Higher SWR When Using Amp.

[pastormg]

I have noticed when I fired up my amplifier on 40 meters that the SWR reading on the K3 when I tune up is higher then when I tune up on standby.  But what is really odd is on my External Wattmeter the SWR is the same when the amp is active versus standby. On standby my SWR on 40 meter SSB is 1.0 to 1.  When I tune the Amp it goes up to 1.6 to 1.  I realize that it isn't that high but just would like to know if there is an explanation as to why this occurs.  Mark KB3Z
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[Don Wilhelm-4]

  Mark,

Where do you have your external wattmeter?  On the output side of the
amp?  If so, then your external wattmeter and the K3 are reading the SWR
of your antenna system when the amp is on standby.  When you put the
amplifier in-line, the K3 will read the input of the amplifier.  In the
meantime your external wattmeter is still reading the antenna system,
and should not be expected to change.


73,
Don W3FPR

On 11/15/2010 6:43 AM, [hidden email] wrote:
> I have noticed when I fired up my amplifier on 40 meters that the SWR reading on the K3 when I tune up is higher then when I tune up on standby.  But what is really odd is on my External Wattmeter the SWR is the same when the amp is active versus standby. On standby my SWR on 40 meter SSB is 1.0 to 1.  When I tune the Amp it goes up to 1.6 to 1.  I realize that it isn't that high but just would like to know if there is an explanation as to why this occurs.  Mark KB3Z
>
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[Cookie]

Mark, when you tune your amp you are reading the input impedance of the amp SWR,
not the antenna SWR.  The only way that your external SWR meter and the
transceiver SWR meter should match is if the external meter is between the amp
and the transceiver.  Even then, the SWR meters available to us are not
precision instruments and are usually 5 to 10% of the max reading, so a 2kw watt
meter could read 200 watts in error and still be in tolerance for an inexpensive
meter.  The better meters could be 100 watts in error, so the error can equal or
exceed the output of the transceiver.  If the 1.6 bothers you, you can adjust
the input impedance matching circuit of the amplifier and perhaps reduce it, or
you can activate your antenna tuner on your transceiver if you have one.  You
can also expect the SWR to change some as you tune your amp.  If you have an old
amp from the tube final days when the transceivers all had a pi network to match
the amp, such as my Dentron Clipperton L, you may not even have an input
matching circuit.  I use the tuner in my K3 to match the input of the
Clipperton.  I don[t think you have anything to worry about.
Willis 'Cookie' Cooke
K5EWJ




________________________________
From: "[hidden email]" <[hidden email]>
To: [hidden email]
Sent: Mon, November 15, 2010 5:43:49 AM
Subject: [Elecraft] Higher SWR When Using Amp.


I have noticed when I fired up my amplifier on 40 meters that the SWR reading on
the K3 when I tune up is higher then when I tune up on standby.  But what is
really odd is on my External Wattmeter the SWR is the same when the amp is
active versus standby. On standby my SWR on 40 meter SSB is 1.0 to 1.  When I
tune the Amp it goes up to 1.6 to 1.  I realize that it isn't that high but just
would like to know if there is an explanation as to why this occurs.  Mark KB3Z
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[Jim Brown-10]

On 11/15/2010 3:43 AM, [hidden email] wrote:
> I have noticed when I fired up my amplifier on 40 meters that the SWR reading on the K3 when I tune up is higher then when I tune up on standby.
When a rig drives an amp, it does not "see" the antenna, it sees the
input circuit of the power amp. When the amp is in bypass mode (that is,
not in the OPERATE mode), your rig DOES see the antenna. What you are
seeing  is some mismatch between the K3 and the power amp, which is the
result of the input circuit of the power amp not being a perfect match
to 50 ohms. This is QUITE NORMAL for most power amps. You should only
worry about this if that SWR is much more than about 2:1. Most rigs,
including the K3, will "fold back" (reduce power) to protect themselves
if the SWR gets too high.

As long as the K3 provides enough power to drive the amp to the desired
output level, don't worry about it.  With some amps, you can engage the
K3's antenna tuner and tune it to provide a better match. That depends a
bit on how the amp is being keyed.  It is also common for the mismatch
to vary from one band to another, and from one place on the band to
another.  In most power amps, the input circuit includes some form of
bandpass filter.

73, Jim Brown K9YC
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