[Elecraft] KPA-500 Question

[Edward R. Cole]

Something to remember is that your coax switching needs to handle 
high-power.  If it is rated at 500w it will likely fail; go for at 
least a kilowatt.  Manual multi-position switches are common.  But 
using coax relays is an option if you need fast switching for contesting, etc.

Just remember not to switch "hot".  That means not when the KPA-500 
is transmitting.  This will cause arcs, pitting of contacts, possible 
failure of the output devices in the amp.

I recently had an experience running 1250w at 144-MHz where a coax 
relay overheated and melted the movable contact shorting it out and 
causing extreme anode and grid current on my 8877.  The relay was 
rated for 1200w at 150-MHz.  It was not "hot-switched" as it is 
controlled by a sequencer.  None the less the contact got so hot that 
is folded 180-degrees back on itself so that it contacted ground in 
the coaxial cavity of the switch.  It is being replaced with a coax 
switch rated for 3kW.  Oh the 8877 is probably OK as it can handle 
2500w for short durations.
http://www.kl7uw.com/Switch-001_1.jpg

So not all manufacturers are equal - check the rating of switches 
used for QRO.  Ratings are all for low SWR and derate when SWR 
climbs!  Also often overlooked is insertion loss in relays (more 
significant as freq. rises into VHF/UHF).  I find the simple 
open-frame coax bypass switches that are used in most commercial amps 
are not low loss or flat SWR.  My 6m Mirage presents a 1.4 SWR to the 
K3 when its in bypass and I have a 50-ohm termination on the 
amp.  Power loss is about 1w (of the 8w output of the K3/10).  That 
is about 0.6 dB.  Typical loss of VHF coax relays is 0.05 dB  No one 
would confuse a Mirage with "quality", of course.

The simple fool-proof switch is to connect cables manually.
--------------------------------------
Message: 41
Date: Sun, 10 Jul 2011 07:58:47 -0700
From: "Dick Dievendorff" <dieven at comcast.net>
Subject: Re: [Elecraft] KPA-500 Question
To: "'Fred Maas'" <just.one.hill at gmail.com>
Cc: Elecraft at mailman.qth.net
===snipped====
The Antenna1 output of the K3 goes to the KPA500 transceiver in.  The KPA500
output goes to the antenna switch common. The antennas connect to the
antenna switch.
=============
Some amplifiers have a couple of antenna outputs.  Ours doesn't.  Whatever
number of antenna connectors we chose, other than one, would be too few for
some and too many for many.

73 de Dick, K6KR




73, Ed - KL7UW, WD2XSH/45
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