SWR and Power readings RE: K2

[Charles Greene-2]

Hi,

I have a question concerning antenna power and SWR.  I am setting my K2
power at 5 watts via CAT.  The K2 display reads 4.3 F, 0.0r.  An external
watt/SWR dual meter reads 5.9  +/- 0.2 Fwd and a small value which can't be
read accurately, Rev.  A Tandem SWR/Power meter reads 7.2 +/-  0.2 watts
and a SWR of 1.2 +/- 0.1.

The question is, if the SWR is not 1:1, does the power reading which is
based on the voltage in a 50 ohms load read high or low?   I don't think
any of my meters have  SWR compensation in the power level reading, if
there is any.  The Tandem has compensation for diode nonlinearity in low
power readings.

I had measured the following on the antenna a few days ago using a AIM430
antenna analyzer: http://w5big.home.comcast.net/sitemap.htm  SWR 1.1:1 +/-
0.1, Zmag 48.5 +/- 0.5 ohms, Theta +5 +/- 2 degrees, Xs, a small + value,
can't read it accurately.  I am using the KAT2 so what the K2 display reads
on r is probably correct or it is less than 0.1 which is the LSD.

Also, if the power level is set by the K2 CAT at 5.0 watts, why is the K2
display reading 4.3 watts?  If I have a very accurate wattmeter, can I
calibrate the K2 wattmeter?  How about calibrating the CAT setting?  I
probably can get a more accurate reading on the Tandem by using a DVM set
on current in series with its meter(s).

73,  Chas,  W1CG

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[KK7P]

> The question is, if the SWR is not 1:1, does the power reading which is
> based on the voltage in a 50 ohms load read high or low?

It depends :-)

If the SWR is 2:1 and is caused by low impedance (e.g., 25 ohm load)
then the voltage will be lower for a given power.  If the SWR is caused
by a higher impedance (e.g., 100 ohm load)) then the voltage will be higher.

Lyle KK7P

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[Don Wilhelm-3]

Chas,

Since you have the KAT2 installed, it may be simply a matter of calibrating
the internal wattmeter in the KAT2.  If you are using TUNE, be aware that
TUNE may produce a power output that is different from the requested power
by up to 0.5 watts (up to 5 watts with the KPA100).

While you can use an external wattmeter for calibration, I really don't
trust any of them (check their specs - usually 20% of full scale is the
speced accuracy, and that is 2 watts on a 10 watt scale).  I use a 'scope
across a good 50 ohm pure resistance and the formula Vp-p^2/400 when
calibrating the forward power (an RF probe could be used instead).  I also
have a 25 ohm and 100 ohm non-reactive dummy load that I use when setting
the SWR at the 2:1 point.  In general, I have found that the REFL pot on the
KAT2 must be set more clockwise than the FWD pot to obtain the proper 2:1
SWR calibration.  Additional note - if the SWR with the 25 ohm load is
different than the 100 ohm load, the balance trimmer cap is not set
properly - if you have similar loads, you can nudge the trimer a bit until
the SWR reported is the came with both loads.

Additional note - the KPA100 and the KAT100 reflected power pot WILL be set
nearly the same as the position on the FWD pot - but the KAT2 settings will
differ quite a bit.

73,
Don W3FPR

> -----Original Message-----
>
> I have a question concerning antenna power and SWR.  I am setting my K2
> power at 5 watts via CAT.  The K2 display reads 4.3 F, 0.0r.  An external
> watt/SWR dual meter reads 5.9  +/- 0.2 Fwd and a small value
> which can't be
> read accurately, Rev.  A Tandem SWR/Power meter reads 7.2 +/-  0.2 watts
> and a SWR of 1.2 +/- 0.1.
>
> ...
>
> Also, if the power level is set by the K2 CAT at 5.0 watts, why is the K2
> display reading 4.3 watts?  If I have a very accurate wattmeter, can I
> calibrate the K2 wattmeter?  How about calibrating the CAT setting?  I
> probably can get a more accurate reading on the Tandem by using a DVM set
> on current in series with its meter(s).
>
> 73,  Chas,  W1CG
>
> _______________________________________________
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