[Elecraft] T4 PA XFRMR winding question in K2

Sat Aug 2 14:14:00 2003

That mod reduces the DC current demand by 25% or so when running 5 watts
output. If you are backpacking the K2 or otherwise want to stretch the
last minute of operation out of it with a battery pack at 5 watts CW
output, it's a useful mod. 

With the mod your current drain above 5 watts will be HIGHER than normal
and SSB signals may be distorted because the amplifier isn't operating
in its most linear mode.

Here's a little background - 

Transistor power amplifiers (like all PA's) need to look into the right
load impedance to work efficiently. Modern rigs like the K2 have "fixed
tuned" output networks. That means that they are set up for the power
amplifier to be running ONE power level looking into ONE output load
impedance. Any departure from this causes the efficiency to go down and
sometimes for the distortion products to go up.

Old "boat anchor" rigs didn't suffer this problem because you had to
"dip the plate and set the loading" - which is to say you had to
manually adjust the output matching network for the power output and
load impedance the rig "saw". What we "lost" when we got the modern "no
tune" rigs was the ability to compensate and work efficiently at various
power levels and into various load impedances.

Nowaways we use an external "Antenna Tuner" to convert the impedance the
antenna presents to the rig to the optimum value the rig needs. Using an
"automatic antenna tuner" (ATU) like the KAT2 or KAT100 keeps us from
having to make that adjustment manually. 

Even with an antenna tuner, we are still "stuck" at one power level for
the most efficient operation. The output networks in the K2 are still
"fixed tuned" and want to see a certain impedance at BOTH ends. One end
is the antenna, which now is the antenna tuner, and the other end is the
power amplifier stage. The power amplifier stage will only show the
right impedance to the network when it is running at a certain power.

For the K2 that's about 10 watts output and for the K2/100 that's about
100 watts output. Drop below those levels and the efficiency suffers.
Normally the only problem is that the transmitter draws more current
than necessary at lower power levels. Sure, the current demand drops as
the power is decreased, but it will be higher than absolutely essential
at power levels below the design value. That's why the mod was
documented for QRP operators who NEVER go over 5 watts output. It moves
the optimum-efficiency power output down to 5 watts. The trade-off is
increased distortion and limited maximum power output available above 5
watts. 

Bottom line - 

Do not make this mod if you contemplate ever exceeding 5 watts output.
It gains you nothing except a very small saving in power supply drain
when you do run 5 watts. 

Ron AC7AC
K2 # 1289

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Schaffer, Myron
Sent: Saturday, August 02, 2003 10:21 AM
To: Elecraft (E-mail)
Subject: [Elecraft] T4 PA XFRMR winding question in K2

Hello,

I have recently purchased the K2 and I was wondering about the
advantages and disadvantages of winding the PA output transformer T4 to
different ratios.  Can anybody highlight the pro's and con's of the
different ratios?  Thanks in advance.

WV0H
Myron Schaffer
Westminster, CO

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