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.

Red Pitaya SDR TX / RX

The Red Pitaya hardware is the first low cost RX / TX capable SDR hw to come onto the market that is open source and can match the Ettus Research USRP periperhal. It has a combined CPU and FPGA signal chain with  two channels 14 bit 125 MSPS A/D and D/A. It also has a Dual core ARM Cortex A9+ FPGA (Xilinx Zynq 7010 system on chip). Only a few years ago this caliber of hardware had to be custom designed and was typically used in radar antijamming systems, radar signature classification systems, ultrasound, sonar and in high end vibration analysis tools (as examples). The ARM CPU on board can run Linux and it has GNU-Radio support. For fast data transfer there is a GBE (Gibabit Ethernet) interface to other host systems. With a a RTOS on the ARM core or a zero copy IP stack under Linux it should be possible to approach fairly close to 1 Gbit/sek transfer rates to host systems (if needed).



Contesting from CR3L (Madeira Island) with RRDXA

I am back from the megalocation at CR3L Madeira Island after working CQWW SSB with the R25 RRDXA team. It was a great weekend and we achieved good run rates however our targets were set at a higher QSO number. Since CR3L is in AF zone 33 we worked three pointers both to Europe and US and got a good score. That was fun. Claimed score looks like we will be around number 9 worldwide (a bit above 15M points). The CR3L Madeira station is located at Santana in a mountain side on Madeira overlooking salt water.  Thanks to the team! Ops: DF4UM, DH4JQ, DJ5KW, DJ8VC, DL1EK, LB3HC . Stay tuned for 2013 events.

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Claimed #9 WW score in CQWW 2012 contest from CR3L (Madeira) LB3HC in the foreground and DH4JQ in the background (we are working pileup) DF4UM in the foreground and DJ5KW in the background (working pileup)
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DH4JQ on rig A DL1EK in the foreground and DL8VC in the background (working pileup) N1MM logger
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RIG B Weather changes fast. Here the shack and the lower antenna in fog! The team after the contest
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The 80m vertical The 10m 5 el tower is “adjusted” by DL8VC! The lower Threebander and 160m ant
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Ready for takeoff from Madeira Island Approaching coastline from flight over Atlantic Ocean On the way back to mainland Europe

10, 12 and 15 meter openings? The secrets of solar data.


Many amateur radio websites show the Solar Terrestrial Data “ticker” from N0NBH. However, many HF radio enthusiasts just look at the SN (solar spot number) and perhaps the K index. The truth is that the SN does not necessary say too much about the current radio conditions. So what should you look at to check the current conditions (more realtime information)? solarvhf.php Here is the secret: I have personally found that the SFI and the 304A as well as the X-Ray values are very good realtime clues. One example is from 21.Oct.2012. This Saturday in October 2012, the 10 meter band was wide open from early morning even as far north as Oslo, Norway. The conditions on 12 and 15 meters was also fantastic. A screenshot of the N0BNH “ticker” shows SFI well above 120, 304A well above 140, K like zero and A like three as well as an X ray index of B9,9. Note that SN (sunspots) is not near abnormally high at around 112 (keep in mind we are closing in on sunspot maximum in cycle 24 so the SN will likely be around 100 all the time). One could expect average conditions by just looking at the SN. However the conditions was nothing like average. They were very very good. What was notable from that day is the following: SFI was well above 120 so there was a strong flux, K was zero so the mag field was still undisturbed, A was 3 so the mag field had been holding undisturbed for some time, X-Ray was B9,9 and this meant that there was quite strong X ray radiation coming in, 304A was 172,5 and this meant there was also quite strong UV radiation coming in, Aurora was 1 so there was practically no Aurora activity. The conditions was fantastic on this date on all bands above 7 Mhz. So again here is the secret: don’t just look at the SN for realtime information about the HF radio conditions.

G4WPW microphone connector database


G4WPW has made a nice database covering microphone adapters for most amateur transceivers. This is a good tool for contesters and DX-ers adapting microphones and headsets for their new radios. You can check out his webpage here

Live status of amateur satellites


Many radio amateurs find it difficult to find the real-time operational status of amateur satellites. The AMSAT website is not very good at showing real-time status in my personal opinion. If you have been looking for a way of finding the status of the birds like me, head over to for a real-time status based on thousands of reports from radio amateurs all over the world.

Currently these satellites are active with transponders (FM / SSB / CW) : AO-7 (launched in the seventies!), AO-27, FO-29, SO-50, VO52