[Elecraft] KPA500 swr tolerance
Jack Brindle
jackbrindle at me.com
Wed May 6 18:16:44 EDT 2015
This may be a case where two directional couplers (or SWR bridges) may not be created equal.
The KPA500 uses a directional coupler, which needs no calibration, and measures the forward
and reflected voltages that are sampled simultaneously. This allows us to have a rather accurate
measurement and calculation. The limits are in the directional coupler itself, as to how well the
windings are coupled.
Having said this, the KPA500 makes most of its decisions based on reflected power, not on SWR.
The only SWR decision is for very high SWR protection - the amplifier will fault at or above 18:1.
This provides short- or open-circuit protection. The amplifier protection kicks in with 60 watts
reflected power (for a short while), and we will fault at 100 watts of reflected power. What this really
means depends entirely on the output power, antenna matching and any other signals in the area
that are going in to the KPA’s antenna port. If you are running relatively low power into a relatively
high SWR, the KPA probably won't fault unless it sees an SWR above 18:1. Other than that, what is
coming back from the antenna determines what happens. The higher the output power,
the more reflected power for a given match. Want to test it? Set things up with 65 watts reflected
power, then watch as the KPA protests itself by inserting the attenuator, cutting output power in
half. The KPA will continue amplifying, periodically dropping the attenuator to see if the antenna
match changed, then reprocessing itself when it sees that it hasn’t.
Something else of note. Non-choked RF currents on the outside of the coax shield will also have
an effect on reflected power (usually makes it worse). Proper choking will have a very good
effect when it is needed (and it usually is). Also, strong nearby signals may add to the reflected
power, causing a higher reading, and thus causing the KPA to protect itself more quickly than
if that signal is not present. Folks who build high-power multi-transmitter stations routinely deal
with this scenario.
This is a bit long-winded, but I hope it answers the question. We simply cannot give a valid
answer to the question of SWR limits, but rather do have very valid specifications for reflected
power limits. As usual, then answer is in your antenna system. Design it properly and the KPA500
will run just fine for a long time.
- Jack Brindle, W6FB
> On May 6, 2015, at 2:31 PM, jim <jbollit at outlook.com> wrote:
>
> In my experience, it does not like anything near 2:1.
>
> It can trip out at just over 1.6:1, depending on the band and the type of
> load (reactive part) and power out.
>
> I got the KPA-500 which makes the amp happy.
>
> Jim
> W6AIM
>
>
>
>
> .
>
> -----Original Message-----
> From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim
> Hoge
> Sent: Wednesday, May 6, 2015 1:43 PM
> To: Elecraft Reflector
> Subject: [Elecraft] KPA500 swr tolerance
>
> Greetings,
> In viewing the info on the KPA500, I see a spec for input swr of 1.5/1 but I
> cannot find any numbers for what the amp will tolerate from the antenna
> load. I assume it is low because it's a solid state amp but what exactly
> will it tolerate before it soft or hard faults? Perhaps I am
> misunderstanding the input swr as being from the transceiver?
>
> Tnx es 73,Jim W5QM
>
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