So sorry, my fault...

73 - Petr, OK1RP

> Sent from my iPad

>

> > On Nov 30, 2018, at 6:29 AM, Petr, OK1RP/M0SIS <indians@xsmail.com> wrote:

> >

> > Hi Wes,

> >

> > excellent!

> >

> > Many thanks for this post which is nicely explaining what is going on about

> > the measurement in K3s.

> >

> > In fact until now no one talked about the calibrations, uncertainties,

> > errors, accuracy, reading errors, uncertainties A, B and combined

> > uncertainties etc. In that case there is several error sources and factors

> > which need to be calculated in order to get some more precise values ...and

> > in all cases the uncertainties must be calculated together with measured

> > value if we would like to talk about scientific or sophisticated

> > measurement.

> >

> > Thanks for nice explanation Wes to all.

> >

> > Best regards.

In reply to this post by Bob McGraw - K4TAX

I would be surprised if two garden variety instruments, even placed at the same

point, would agree. The directional bridges/couplers in most "(V)SWR" meters

that hams routinely use externally or which are built into our radios are not

precision instruments. There are a number of error sources in reflection

measurements; source match, diode non-linearity, coupler tracking errors and

often the most significant, directivity error.

In an ideal coupler, (i.e signal separation device) one port measures the

forward (incident signal) and another measures the reverse (reflected) signal

and there is no coupling between ports in the unwanted direction(s). In other

words there is no signal at the reverse port due to the forward signal. In a

real world coupler there is some leakage signal appearing at the reverse port

due to the forward signal, absent any reflected signal. The "goodness" of a

directional coupler in this instance is called "directivity" and the error

signal is directivity error. Directivity is usually specified in dB. Really

good couplers might have directivities in the 40 dB neighborhood. Really really

good directional bridges can be 50 dB, but so-so units might be 25-30 dB. Not

ready for prime time units are lower than this.

Now I have no way of knowing what the directivities are of the couplers built

into K3s, KPA500s, KAT500s, etc. but considering that they have to work over

about 5 octaves, I'm going out on a limb and saying that 25 to 30 dB is a fair

estimate. If I'm wrong, I'm sure I'll hear about it. For sake of discussion

I'm going to use 26.5 dB. What this means is that if I terminate the output

spigot of one of these radios with a perfect 50+j0 load, I'm going to measure a

leakage signal (directivity error) that is 26.5 dB below the incident value.

I'll introduce the concept of return loss here.

We hams usually speak in terms of SWR. SWR = (1 + p) / (1 - p) where p is the

reflection coefficient. Here the p = the voltage measured at the reflected port

and the constant 1 represents the incident signal. In reality both of these

quantities are complex numbers, they have both magnitude and phase but SWR

measurements are scalar, we throw away the phase (since it's difficult to

measure) and just use the magnitude. (In fact the symbol "p", which is really

the Greek letter rho, indicates the magnitude of the reflection coefficient in

normal usage) We can also express this ratio as return loss, which is -20 *

log10(p). So return loss, SWR and reflection coefficient are just different

ways to express the same thing; the ratio of incident to reflected signal.

Let's return to our example; the coupler with 26.5 dB directivity, which

indicates a return loss (RL) of 26.5 dB even with a perfect termination. Doing

the math and converting RL = 26.5 dB to SWR we get 1.1:1. Our perfect load

measures 1.1:1 with our imperfect instrument. And this assumes that there are

no other errors, which there always are. But it gets worse.

Let's say that the load we want to measure really is 1.1:1. We now have two

(apparent) reflections, 1) the real one and 2) the directivity error and they

both have the same magnitude. In our simple detector, they sum together. Now I

said earlier that we don't measure phase, only magnitude, but just because we

don't, or can't measure the relative phases doesn't mean they aren't there. We

will examine two cases to determine the limits of error. Case 1) both

reflections are in phase, they add up to p + p or 2p, RL = 20.5 and SWR

~1.21:1. Case 2) they are exactly out of phase, they sum to zero. p = 0, RL is

infinite and SWR = 1:1. The possible RL error is then -6 to +infinity dB!

