[Elecraft] K3 SWR Accuracy: Final explanation

[Wayne Burdick]

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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