email]>

>It appears that your coax does not exactly match the impedance of your
>dipole (It rarely does)....
-------------------------------------

Thank you Steve for your comments. I fully accept about a probable mismatch
between dipole and coax (and of course the performance of the antenna
remains to be seen), also that there are many factors affecting SWR and that
sampling at different points along the coax will produce different results
if there is reactance in the load. In this case the antenna feeder was
connected to the LP100A about 2.5 inches down the line from the ANT 1
socket. To my mind that seems a short length to account for an SWR
difference at 7 Mhz as big as 1.2:1 (K3) vs 2.0:1 (LP100A and MFJ 259B). We
all need to know what sort of load the rig or the amplifier is looking into
and a discrepancy like that is unsettling.

All three measuring devices see no reactance and 1.0:1 at 7.145 MHz. What
seems to happen is that as one moves up or down in frequency from that point
(and the load gains reactance) the K3 indication remains significantly
lower than than other two. I doubt if 2.5 inches of coax would account for
the difference and the MFJ259B, which agrees with the LP100A,
connects to the antenna feeder at exactly the same point as the K3's
ANT 1 socket so there is not even that extra 2.5 inches.

It leads me to think that maybe my K3 is not measuring SWR as well as it
should either due to a fault in my particular one or to the way it is set up
to work.

73 to all

Geoff
G3UCK

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Steve Ellington

May 04, 2009; 2:22pm

Re: K3 - SWR Indication

Geoff
Try putting a 50 ohm dummy load in line and see if the K3 and your ext.
meters read the same swr. This will take reactance and inbalance out of the
picture.

Steve Ellington
[hidden email]
----- Original Message -----
From: "Geoffrey Downs" <[hidden email]>
To: <[hidden email]>
Sent: Monday, May 04, 2009 10:14 AM
Subject: Re: [Elecraft] K3 - SWR Indication

> ----- Original Message -----
>
> From: "Steve Ellington" <[hidden email]>
>
>
>>It appears that your coax does not exactly match the impedance of your
>>dipole (It rarely does)....
> -------------------------------------
>
> Thank you Steve for your comments. I fully accept about a probable
> mismatch
> between dipole and coax (and of course the performance of the antenna
> remains to be seen), also that there are many factors affecting SWR and
> that
> sampling at different points along the coax will produce different results
> if there is reactance in the load. In this case the antenna feeder was
> connected to the LP100A about 2.5 inches down the line from the ANT 1
> socket. To my mind that seems a short length to account for an SWR
> difference at 7 Mhz as big as 1.2:1 (K3) vs 2.0:1 (LP100A and MFJ 259B).
> We
> all need to know what sort of load the rig or the amplifier is looking
> into
> and a discrepancy like that is unsettling.
>
> All three measuring devices see no reactance and 1.0:1 at 7.145 MHz. What
> seems to happen is that as one moves up or down in frequency from that
> point
> (and the load gains reactance) the K3 indication remains significantly
> lower than than other two. I doubt if 2.5 inches of coax would account for
> the difference and the MFJ259B, which agrees with the LP100A,
> connects to the antenna feeder at exactly the same point as the K3's
> ANT 1 socket so there is not even that extra 2.5 inches.
>
> It leads me to think that maybe my K3 is not measuring SWR as well as it
> should either due to a fault in my particular one or to the way it is set
> up
> to work.
>
> 73 to all
>
> Geoff
> G3UCK
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
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... [show rest of quote]

Terry Schieler

May 04, 2009; 2:45pm

Re: K3 - SWR Indication

In reply to this post by Geoffrey Downs-2

Geoff,

I experience the same issues between my factory aligned K3/100 (#474) and my
LP-100A (aligned by Larry at Telepost, Inc). Firmware 3.11 here as well.

Terry, W0FM

-----Original Message-----
From: Geoffrey Downs [mailto:[hidden email]]
Sent: Sunday, May 03, 2009 6:12 PM
To: [hidden email]
Subject: [Elecraft] K3 - SWR Indication

In the past week I've been tuning and pruning an inverted V dipole I've put
up for 40m, fed with coax. The job isn't finished yet but I'm noticing
significant differences between SWR readings on my K3 and an external
LP100A.

For example: At 7.000 MHz the K3 currently shows 1.2:1 and the LP100A
2.00:1. At 7.200 MHz the K3 shows 1.0:1 and the LP100A 1.33:1. Both agree
that it's 1.0:1 at 7.145 MHz. My MFJ 259B shows similar readings to the
LP100A.

The differences seem more than one would expect from stray impedance in the
KAT3 as referred to when this subject was brought up on the reflector in
January this year. I've not made direct comparisons like this before so I
don't know if it's a recent phenomenon with my K3 (S/n 266) or has always
been so.

