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

[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, 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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>