[Elecraft] K3 SWR Accuracy: Final explanation

[Duncan Carter]

Does part of the K3 spec mention that it's supposed to be a precision 
swr indicator?  If so, I missed that.  I thought that I was merely 
getting an outstanding transceiver, especially for cw, at a relatively 
low cost.

This year, I became active on ham radio after a multi-decade hiatus.  
Before I left, the normal assumption about swr meters with diodes was 
that they had reduced accuracy at low power levels because of diode 
non-linearities.  Has this changed while I was away?

73, Dunc, W5DC

Wayne Burdick wrote:
> There's no way to avoid some increase in SWR reading error at low  
> power, since the bridge transformers have a low coupling ratio to  
> avoid heating at full power. We're already using very low-drop diodes  
> and pre-biasing them.
>
> That said, I am going to look at a correction based on power level if  
> possible.
>
> tnx
> Wayne
> N6KR
>
> On Nov 9, 2009, at 9:06 AM, Jack Smith wrote:
>
>   
>> Wayne:
>>
>> There's also an SWR versus output power issue, or is that part of  
>> the first correction factor? The SWR error increases significantly  
>> at lower power. I have not had a chance to plot the data because it  
>> was not part of the original question, but there is a clear power  
>> related error seen in the raw data, which I assume is tied to the  
>> detector diode linearity at low voltages.
>>
>> Jack K8ZOA
>>
>>
>> Wayne Burdick wrote:
>>     
>>> There are two sources of the reported SWR inaccuracies:
>>>
>>> - An SWR calculation scalar that I'll be adjusting in firmware;  
>>> this  is what's causing a 2:1 SWR to read closer to 1.8:1 on some  
>>> K3s.
>>>
>>> - The KAT3 itself, and to a much lesser degree, the KANT3, which  
>>> is  present if the ATU is not installed. Both modules have inherent  
>>> stray  reactance, part of which is due to the 8.2 MHz first-IF  
>>> trap. These  strays slightly alter the impedance seen by the K3's  
>>> SWR bridge.
>>>
>>> Even when the scalar correction is applied, there will still be a   
>>> small frequency-dependent variation in SWR reading between the  
>>> K3's  bridge and an external instrument. This has no impact on K3  
>>> operation.
>>>
>>> 73,
>>> Wayne
>>> N6KR
>>>
>>>
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>
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