The readings were taken with PWR at 15w and the LP100A coupler connected to
ANT1 with a back-to-back PL259 adapter. ATU was of course in bypass mode.
Firmware 3.11.

What do others see?

73 to all

Geoff
G3UCK

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Geoffrey Downs-2

May 04, 2009; 4:48pm

Re: K3 - SWR Indication

In reply to this post by Steve Ellington

----- Original Message -----
From: "Steve Ellington" <[hidden email]>

> Geoff
> Try putting a 50 ohm dummy load in line and see if the K3 and your ext.
> meters read the same swr. This will take reactance and inbalance out of
> the picture.

Thanks Steve. I did try that and with a 50 ohm dummy load all three meters
show the same 1.0:1 SWR. It seems that the K3 meter responds differently
from the other two to reactance in the load.

73 to all

Geoff
G3UCK

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Robert Friess

May 04, 2009; 6:09pm

Re: K3 - SWR Indication

Hello Geoff,

Which bands were you making the comparison on?

Bob, N6CM

On Mon, May 4, 2009 at 9:48 AM, Geoffrey Downs
<[hidden email]> wrote:

> ----- Original Message -----
> From: "Steve Ellington" <[hidden email]>
>
>
>> Geoff
>> Try putting a 50 ohm dummy load in line and see if the K3 and your ext.
>> meters read the same swr. This will take reactance and inbalance out of
>> the picture.
>
> Thanks Steve. I did try that and with a 50 ohm dummy load all three meters
> show the same 1.0:1 SWR. It seems that the K3 meter responds differently
> from the other two to reactance in the load.
>
> 73 to all
>
> Geoff
> G3UCK
>
> ______________________________________________________________
> 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
>
>

... [show rest of quote]

Don Wilhelm-4

May 05, 2009; 5:04am

Re: K3 - SWR Indication

In reply to this post by Geoffrey Downs-2

Geoff,

That test at 50 ohms substantiates that the wattmeters are properly
balanced for a 50 ohm system, and nothing more - read on if you are
interested in more skepticism.

Contrary to several statements made, it is quite possible that *all*
those wattmeters can be wrong when reporting SWR that is greater than
SWR = 1.0. Although I have faith in Larry's calibration of forward
power for the LP100 meter, I don't have much information about what he
does for the reverse power calibration, so I cannot comment with any
degree of certainty - I can only speculate that some error is possible.
Likewise, I do not have the details of the alignment procedures for the
K3 reverse power calibration, so I can be just as skeptical of the K3
SWR indication as I am of the LP-100.

Before I stick my head further onto the chopping block, I can say that
the forward power indication for both the K3 and LP-100 is quite good if
properly calibrated (and most are - so they should have reasonable
agreement).

Most any wattmeter is balanced to produce zero reflected voltage when
terminated in its design impedance (normally 50 ohms resistive). But
when the load is different than 50 ohms, there will be some reflected
voltage indicated. The gain of the amplifiers following the detector
must be set to indicate the proper SWR, just like the gain of the
forward voltage must be calibrated to indicate the correct forward
power. To do that SWR calibration properly requires one or more loads
of known impedance. When I calibrate a wattmeter (in the KXAT1, KAT1,
KAT2, KPA100 or KAT100), I use a precision 25 ohm and 100 ohm pure
resistive dummy load (connected with the shortest possible connection)
to set the SWR = 2.0 point. The resistors I use for these dummy loads
are Caddock thick film 50 ohm 1% resistors (connected in parallel for 25
ohms and connected in series for 100 ohms). These resistors are known
to be pure resistances up to 500 MHz, so I know I can trust them (and
they have been measured on a VNA).

So bottom line, I cannot vouch for the accuracy of any wattmeter's SWR
indication until I can test them with my precision loads (Trust no tool
until it has proven its worth).

I do not believe the K3 SWR indication is intended to be a precision
measurement tool, it's major purpose is to find the lowest SWR while the
KAT3 is tuning, and it will do that task quite well without precise
calibration. Larry will have to speak for the LP-100 SWR accuracy.

Actually, there is more to forward power and reverse power calculation
that must be handled in an instrument that will correctly indicate both
(and 2 different instruments may do it differently). The actual power
must take the voltage developed by the forward detector and subtract the
reflected power to obtain the actual forward power. The voltage
developed by the reverse detector must also be subjected to calculations
to determine the actual SWR (which also has a relationship to the
forward power). The one meter that I am familiar with which takes all
this into consideration is the KI6WX Tandem Match. The Tandem Match
does the calculations necessary by using operational amplifiers to do
the required summing, subtraction, and conversion to logarithms so we
can view the actual forward power and SWR. That task can also be done
with a pic (firmware), but even so, it still requires some calibration
steps to get everything right and calibrated. and not all wattmeters go
to the trouble of doing all the necessary math - some can simply report
the reflected power directly on some "SWR" scale - which will only
accurately report the SWR when it is 1.0:1.

