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: [email protected] [mailto:[email protected]] 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
