[Elecraft] TxMon and K3 (tuner bypassed) SWR missmatch

Sat Nov 5 13:03:21 EDT 2016

Quite so, Alan and Matt; the SWR *IS* the same all along the line, but it is
still good practice to compare two couplers at the same place along the
transmission line. 

Since the outside of the coax shield is exposed to the RF field from the
antenna, it's hard to imagine a situation where there are no RF currents
flowing on the outside of the feed line unless specific measures are taken
to suppress them.

Also, any transmission line that is not terminated exactly in its
characteristic impedance will act as an impedance transformer presenting a
different impedance to any device along its length depending upon the
frequency and the line's length. To ensure two couplers being compared are
faced with the same impedance, I would compare them connected together at
the same place. Although, as Alan noted, a well-designed coupler should show
the same SWR anywhere along the line, putting them together eliminates
another variable. 

73, Ron AC7AC

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Alan
Bloom
Sent: Friday, November 4, 2016 11:03 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] TxMon and K3 (tuner bypassed) SWR missmatch

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, dmoes at nexicom.net 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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