So, my bottom line advice is that most any wattmeter designed for a 50
ohm system can be trusted when it indicates SWR = 1.0 with a 50 ohm
load, but beyond that point, it is difficult to say how accurate it
really is until it is proven to have accuracy within reason for SWR
indications.

OK, my skepticism about wattmeters is showing once again - only this
time with SWR rather than my usual tirades about the inaccuracies of the
forward power indications of wattmeters (+/- 20% of full scale specs).

73,
Don W3FPR

Geoffrey Downs wrote:

> ----- Original Message -----
> From: "Steve Ellington" <[hidden email]>
>
>
>
>> Geoff
>> Try putting a 50 ohm dummy load in line and see if the K3 and your ext.
>> meters read the same swr. This will take reactance and inbalance out of
>> the picture.
>>
>
> Thanks Steve. I did try that and with a 50 ohm dummy load all three meters
> show the same 1.0:1 SWR. It seems that the K3 meter responds differently
> from the other two to reactance in the load.
>
> 73 to all
>
> Geoff
> G3UCK
>
>

... [show rest of quote]

Jack Smith-6

May 05, 2009; 9:58am

K3 - SWR Indication

A wattmeter built around a directional coupler always has to deal with
finite coupler directivity. Making the problem more difficult is that we
expect a wattmeter to be accurate over a rather wide frequency range,
1.8 to 30 or even 50 MHz. This places an even greater burden upon the
directional coupler.

If the directional coupler has 30 dB directivity--a very good number to
be maintained over a wide frequency range--then 1 KW forward power into
a perfect load will show 1 watt reflected power, corresponding to an SWR
of 1.065:1 instead of the expected 1.0000... for the theoretically
perfect load.

It is possible to measure the phase and amplitude of the coupled signal
to "calibrate out" coupler imperfections. This is what is done with a
vector network analyzer when the standard "open/short/load" calibration
is applied. The VNA measures the phase and amplitude of the coupled
signal when the through port is operated into an open circuit, a short
circuit and a known value (resistance and stray L & C known) termination
for each test frequency. The VNA then computes and applies an
appropriate correction factor to correct for coupler errors. O/S/L
calibration has been supplemented by more advanced techniques in newer
VNAs. (There's a very good Application Note AN 1287-3 from Agilent on
this subject available at
http://cp.literature.agilent.com/litweb/pdf/5965-7709E.pdf. Bird
Electric has a less technical Application Note on the effect of coupler
directivity on SWR at
http://www.bird-technologies.com/techapps/app_notes/StraightTalkAboutDirectivity.pdf)

A wattmeter using diodes to measure RF voltage used with a directional
coupler cannot apply sophisticated error correction to compensate for
finite coupler directivity. At most, one can tweak a balance pot or
trimmer cap to null the reflected signal at a single frequency and power
level. Further complications result from the forward and reverse diode
detectors being operated at different points on their sensitivity curve,
etc.

Hence, it is far from surprising that different wattmeters will show
different SWR under ostensibly identical test conditions.

Larry's LP-100 wattmeter operates with a different methodology and I'll
leave it to him to explain the differences and how coupler directivity
is considered.

Jack K8ZOA
www.cliftonlaboratories.com

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N8LP

May 06, 2009; 2:08am

Re: K3 - SWR Indication

Several interesting topics have arisen in this discussion. Here are a couple comments...

1) Most wattmeters use a diode peak detector for each of the coupler ports (FWD, REF). The forward voltage drop of the diodes becomes an increasing source of error as power is lowered. The means than for fairly low SWRs, there is more error in the REF voltage sample, which is lower to begin with. Squaring the voltage to calculate power exaggerates the error. Very few commercial meters try to compensate for the diode drop. The LP-100A does, using a circuit similar to the one in the Tandem Match design, as well as using a dual Schottky diode on a common substrate to minimize temp variations between the detector diode and the correction diode. It is possible to get a fairly good correction for static voltage drop errors, but errors due to the dynamic inverse impedance of the diode, which is frequency dependent, can't be easily compensated for. Still, it's much better than nothing.

