[Elecraft] K2: Tune For Low SWR or Max Power?

[Ron D'Eau Claire]

Jon, I suspect the SWR is "fooling" the power measurement system somehow. In
any case, the difference is extremely minor - 16 to 20 watts is only 0.9 dB.
Note that 1 dB is the smallest difference detectable by the human ear under
ideal, controlled laboratory conditions. Typically it takes a much bigger
difference in power or signal level to hear the difference in HF radio
communications.

If your K2/100  is aligned correctly, transmitting into a dummy load (not
your ATU/Antenna combo) should show a 1:1 SWR. If not, you need to realign
the SWR bridge in your KPA100.  If you are using the same dummy load used to
set the null in the KPA100 SWR bridge, you might check it with your DMM to
see if its resistance is close to 50 ohms (say between 45 and 55 ohms d-c
resistance). If so, trust your SWR to indicate optimum ATU tuning. 

A radiator length down to 1/8 wave will work FB when used with a matching
network (tuner) that brings the system to resonance. The biggest issues with
a short center fed (or, in your case, off-center fed) wire are transmission
line losses and tuner losses. "Windoms" using some sort of open wire or
ladder line for the bulk of the feedline usually do not have excessive
losses in that part of the feed line, but can experience large losses in any
baluns or coaxial line used to complete the run to your rig when the
antenna. 

As Don mentioned, if you are using a T-network tuner it can also introduce
large losses. T-nets are extremely versatile, matching a huge range of
impedances with few components, but their losses go up very fast when
matching to a very low impedance load. Although a short radiators such as
you have will show very low impedance where the feed line attaches, the
impedance the tuner sees is hard to predict since it is also a function of
the transformations occurring in your feeder system. 

Also as Don mentioned, T-networks sometimes show more than one combination
of settings to affect a match, but only one of those settings will provide
good efficiency. MFJ usually puts a table of recommended starting points for
each band in their tuner manual. Be sure you are starting with their
recommendation to help avoid accidentally finding the "wrong",
low-efficiency, combination. 

Ron AC7AC




-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jon Perelstein
Sent: Saturday, December 25, 2010 2:13 AM
To: elecraft reflector
Subject: [Elecraft] K2: Tune For Low SWR or Max Power?

I'm running a K2 (with KPA100) with an MFJ941E Versatuner.

My antenna is a multi-band Windom that will load up on 80, but is less than
a 
half-wave on 80.  On most bands, lowest SWR per the tuner coincides with the

highest output power showing on the K2 (usually 20 or 21 watts).  However,
when 
in TUNE mode in the CW portion of 80, the lowest SWR per the tuner shows a 
reduced power output per the K2 (about 16 or 17 watts).  In that portion of
80, 
I get the maximum power output showing at about a 2.5:1 SWR per the K2.  The

readings on the tuner's forward and reflected power meters are consistent
with 
what the K2 is showing, with a noticeable dip in forward power when I get 
minimum SWR.

Receive signal strength is also consistent with what the K2 is showing.  By
ear, 
received signal strength is lower at the minimum SWR point than at about a
2.5:1 
SWR setting.  Similarly, the K2's S-Meter shows that the received signals
are 
about 1 S-Unit lower at lowest SWR than they are when I tune for that 2.5:1
SWR 
setting.

I understand that this antenna is not "optimal" for the lower stretches of
80, 
but given that it's all I can put up at the moment, am I better off tuning
for 
the best output power or for the lowest SWR?  If I tune for the best output,
how 
much should I back off on the output power to adjust for the fact that I'm 
getting more reflected power coming back into the rig?

Thanks

Jon
KB1QBZ
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