[Elecraft] KPA500 and SWR above 1.5:1

[Eric Swartz - WA6HHQ, Elecraft]

The KPA500 bases its SWR protection on reflected power level, not an 
absolute SWR. The KPA500 is happy up to a 2:1 SWR at 500-W out. As long 
as the actual reflected power is below that for a 2:1 SWR at 500W, it 
will not reduce its power. This means you can operate into a higher than 
2:1 SWR load at full amplifier gain if you reduce the amp's output power 
slightly.

The amp "soft-faults" and reduces output power by about 2.5 dB by 
switching in an input attenuator at the 500W 2:1 reflected power point, 
and it flashes a red LED to alert you to this so you can adjust driver 
power (or SWR via an external tuner) down a bit. Once this is corrected 
below this threshold by the operator, the KPA500 automatically goes back 
to full gain. It also does this for minor overdrive conditions.

Of course, for excessive SWR events (wrong antenna, open antenna etc), 
and severe overdrive, the amp protects itself by immediately hard 
faulting and going into bypass mode. After correcting the fault, just 
push the operate/standby switch to get back on-line. While at Contest 
University, our Italian representative surprised me by demonstrating 
this feature to customers by pulling the coax from the dummy load while 
the amp was transmitting. After I recovered from my surprise, I realized 
the amp was perfectly happy and had protected itself as advertised :-)  
But don't try this at home, as he also received a minor RF burn to help 
him remember this event..

73, Eric   WA6HHQ
www.elecraft.com
====

On 5/11/2011 9:24 AM, Geoffrey Downs wrote:
> The manual says the bargraph shows green ("normal") up to an SWR of 1.5:1,
> yellow ("marginal") from 1.5:1 to 2.1:1 and red ("excessive") above that.
> Does this mean no inhibition of output  by the protection system (and no
> need for it) up to 1.5:1 but progressive reduction of output above? If so,
> will the amp operate without damage in the yellow zone and what %
> reductions in output might be expected up to 2.1:1? Also in what conditions
> of SWR does the intelligent 2.5db output reduction apply; and at what SWR
> does output shut off altogether - ie the REFL HI fault condition.
>
> 73 to all
>
> Geoff
> G3UCK