[Elecraft] K3 - SWR Indication

[Don Wilhelm]

Geoff,

That test at 50 ohms substantiates that the wattmeters are properly 
balanced for a 50 ohm system, and nothing more - read on if you are 
interested in more skepticism.

Contrary to several statements made, it is quite possible that *all* 
those wattmeters can be wrong when reporting SWR that is greater than 
SWR = 1.0.  Although I have faith in Larry's calibration of forward 
power for the LP100 meter, I don't have much information about what he 
does for the reverse power calibration, so I cannot comment with any 
degree of certainty - I can only speculate that some error is possible.  
Likewise, I do not have the details of the alignment procedures for the 
K3 reverse power calibration, so I can be just as skeptical of the K3 
SWR indication as I am of the LP-100.

Before I stick my head further onto the chopping block, I can say that 
the forward power indication for both the K3 and LP-100 is quite good if 
properly calibrated (and most are - so they should have reasonable 
agreement).

Most any wattmeter is balanced to produce zero reflected voltage when 
terminated in its design impedance (normally 50 ohms resistive).  But 
when the load is different than 50 ohms, there will be some reflected 
voltage indicated.  The gain of the amplifiers following the detector 
must be set to indicate the proper SWR, just like the gain of the 
forward voltage must be calibrated to indicate the correct forward 
power.  To do that SWR calibration properly requires one or more loads 
of known impedance.  When I calibrate a wattmeter (in the KXAT1, KAT1, 
KAT2, KPA100 or KAT100), I use a precision 25 ohm and 100 ohm pure 
resistive dummy load (connected with the shortest possible connection) 
to set the SWR = 2.0 point.  The resistors I use for these dummy loads 
are Caddock thick film 50 ohm 1% resistors (connected in parallel for 25 
ohms and connected in series for 100 ohms).  These resistors are known 
to be pure resistances up to 500 MHz, so I know I can trust them (and 
they have been measured on a VNA).

So bottom line, I cannot vouch for the accuracy of any wattmeter's SWR 
indication until I can test them with my precision loads (Trust no tool 
until it has proven its worth).

I do not believe the K3 SWR indication is intended to be a precision 
measurement tool, it's major purpose is to find the lowest SWR while the 
KAT3 is tuning, and it will do that task quite well without precise 
calibration.  Larry will have to speak for the LP-100 SWR accuracy.

Actually, there is more to forward power and reverse power calculation 
that must be handled in an instrument that will correctly indicate both 
(and 2 different instruments may do it differently).  The actual power 
must take the voltage developed by the forward detector and subtract the 
reflected power to obtain the actual forward power.  The voltage 
developed by the reverse detector must also be subjected to calculations 
to determine the actual SWR (which also has a relationship to the 
forward power).  The one meter that I am familiar with which takes all 
this into consideration is the KI6WX Tandem Match.  The Tandem Match 
does the calculations necessary by using operational amplifiers to do 
the required summing, subtraction, and conversion to logarithms so we 
can view the actual forward power and SWR.  That task can also be done 
with a pic (firmware), but even so, it still requires some calibration 
steps to get everything right and calibrated. and not all wattmeters go 
to the trouble of doing all the necessary math - some can simply report 
the reflected power directly on some "SWR" scale - which will only 
accurately report the SWR when it is 1.0:1.

So, my bottom line advice is that most any wattmeter designed for a 50 
ohm system can be trusted when it indicates SWR = 1.0 with a 50 ohm 
load, but beyond that point, it is difficult to say how accurate it 
really is until it is proven to have accuracy within reason for SWR 
indications.

OK, my skepticism about wattmeters is showing once again - only this 
time with SWR rather than my usual tirades about the inaccuracies of the 
forward power indications of wattmeters (+/- 20% of full scale specs).

73,
Don W3FPR

Geoffrey Downs wrote:
> ----- Original Message ----- 
> From: "Steve Ellington" <n4lq at carolina.rr.com>
>
>
>   
>> Geoff
>> Try putting a 50 ohm dummy load in line and see if the K3 and your ext. 
>> meters read the same swr. This will take reactance and inbalance out of 
>> the picture.
>>     
>
> Thanks Steve. I did try that and with a 50 ohm dummy load all three meters 
> show the same 1.0:1 SWR. It seems that the K3 meter responds differently 
> from the other two to reactance in the load.
>
> 73 to all
>
> Geoff
> G3UCK 
>   
>