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

[Vic Rosenthal 4X6GP]

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