Mike, First, I am pretty sure HIGH SWR really means high reflected power. The K3 does what it needs to do to protect itself, which is why you can get away higher SWR with lower power. Second, the Johnson Matchbox is a link coupled high-Q parallel tuned circuit which acts as a narrow bandpass filter. It has a lot of attenuation when tuned off resonance. Using the K3 tuner to broaden the range of acceptable SWR after a Matchbox will result in rapidly increasing loss as you move away from the point the Matchbox is tuned to.
Victor 4X6GP On Thu, Mar 13, 2025, 00:16 Michael Carter via Elecraft < elecraft@mailman.qth.net> wrote: > With many thanks to those who offered suggestions on this topic, I am > reporting back what I've learned from myriad experiments over the past > several days. I apologize for the length of this post. > > As I'd commented in a separate post, the arrangement of a Johnson Matchbox > in front of the KAT3 ATU is not a preferred or permanent configuration for > me. However, because it was the configuration I used when I first noticed > the TX power foldback upon the flashing of the 'HI SWR' message on the > display, I kept it in place for the experiments. At no point during the > ARRL DX contest weekends did I observe the 'HI CUR' warning even when > operating at 100 watts. > > Experiments I performed: > > I experimented on several bands (40, 20, 15 and 10 meters), but report > only my observations on the 20 meter band. I first adjusted the Matchbox > controls for minimum SWR at the midpoint of the U.S. phone subband (14.250 > MHz). Although unique to my antenna and feedline length, that minimum SWR > was 1.95. The SWR increased to about 2.8 at either end of the phone > subband (14.150, 14.350), and did not exceed 3.0 at any frequency within > the subband. I cleared the KAT3 ATU memory, then executed a single tune > cycle at 14.250 MHz. The ATU easily attained a 1.0 SWR. > > I set the TX Power control to 100 watts, set MIC GAIN for proper ALC > indication, and transmitted USB voice on 14.250 MHz. As expected, no > indication of 'HI SWR' or power foldback. I then moved in frequency up and > down the band without an ATU retuning. At 14.290 MHz the 'HI SWR' message > flashed and the TX power was automatically reduced by the K3. The measured > SWR as reported by the K3 was 2.8 . I put the KAT3 in 'BYPASS' mode and > measured SWR directly at the Matchbox output at 14.290 MHz as 2.3 using > my AA-55 impedance analyzer. I then tested at 14.155 MHz and again the 'HI > SWR' message was flashed. The measured SWR via the AA-55 analyzer was 2.4 . > > I was curious if the same 'HI SWR' message would be displayed at the lower > TX power level of 10 watts at the same frequencies where the 'HI SWR' > message was displayed with high TX power. At both 14.290 MHz and 14.150 > MHz, no 'HI SWR' message was displayed using USB voice. The raw SWR from > the Matchbox output remained as before and the ATU was not re-tuned. > > There are many more experiments I performed on other bands, but the > description above is typical of my K3 response to frequency change from the > ATU tuning point. > > My inferences: > > I don't know the K3 MCU algorithm's threshold for 'HI SWR' to be > displayed, but it appears to be dependent on the TX Power setting. I tried > only two test power levels, but I suppose that TX Power settings between 10 > watts and 100 watts would result in different responses. It appears that > SWR alone, which doesn't change at the selected measurement frequencies, is > not the sole determinant of the 'HI SWR' condition and TX power foldback. > While high SWR is subjective, it appears that SWR values greater than 2.3 > are sufficient to trigger the 'HI SWR' message and TX power foldback, but > only if the 100 watt PA module is engaged and TX Power set to 100 watts. > The same SWR values presented at low TX power do not result in the 'HI SWR' > message and TX power remains at the original setpoint (10 watts here). > > A more curious observation is that switching out the KAT3 (by placing it > in 'BYPASS' mode) enabled transmission at 100 watts without the 'HI SWR' > warning at the extreme ends of the phone subband. The KAT3 does not have a > true bypass relay like the KAT3A ATU, but instead routes the signal around > the series inductors and leaves the shunt capacitors disconnected except > for their stray capacitance via the relay contacts. The inference here is > that the combination of the impedance presented to the KAT3 ATU by the > Matchbox away from the tuning setpoint, and the impedance transformation > effected by the KAT3 ** without an ATU re-tuning **, can present an > impedance to the 100 watt PA module that is sensed as 'HI SWR.' This is > only mildly surprising because the L-network of the ATU remains fixed away > from its nominal ATU TUNE frequency, and the impedance at the output of the > Matchbox varies primarily in its reactive part over the phone subband. The > resistive part of that impedance > is about 28 ohms and virtually constant across the band, whereas the > reactive part varies from minus 25.4 ohms at 14.150 MHz to plus 34 ohms at > 14.350 MHz. These numbers are, of course, unique to my particular antenna > and feedline length. > > Jim, K8MR kindly provided the practical solution to restore full TX power > output when the 'HI SWR' message is flashed, and I'll remember it for the > next time it occurs. My curiosity was how high must SWR be to trigger that > warning, and does it depend upon whether the 100 watt PA module is engaged > or not. > > Does the K3 MCU algorithm for protective TX power foldback take account of > the current demanded by the PA finals as well as the SWR information coming > from its SWR bridge? I don't know the answer to this question and I don't > have an easy way to measure the peak RF current at the PA transistor > collectors (or drains, for the KPA3A module). > > Bottom line: 'high SWR' is a relative condition based on one's experience > and expectations. TX power foldback occurs at modest SWR values in the 2.x > range, at least in my experience, and may occur more frequently if the KAT3 > internal ATU is trained/tuned only at a single frequency in the phone > subband. If the KAT3 is bypassed, the very same raw SWR presented at the > shack end of the feedline may be tolerated without the TX power foldback > occurring. > > I'll refrain from any more posts on this topic, and thanks again to all > for their suggestions and observations. > > 73, > Mike, K8CN > > > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to k2vco....@gmail.com > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
