LDG AT-100PROII max C / max L

LDG does not specify either max C or max L of their antenna tuners in the “pro” series. They specify a vague “it can tune 1000 ohms”. This basically means nothing as no frequency is given and it is not given if they mean R or X or Z by “1000 ohms”. There is also no service manual or schematic available. Very disappointing from a serious (?) supplier.

Well, since LDG doent specify anything, I have measured it.

The LDG AT-100PROII is a series L, shunt C configuration.

Min L is 3uH, max L is 11,8uH
Min C is 85pF, max C is 250 pF
This is measured at 2MHz.

This shows that the capability of tuning an 2x20m doublet via “10-12m” of 590 ohm ladder line on 80m seems to be limited, and that tuning on 160m is very difficult.This is mainly due to limited Lmax. Also the limited Lmax gives limitations on other bands as well. Having the proper length of transmission line is important to overcome this limitations. You MAY also use a 4:1 or 9:1 balun. However it is important that the balun does have a very low loss. (Otherwise you you end up with a good SWR but poor efficiency).

Designing L and T antenna tuners for HF on the Smith chart

The Smith chart is a tool used a lot by professional RF engineers for solving transmission line stub matching problems and all sorts of quick calculations.

The Smith chart can also be used for quick back of the envelope L and T antenna tuner engineering calculations.

I have on the picture above plotted a T configuration antenna tuner with the first capacitor set to a so big value that it is shorted as seen by the RF voltage (large C – low |Z|). Then the configuration becomes a L tuner in practice with a shunt L followed by a series C when seen from the load in towards the generator.

I measured the Z in the shack end of the ladder line feeding my doublet antenna to be Z = (24.1 – j35) ohms at 14.200 MHz by a Vector Network analyzer. That can be plotted as a point in the lower part of the Smith chart (capacitive Z).

(1) Since we have now first an inductor (in the tuner to ground) as observed from the load towards the generator, we can use this inductance to move along a constant Conductance curve in the Y plane (upwards in the Z plane). The conductance is constant but the Susceptance varies. (We remember from the RF engineering classes at engineering school that Y = 1/Z – of course).

(2) Then we use a series capacitor to move down inside the 1.25:1 SWR circle. We dont have to hit the center because anything inside the inner 1.25:1 circle is good enough. (We move while the R part of R + jX is constant, while the X part is changing to become more negative. This means we move on a constant resistance circle in the Z plane).

Determination of component values can be done easily by hand in a tool like this while still retaining an intuitive understanding of what is going on.

Black magic! Especially with a digital smith Chart tool.

Why you should choose a lowpass configuration on your L antenna tuner

K6JCA has analyzed the needed components values for matching a load while moving on a constant reflection coefficient circle. The plot below shows that in case you select the high-pass configuration for your tuner, certain angles of the reflection coefficient will give you skyrocketing component values.

Component values for the highpass and lowpass configurations

Above you can see that the LsCp &CpLs configuration keeps the max component values quite flat. LsCp and CpLS  are therefore the best engineering choices based on cost and realistic component values.

Repair of LDG automatic antennatuner

I blasted my LDG antennatuner some time ago. Or …. I thought I blasted it….. It appeared that it was only the resistor in the SWR detector circuit that got burned out. I replaced that resistor and now its ok again.It was easy to repair. However these small LDG tuners dont take more than 100W max. The designers have used ferrite cores, whereas it would have been a much better idea to use carbonyl cores or air core inductors. The latter doesnt get so easily saturated.

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However I must say that the design of the LDG equipment I have seen so far is not very impressive. Why use that BIG chasis when you dont need it? Why use DB9 style connectors on a chassis that is supposed to be watertight? Look at that coax termination there. Both on the board and on the PL259 chassis connector. Why use RG174 teflon coax when you have such crappy terminaions? Perhaps it would be better with no coax at all 🙂 However when the tuner works it works fairly OK. Just dont trust this kind of equipment in a contest or on a dx expedition.

RF current amperemeter with log detector

I have long had the need to measure antenna current. This can be done by a rf-current transformer and a rectifier made by a shottky diode or fast silicon diode and an integrator driving a mechanical instrument. However the dynamic range is very limited with such a setup.

Therefore I made a new rf-amperemeter design with a log detector chip from Analog Devices driven from a current transformer made out of a split core ferrite material that has response in the HF and lower VHF frequency range.The current trafo is terminated in a few ohms and the RF current conductor (actually the primary) sees only a fraction of an ohm so very little influence is done on the circuit you are measuring. (Apart from capacitive coupling and a slight leakage inductance from the current transformer windings/ferrite combination). The ferrite core is a split type and is epoxied to a clip.

I calibrated the instrument by terminating the generator in a fancooled 100W dummy load and measured the voltage over the load by an oscilloscope. Since it has a log output the dynamic range from milliamp range up to 1,4A RF current. That would peg a mechanical instrument if you would at the same time want to be able to detect a significantly lower RF-current without having to change scales with switches / pot meters etc.

Next I plan on making a OLED display on this design with an arduino controller. Its a sparetime project so lets see how long tiem it takes before I implement that.

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Contesting @ SJ2W

Edit 15. April 2015: since last time I also operated Russian DX contest from SJ2W. Good results. We expect to be among the top stations in Europe in our class.

