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.

Azores Island Hunt. Captioned pictures from CU2ARA

The teams are arriving at the airport in Ponta Delgada at Sao Miguel island. A lot of luggage was brought in. Here the
Danish and German teams are waiting for their taxis. The DARC journalist is checking his photos on the digital camera as well to the left.
A too small car for rigs, linears and antennas when 9  teams arrive at the same time…
A team photo was taken in the backyard of the CU2ARA club station before teams were departing to their individual islands
Antenna and rig discussions are taking place in the backyard. The short CU2ARA tower that we used can be seen in the middle of the picture
Our guide Mr. Rui is also a pro photographer. Just look at all the cameras!
Ghis ON5NT is busy adjusting the inverted vee antenna to resonance
Marius, LB3HC is using his DG8SAQ Vector Network Analyzer to check the multiband antennas before the event.The CU2ARA members CU2IF and CU2CN and are helping out
The organizing committee is formally opening the event!
IMG_1080 Since we had a city location with some noise, we wanted to do a remote hilltop station experiment to learn from that experience for future events. After first checking with the official organizers for  approval, we travelled to one of the points in the island where there is almost 360 degrees negative horizon and no broadcast installations.This would enable a good remote location. On the above picture you can see the takeoff towards Europe. Wow! We did have some technical challenges that were solved, but the main QSO amount by far was made with the main stations down at CU2ARA. The remote station was left operational so the CU2 ops could do more work on it after the event IMG_1127  Above:
CU2CN climbs the tower on the mountaintop to put up the highest point of our sloping antenna. The antenna was sloping towards west (US).
Here is the house where the experimental remote station was mounted. We had a 100mbit/sec WLAN connection down to CU2ARA.
Another picture of the takeoff to the east (against Europe). A pretty good QTH for the remote site.  (The Azores are full of beautiful views like this. Visit to see for youself!)
Our remote station is located inside the hilltop house. A Yaesu FT857 and HRD was used. More dedicated remote systems should be used in a future event it was decided.
Here is the HRD remote screen where we controlled the hilltop remote station. This was done down in the city where the CU2ARA shack is located. (As OH2BH encouraged, we did it the innovative Arcala way!). Notice the Norwegian flag by the way!
ON5NT is working pileup
LB3HC is working pileup
The CU2ARA residents are working pileup
Our antennas downtown at CU2ARA. We used a 3 el yagi for 20 meters and inverted vees for the other bands (17 and 40).


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.

PL519 HF linear amps by PA0FRI and EA6AFJ

PA0FRI and EA6AFJ has made some cool low power HF tube amps with the old and thrusty PL519. Here is a picture taken by EA6AFJ.


This is actually a nice amplifier that should be quite easy to build for the experienced engineer and experimenter. The benefit of using lower anode voltages is that the tank circuit capacitors can be of a low cost variable type. The PSU can also be integrated more easily in the same cabinet as the amplifier circuit. You can check out more information over at PA0FRI’s webpages:

Good HF condx. A lot of DX worked lately.

The conditions lately have been very good on 20, 17, 15 and 12 meters. I have worked a lot of new stations on the HF bands above 40 lately. Finally the nice sound of a wide open high band that I remember from last time the SSN stayed above 100 is here again.

The SSN peaked above 100 between 12. and 13. April. The SFI also peaked around 14.-16. April. The K-index also stayed low and the auroral activity has been modest. Perfect settings for working DX!

Antenna: broadband vertical with computer controlled tuner. Large groundplane.
Rig: Yaesu FT-1000MP MKV
Power: 200W