In other words, an actual SWR of 1.1:1 can be measured anywhere between 1.0:1

and 1.2:1. Is it any wonder that we often read about concerns that one device

measures one thing, while another located at the same, or close location

measures something different. Of course all of this is predicated on a

directional coupler with 26.5 dB directivity and no other error sources. It's

entirely possible that the Elecraft couplers are better than this. They are

certainly no better than 40 dB since the internal reference resistors are 51

instead of 50 ohm. Plus the "Tandem Match" configuration is in itself not a

great match to the transmitter output.(1) Furthermore, the coupler, at least in

a K3 is driven by a LPF, which isn't a great 50 ohm source. Plus the coupler

output port isn't connected directly to the coax connector..... and so on and so

forth (2). All of this creates "uncertainty."

In a metrology lab heroic efforts are made to reduce uncertainty but do we, or

should we, really care in this situation? In my opinion, no, but everyone is

free to differ.

Wes N7WS

(1) See "An HF In-Line Return Loss And Power Meter" by Paul Kiciak, N2PK.

http://n2pk.com/#TP3(2) See "Gauge the Accuracy of SNA Measurements"

http://www.testmart.com/webdata/appnote/763.PDFOn 7/27/2018 1:08 PM, Bob McGraw K4TAX wrote:

> Yes, the K3S SWR display can show a 1.0:1 value. But in my case, it does not

> exactly agree with another instrument in the feed line system. As to why

> you are showing two different values, as minute as they are I might add, you

> are measuring 2 different places in the feed line. In my thinking, it is

> physically impossible to put two SWR bridges in the same place electrically.

>

> In theory the SWR on a given line should be the same at all places, but maybe

> not since there is loss of some minute value in the line. Since you are

> measuring on antennas, common mode current, may be the contributing cause.

> Difference in measurement calibration, may be a factor as well.

>

> VSWR bridges are calibrated with some specific value of load. Ideally, it is

> 50 ohm non-reactive, but it could be 49 ohms or 51 ohms or some other value.

> Just because a load says "50 ohms" on the label is no real indication that is

> actually fact. To that end, I have 3 dummy loads which are "50 ohm" loads

> according to the label but none are not true 50 ohm loads. I do have a

> Celwave load that says 50.5 ohms on the label and measures 50.5 ohms per my

> General Radio bridge. The others are +/- something, but good enough to

> evaluate a ham transmitter or amplifier.

>

> Frankly, a difference between 1.1:1 and 1.0:1 won't make any realistic

> difference in any form or fashion other than to appease the operator.

>

> 73

>

> Bob, K4TAX

>

>

>

> On 7/27/2018 9:58 AM, Dick Dickinson wrote:

>> I've noticed that I'm not showing a reading of less than 1.1:1 SWR on my

>> antennas per K3 SWR Numerical Readout. K3EZ will record 1.0:1 SWR in band

>> sweeps.

>>

>> Can the K3(S) Numerical Display show 1.0:1 SWR? If so, is there a likely

>> reason why K3EZ will show 1.0:1 while my K3 will only go as far down as

>> 1.1:1?

>>

>>

>> Dick - KA5KKT

... [show rest of quote]

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On Fri, Nov 30, 2018, at 4:47 PM, W2xj wrote:

> I wish I could find the original post that started this discussion. Very

> frustrating when there is no quoted material.

>

> Sent from my iPad

>

> > On Nov 30, 2018, at 6:29 AM, Petr, OK1RP/M0SIS <indians@xsmail.com> wrote:

> >

> > Hi Wes,

> >

> > excellent!

> >

> > Many thanks for this post which is nicely explaining what is going on about

> > the measurement in K3s.

> >

> > In fact until now no one talked about the calibrations, uncertainties,

> > errors, accuracy, reading errors, uncertainties A, B and combined

> > uncertainties etc. In that case there is several error sources and factors

> > which need to be calculated in order to get some more precise values ...and

> > in all cases the uncertainties must be calculated together with measured

> > value if we would like to talk about scientific or sophisticated

> > measurement.

> >

> > Thanks for nice explanation Wes to all.

> >

> > Best regards.

> >

> >

> >

> > -----

> > 73 - Petr, OK1RP

> > "Apple & Elecraft freak"

> > B:

http://ok1rp.blogspot.com> > G+:

http://goo.gl/w3u2s9> > G+:

http://goo.gl/gP99xq> > --

> > Sent from:

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> > Elecraft mailing list

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> >

>

--

73 - Petr, OK1RP

--

B:

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