[Elecraft] KXPA100 Mobile Operations -- Power Output

[Edward R Cole]

Matt,

The dc wiring is not doing the job.  You are seeing 11.2-11.8v 
running 60w!  The battery under load should hold 13.2v if it is in 
good condition and fully charged.  That is a voltage drop of 
1.6v.  You should not see that using awg-8 wire.  You mention a relay 
in the trunk.  What is the wiring from battery to relays?

To find the problem first measure your battery voltage with engine 
running and with it off.  Measure voltage at the amp when 
transmitting 100w with engine running and off.  How much drop do you 
see from the battery?  I would expect only about half a volt.  Your 
battery should hold about 13v with engine off and transmitting full 
power.  If it is dropping below 12v then the battery needs replacement.

You said it shows 12.2v at the amp when engine running.  That is too 
much voltage drop from the battery.  You should see at least that 
with engine off and transmitting.  The amp will not perform at full 
power with only 11.2-11.8v.  It is also harder on the transmitting 
transistors as they have to draw more current to get power output and 
that translates to running hotter!

Did you connect the awg wires directly to the battery posts?  You 
should.  Too often folks tie off wiring under the dash at the fuse 
block or connect to the cigarette lighter; those are bad ideas for 
running high power radios.

Is this really awg-8? Resistance in 100-feet of awg-8 is 0.0628 
ohms.  Assuming you used 20-feet that would reduce to 0.013 
ohms.  Double that to include the resistance in the negative lead and 
its 0.26 ohms.  If the amp draws 200w dc load at 12.5v that is a 
current load of 16 amps.  The voltage drop = 0.026 x 16 = 0.4v

Another tip is fusing both pos and neg leads near the battery 
connection.  That protects the radio equipment from high current 
loads from other devices in the car (like a bad ground on the starter).

I run 12v in my shack with a 16-foot dc wire run to a 30amp main 
fuse.  I can run up to 400w dc load with this.  My PS runs 14.2v and 
I have 13.5v at the main fuse under load.  I run welding cable to 
accomplish this.

73, Ed - KL7UW

From: Matt Zilmer <mzilmer at roadrunner.com>
To: Phil Hystad <phystad at mac.com>
Cc: Elecraft Reflector <Elecraft at mailman.qth.net>
Subject: Re: [Elecraft] KXPA100 Mobile Operations -- Power Output
Message-ID: <o3n5d9ttvgbubdkbbq1dpg4tpikvh36u9g at 4ax.com>
Content-Type: text/plain; charset=us-ascii

I've started testing the KX3/KXPA100 mobile configuration, but don't
have all the cabling in yet and it's not permanent.  This KXPA100 has
the ATU.  Antennas are Hustler on a custom vehicle mount.

The vehicle is a Honda Civic with a standard 12V electrical system.
Not seeing any issues with this particular setup.  Wiring to the
KXPA100 is 8 AWG, with two "transfers" to APPs at a relay enclosure
for power control in the trunk.  The source is the Civic's battery.
The KXPA is located in the trunk.

With the engine running, I am getting 100W.  During key down at full
power, measured voltage at the amp is showing 12.2V or more (depends
on whether the regulator decides it's battery charge time, I suspect).

With the engine off, I get between 11.4 and 11.8V at the amp with
power set at 60W or below.  Ymmv, because it would depend on the
battery's state of charge and any other loads on it.  I've run the
KXPA as high as 60W with battery-only, but I'm still in a test phase
and waiting on cabling to arrive.

73,
matt W6NIA



73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     dubususa at gmail.com