SWR - does it matter?

[ANDY DURBIN]

As a counter to those who are asserting that SWR 1.5:1 is a good enough match for an antenna system I offer measured data for the KPA500.

At 14.005 MHz my KPA500, when connected to a 1.44:1 SWR load (R37, X9), had a PA dissipation of over 500 watts when producing 400 watts RF output.   When the load was changed to 1.30:1 (R39, X3) the PA dissipation was about 410 watts.  When the load was changed to SWR 1.07:1 (R49, X3) the PA dissipation was reduced to under 400 watts.   (The plots are far more informative than a text description but I can't post them here).

If you don't mind your KPA500 running hot, and are prepared to accept it shutting down with PA DISS fault, then 1.5:1 SWR may be good enough for you.   I'd rather know the complex impedance of the antenna system load and avoid loads that result in high PA dissipation.

73,
Andy, k3wyc






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[Elecraft mailing list]

 

If you want your antenna match to be correct use aScopeMatch as I describe on my 630M web-site.

Thanks 73 Ken K5DNL

www.k5dnl.com

    On Wednesday, April 10, 2019, 9:03:48 AM CDT, Andy Durbin <[hidden email]> wrote:  
 
 As a counter to those who are asserting that SWR 1.5:1 is a good enough match for an antenna system I offer measured data for the KPA500.

At 14.005 MHz my KPA500, when connected to a 1.44:1 SWR load (R37, X9), had a PA dissipation of over 500 watts when producing 400 watts RF output.  When the load was changed to 1.30:1 (R39, X3) the PA dissipation was about 410 watts.  When the load was changed to SWR 1.07:1 (R49, X3) the PA dissipation was reduced to under 400 watts.  (The plots are far more informative than a text description but I can't post them here).

If you don't mind your KPA500 running hot, and are prepared to accept it shutting down with PA DISS fault, then 1.5:1 SWR may be good enough for you.  I'd rather know the complex impedance of the antenna system load and avoid loads that result in high PA dissipation.

73,
Andy, k3wyc






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[Ignacy]

Assuming that the correct load is 50 Ohms, load with SWR of 1.5:1 could be 33
Ohm or 75 Ohm. Or something more complex.

The load of 33 Ohm would cause bigger dissipation and the load of 75 Ohm
less dissipation but also less power.

Three options here. First, spend hrs/days optimizing your antennas. Second,
use antenna tuner. Third, accept inefficiencies. Personally, I would not
consider a solid-state amp without a built-in tuner.

Ignacy, NO9E



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[Bob McGraw - K4TAX]

And then use the lowest loss feedline one can afford and install.  This minimizes the increased loss due to SWR.

Bob, K4TAX


Sent from my iPhone

> On Apr 10, 2019, at 11:13 AM, Ignacy <[hidden email]> wrote:
>
> Assuming that the correct load is 50 Ohms, load with SWR of 1.5:1 could be 33
> Ohm or 75 Ohm. Or something more complex.
>
> The load of 33 Ohm would cause bigger dissipation and the load of 75 Ohm
> less dissipation but also less power.
>
> Three options here. First, spend hrs/days optimizing your antennas. Second,
> use antenna tuner. Third, accept inefficiencies. Personally, I would not
> consider a solid-state amp without a built-in tuner.
>
> Ignacy, NO9E
>
>
>
> --
> Sent from: http://elecraft.365791.n2.nabble.com/
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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[Wes Stewart-2]

At a given operating point, there is always some load Z that will minimize
amplifier dissipation (and probably another that minimizes IMD).  The problem
is, we don't normally know what it is, so guessing about it is pointless. Hence
specs like X watts into Y SWR (nominal).

Andy provided three data points. Changing the sign of the reactance will leave
the same SWR, but more than likely different amplifier response.  In fact, 50
+j0 might not be optimum.   Who knows, maybe some point on the 1.5:1 circle on
the Smith chart is better :-)

Adding to this uncertainty is the fact that with the typical measurement
circuitry in our PAs and tuners, we don't know the real SWR either.

I, and others, discussed this in an earlier thread
:http://elecraft.365791.n2.nabble.com/K3-SWR-Numerical-Indication-td7643839.html

Wes  N7WS


On 4/10/2019 6:59 AM, Andy Durbin wrote:

> As a counter to those who are asserting that SWR 1.5:1 is a good enough match for an antenna system I offer measured data for the KPA500.
>
> At 14.005 MHz my KPA500, when connected to a 1.44:1 SWR load (R37, X9), had a PA dissipation of over 500 watts when producing 400 watts RF output.   When the load was changed to 1.30:1 (R39, X3) the PA dissipation was about 410 watts.  When the load was changed to SWR 1.07:1 (R49, X3) the PA dissipation was reduced to under 400 watts.   (The plots are far more informative than a text description but I can't post them here).
>
> If you don't mind your KPA500 running hot, and are prepared to accept it shutting down with PA DISS fault, then 1.5:1 SWR may be good enough for you.   I'd rather know the complex impedance of the antenna system load and avoid loads that result in high PA dissipation.
>
> 73,
> Andy, k3wyc
>
>

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