2) Most wattmeters use a simple coupler where samples of line current and voltage across the load are combined to obtain FWD and REF voltage samples. There are some inherent phase errors which are relative to frequency in this simple design, and magnitude errors related to parasitic coupling which limit the directivity of the coupler. Directivity determines the ultimate limit of SWR measurement error. As mentioned by K8ZOA, 30dB is an excellent broadband directivity number, but rarely achieved with high power couplers from 2-54 MHz. The LP-100A feeds the raw current and voltage samples to a gain/phase detector chip for determination of SWR, instead of combining them in the coupler. No diodes are involved, and no FWD or REF voltage samples. The meter has a frequency counter, and any phase or gain errors in the coupler samples can be calibrated out, indexed to frequency. This allows the meter to achieve >40dB directivity from 2-54 MHz.

3) Power circuitry and calculations are completely independent of SWR circuitry and calculations in the LP-100A. As long as there is about 1-2W of power, the gain/phase detector will provide the same result at any power level, with slightly reduced accuracy down to 50mW. The LP-100A uses the differential magnitude and phase samples to calculate the complex reflection coefficient, from which all other impedance and SWR numbers are derived. The meter can display REF power, but it is calculated from FWD power and reflection coefficient, not measured directly.

73,
Larry N8LP

Jack Smith-6 wrote

A wattmeter built around a directional coupler always has to deal with
finite coupler directivity. Making the problem more difficult is that we
expect a wattmeter to be accurate over a rather wide frequency range,
1.8 to 30 or even 50 MHz. This places an even greater burden upon the
directional coupler.

If the directional coupler has 30 dB directivity--a very good number to
be maintained over a wide frequency range--then 1 KW forward power into
a perfect load will show 1 watt reflected power, corresponding to an SWR
of 1.065:1 instead of the expected 1.0000... for the theoretically
perfect load.

It is possible to measure the phase and amplitude of the coupled signal
to "calibrate out" coupler imperfections. This is what is done with a
vector network analyzer when the standard "open/short/load" calibration
is applied. The VNA measures the phase and amplitude of the coupled
signal when the through port is operated into an open circuit, a short
circuit and a known value (resistance and stray L & C known) termination
for each test frequency. The VNA then computes and applies an
appropriate correction factor to correct for coupler errors. O/S/L
calibration has been supplemented by more advanced techniques in newer
VNAs. (There's a very good Application Note AN 1287-3 from Agilent on
this subject available at
http://cp.literature.agilent.com/litweb/pdf/5965-7709E.pdf. Bird
Electric has a less technical Application Note on the effect of coupler
directivity on SWR at
http://www.bird-technologies.com/techapps/app_notes/StraightTalkAboutDirectivity.pdf)

A wattmeter using diodes to measure RF voltage used with a directional
coupler cannot apply sophisticated error correction to compensate for
finite coupler directivity. At most, one can tweak a balance pot or
trimmer cap to null the reflected signal at a single frequency and power
level. Further complications result from the forward and reverse diode
detectors being operated at different points on their sensitivity curve,
etc.

Hence, it is far from surprising that different wattmeters will show
different SWR under ostensibly identical test conditions.

Larry's LP-100 wattmeter operates with a different methodology and I'll
leave it to him to explain the differences and how coupler directivity
is considered.

Jack K8ZOA
www.cliftonlaboratories.com

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Geoffrey Downs-2

May 13, 2009; 8:22pm

Re: K3 - SWR Indication

In reply to this post by Jack Smith-6

Thank you Don (W3FPR), Jack (K8ZOA) and Larry (N8LP) for
your interesting comments on this thread last week and for taking the
trouble to set them down so clearly.

My technical expertise in this area is no match for yours although I did
make my first, fairly unsophisticated, indicator of forward and reflected
power in 1967. In those days it was called a "Reflectometer" rather
than an SWR meter - nothing to do with a Reflector of course :-)

Since then I've used quite a few SWR meters of one sort or another and
although a lot of technology has moved on beyond my understanding I've
picked up enough to know that different meters will give different
indications just as Don and Jack say. Some are, I think, better than others
and I must admit that I have a good opinion of the LP100A but in my
experience most of them agree within a reasonable margin of tolerance.
Of course we could argue till the cows come home about what is a
reasonable margin of tolerance and whether one meter or another falls
within it (but let's not!). The reason I originally posted to the reflector
was that the discrepancy I noted at 7.000 MHz of 1.2:1 on the K3 and
2:1 on two other meters was larger than I had come across before,
and arguably outside that reasonable margin so I wanted to know what
was going on. A number of suggestions have been made both on and off
list but I'm still not sure.

It would be interesting to know what indication the newly announced W2 would
give, but we may have to wait a little while to find out.

73 to all

Geoff
G3UCK

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