Edit 11. February 2015: since last time I have been operating from SJ2W in CQWW SSB and there will be additional images and writeups posted from later visits.

I went to SJ2W in northern Sweden in May 2013 to work the WPX DX CW contest with SM3WMV Micke, SM2LIY Pelle, SM2XJP Peter, and SE2T Kurt. The QTH is fantastic!  Several kms to the nearest neighbor, flat terrain and a very good antenna installations. Stacks on 40, 20, 15 and 10 as well as 4-SQ on 40m / 80m.  I have posted some pictures below. Click on the pictures to see comments for each picture. Stay tuned for more info!




4NEC simulations + VNA measurements for vertical antenna design

image My old vertical tuner can’t withstand QRO power levels as the tuner is limited to approx 150W. I am therefore working on a new multiband vertical for QRO operation. Its best to try to be finished before winter sets in(soon approaching as I write this blog post). Instead of using traps, I will tune the antenna like a Marconi type antenna over a ground plane with switchable or tunable L/C networks down at the feed point. The challenge with multiband antennas that is going to cover all the 40, 30, 30, 20, 17, 15, 12 and 10m bands, is that there will be frequency ranges where the real impedance is very high. A high real impedance is not possible to tune out with L or C and also difficult to match with a L network. It can be fed and matched with a tapped parallel network, but  there will be high voltages present and vacuum capacitors will be needed for QRO operation. L networks are easier on most bands and I try to use only one HV capacitor in one parallel network for 18 MHz (at least that is the plan). The trick is to have a proper length radiator that is tuned so that the impedance peaks will lay outside the ham bands of use. At least one band will have high impedance, but it should be possible to have fairly low impedance above that band. I did a 4NEC numeric antenna simulation to investigate the expected impedance range before sizing the radiator in the real life. What to look for is the zero phase transitions (look at the pink curve above). The first zero phase transition is the quarter wave resonant point simulated in 4NEC to be around 8,5 MHz. The next zero phase transition is around 17 MHz. This is the half wave resonant point. The impedance is very high at this frequency. Then there is a zero phase transition  around 25,5 MHz. This is the 3/2 lambda resonant point. Here the impedance is low again. From the simulation graph it can be seen that 7, 10, 14, 21, 24 and 28 MHz will be possible to match with a L network. 18 MHz will have to be voltage fed because the impedance is very high.


To verify the simulations made with the Numerical Electromagnetic Code (4NEC) simulator I did some vector network analyzer measurements in the feed point end of the self supporting fiberglass mast that supports the vertical. The VNA S11 plot can be seen on the PC. The VNA unit is placed inside the tuner enclosure. (The ground plane is buried and is relatively extensive). The impedance peak of the half wave resonant point can be seen on the PC. However, there were some unexpected effects that affected the VNA measurements. I suspect that the master calibration was not good. Will have to look at that later. (The blue plastic sheet placed on the ground is laid there to be able to more comfortably work on the ground without becoming wet and dirty. The gray “ring” to the right is a concrete support for my soldering iron (ELRA ca. 1980 model still in good shape). I use a chair as a “PC support” to avoid placing the laptop on the ground. Cables to the house and control cables are routed below the surface in tubing.

How to get ARRL AAT.EXE working under XP SP3


I recently started to have problems with AAT.EXE from ARRL under XP. AAT.EXE is a DOS program used for calculating efficiencies of antenna tuners of L and T types. The user specifies QU of the inductor and capacitors, capacitor capacitance range, max voltage that the capacitor can tolerate, and some other parameters. AAT.EXE then makes two nice tables identifying where the tuner is most effective, where the tuner has a high loss, where the tuner will see a too high voltage values over the capacitor etc. The tables are generated in a .SUM file and a .LOG file. The problem is that the keyboard suddenly ddid not work in the XP dos box that is opened. I think this happened after upgrading to SP3 or after some security upgrade. I am not sure. What I did to solve it was to make a .BAT file containing this text:

mode con: cp select=865

This selects the codepage that is Norwegian. This solved the problem! If you have another keyboard layout you can find the codepage you will have to try here: http://www.kostis.net/charsets/

Here is an example of the output from AAT:


Does DX-WIRE DXW-174 meet published specs?

I Recently bought some DX-Wire DXW-174 cable from http://www.dx-wire.de/brit/ I was eager to check out if the cable meets the published specifications. (See below picture in yellow. This is the published specifications from the manufacturers DX-WIRE’s website )

To  verify the published specs, I took a roll of 100m and did a S21 measurement with my vector network analyzer. The VNA was calibrated with open, short, 50+jØ, crosstalk, thru, completely open loads. The S21 plot with dB scale and 5dB/division is shown below.


Results: the loss at 100kHz measures 1,29dB and the loss at 28 MHz measures 13,51dB.
Conclusion: this is almost exactly the spec that DX-Wire states on his webpage. I am satisfied with the accurate data published by this supplier. I therefore can recommend his product. However be aware of that 100m on HF with this cable will give you far too high loss even at 80meter. 10-15 meters should be no problem though. The loss on lower frequencies is lower than other cables made for higher frequencies with no solid copper core.