I have been using SGC-230 autotuners to match various wire antennas going back over 15 years and four QTHs. In particular, the only antenna I had at several of those QTHs was an Inverted L. The SGC was always able to match the wire lengths I used until I moved to my present QTH.

The inverted L here consists of about 135 feet of insulated, 18-ga. “stealth” antenna wire. The first 60 to 65 feet of wire run vertically up into the topmost branches of a large oak tree, and the remainder goes across an open space to another treetop that's about the same height above ground. The SGC tuner is attached to the first tree trunk with the bottom about a foot off the ground. The operating ground system consists of a short length of heavy braid, a single ground rod, and seven radials, all attached to a stainless sink strainer “radial plate” as described in http://www.eham.net/articles/21438.

The SGC autotuner readily finds a match on almost every band. The 160-Meter band is the sole exception. On that band the tuner can find a match only at the low end. As you move up in frequency the SWR rises until it reaches the trigger point where the tuner attempts to find a new match. At this point the tuner goes into an endless search, and you can watch the SWR meter bounce up and down as the tuner switches in various series L and shunt C combinations in a futile attempt to achieve a match.

I initially thought this was some kind of stray RF problem, but all attempts to solve it failed. Then I started reading the manual to see if it addresses this problem. It doesn't. But I learned a few things about the tuner as I read the theory of operation section. The SGC-230 tuning elements are arranged in what appears to be a conventional Pi-network, with a separate bank of relay-switched shunt capacitors located on either side of a bank of relay-switched series inductors. However, the manual explains that the tuner is actually an L-network configuration. Either the input side or the output side capacitors are selected, but never both.

When initiating a tuning cycle, the SGC first determines whether the resistive part of the load impedance is higher or lower than 50 Ohms. If it's higher, the capacitors on the output side of the inductors are used. Otherwise the capacitors on the input side of the inductors are used. I don't know why SGC didn't use a relay to switch a single bank of capacitors to either the input or output side of the inductors, but at least in the SGC-230 there are two separate banks of capacitors.

My next step was to use EZNEC to model my inverted L, more specifically to determine the feedpoint impedance. The resistive portion of the impedance is higher than 50 Ohms on all the bands 80 and up. On 160 Meters the antenna is approximately a quarter wave so I expected the resistive portion to be low. Sure enough, EZNEC said the impedance was approximately 18 - j13 Ohms at 1.8 MHz and 24 + j78 Ohms at 2.0 MHz. Notice that the reactance goes from negative to positive somewhere between 1.8 and 2.0 MHz. That means the antenna is slightly less than a quarter wave at 1.8 MHz, and is a bit longer than a quarter wave at 2.0 MHz.

I then plugged those numbers into a DOS-based L-Network calculator program I found on the Internet some time back. You'll find it at http://webpages.charter.net/crstrode/calcs/RFcalcs.htm, which is maintained by WA7CS. The program is very simple to use, and provides values for both possible L-network configurations- series L/shunt C and series C/shunt L.

At 1.8 MHz, the program shows the required values to be 2358 pF on the input side of the network, and 3.01 uH of series inductance. This is within the range of the SGC-230. At 2.0 MHz, the required values were 1529 pF shunt capacitance and 1304 pF series capacitance. Whoa! At this frequency the series element must be capacitive, not inductive!

The SGC-230 can't insert a series capacitive reactance, so that explains why the tuner couldn't find a match above the low end of 160.

When the resistive value of the antenna's impedance is above 50 Ohms, it doesn't matter whether the sign of the reactance is positive or negative. An L network consisting of series L and shunt C can always match the load.

So here's what you need to be aware of if you deploy this type of tuner to match an end-fed wire antenna. Don't choose a wire length that is slightly longer than a quarter wavelength on any frequency of interest. Make the wire length at least 3/8 wavelength, or else make it at least a little bit less than a quarter wavelength.

I hope this will help other hams solve any failure-to-match problems they may have encountered when using this kind of tuner, and help them select a wire length that will allow proper operation of the tuner on all desired frequencies.

K3AN