What Tuners Match

I am reaching the end of a long delayed project to rewire all the coax and control cables coming into the house and shack. One task was to dig a trench from the nearest tower -- where all the transmission lines and control lines from the antennas terminate -- to the house. All cables but one are now underground.

The coax runs from the 2×8 antenna switch have been drastically improved. The underground sections are now Andrew LDF5 Heliax, which is burial grade, with LMR400 jumpers to the switch and the runs through the house into the shack. Once the burial grade control lines were run and jumpers made I was happy to find that everything worked; I had been QRT for 5 days after cutting and removing all the overhead runs of ancient and not outdoor rated cables.

Although I ought not to have been surprised, when I got on the air that night on the low bands I discovered that the SWR on a few of the antennas was inordinately high, as much as 3:1. Yet they all received well and showed appropriate directivity. After several minutes of puzzlement I had to kick myself for not seeing the obvious. There was in truth absolutely nothing amiss.

The clue was that the antennas displaying the anomalous SWR were those that have a significant mismatch at one or all band segments. The antennas that already have a low SWR continued to have a low measured SWR with the new set of transmission lines.

The problem is directly related to a previous article I wrote on the importance of low SWR antennas to contesters. Included with that article was the following Smith chart. We'll use it to help understand what I was experiencing. If you haven't read that article you may want to do so now.

My 80 meter 3-element vertical yagi currently has an SWR in excess of 3 at the feed point because I have not yet built the switching system for the array which will contain the switchable L-networks to transform the low impedance to 50 Ω. Mismatch loss due to the high SWR over the long transmission line (LMR400 and LDF4) is acceptable for now due to the low frequency and the good quality coax.

The mismatch is dealt with in the shack with the rig's internal ATU. In each of the tuner's "steps" of 10 kHz across the band of interest to me -- in this case 3.500 MHz to 3.580 MHz -- the ATU finds the correct match and stores it in its memory. During normal operation and especially during contests I can roam across the band and be assured of a perfect match. When I switch to the inverted vee, which has an excellent SWR across the CW segment of 80 meters, the ATU is disabled.

Tuners transform impedance, not SWR

For an antenna with a perfect match -- Z = 50 + 0j Ω -- no matter the length of transmission line the SWR remains exactly 1. For all other impedances the impedance at the shack end of the transmission line is sensitive to the length of the transmission line. As the Smith chart makes clear there are infinite R and X combinations for every SWR greater than 1. Every electrical ½λ of transmission line (multiply physical length by the transmission line's velocity factor) makes one complete circle of the Smith chart circle for that SWR.

The Smith chart is in this case normalized to 50 Ω so that R and X values must be multiplied by 50. Capacitive reactance is negative and inductive reactance is positive. The higher the SWR the greater the circle diameter. The outer diameter of the chart represents an infinite SWR. There are no closed circles on real transmission lines; attenuation causes the circular path to spiral inward, at a slow rate for low loss transmission line and faster when the loss is high. Interesting fact: a sufficiently long transmission line is equivalent to a dummy load.

The SWR in the shack for this antenna is approximately 3, a little lower than at the feed point. It is the same before and after I changed the coax configuration. It was the impedance that changed due to the shorter length of coax. However the ATU was set to transform the previous impedance. This is why the SWR seen by the transceiver after the change was high.

Solution 1

By now you may have guessed the solution to the problem: reprogram the ATU. It was just that simple. I did the same for the XM240 at the bottom of the 40 meter CW segment for the same reason. Problem solved. Antennas with matches close to 50 Ω exhibit a low SWR do not require the ATU and therefore the measured SWR remained the same.

Those with vacuum tube amplifiers might not even register the existence of a problem since changing antennas and frequency always require adjustment of plate and load capacitors to the transform the amplifier's high impedance output to 50 Ω. Contesters have a habit of defacing amplifiers with stickers on which they pen or pencil marks for each band, band segment and even antenna to speed up tuning after band and frequency changes. Had I been using an amplifier the marks for high SWR antennas would have needed to be redrawn.

Solution 2

As that earlier article on SWR and contests demonstrated the ideal solution is antennas with broad SWR bandwidths or feed point matching networks. Once I complete the 80 meter yagi the SWR will be low and the transmission line length irrelevant. The XM240 will always require solution 1, and therefore ATU or amplifier tuning whenever the transmission line changes. But that is a rare event in most stations.

Even with these solutions danger still lurks, waiting for a chance to manifest. Solid state amplifiers with broadband output networks are not so easy to match to high SWR antennas. Kilowatt rated tuners are expensive and require RF sensing circuits or communication between tuner and rig to adjust the match to the antenna and frequency. It is better to avoid this level of complexity with improved antenna matching.

Another problem to consider is that as the SWR increases you may find that the tuner is incapable of obtaining a match. The extreme values of R and X with a minority of transmission line lengths can exceed the range of the tuner. Replacing the tuner or altering the transmission line length are not reasonable options.

Strive for the best match obtainable at the feed point and all these problems vanish. Avid contesters have learned this lesson. Others can benefit as well. As you have seen, I became complacent, almost forgetting the implications of the operational transparency of how I dealt with high SWR antennas. Had I been using an amplifier I risked arcing or worse.