Testing of a HF current transformer with a vector network analyzer

I wanted to check the load that a ferrite core with a secondary winding presents to the common mode current carrying conductor (the outside of a coax) when set up as a current transformer. The coax runs thru center, the secondary winding could be one or several turns loaded by a R+jø load.  (+jø load but the windings of the secondary will give some +jx component).

I did the following three S11 measurements with the VNA (M1, M2, M3):

M1) No secondary winding is present but the single turn is running thru the core

M2) A shorted two turn secondary and an open two turn secondary (winding is pulsed in and out off to be able to better detect a difference). The time domain is captured by the slow scan and sampling rate of the VNA.

M3) Only the primary winding is attached to the calibrated S11 measuring plane. (to measure the self inductance of the single turn in itself. This is an air-core measurements.)

Above is a similar test setup. The core is an “unknown” Amidon toroid core. The red conductor is the simulated coax common mode current path (a one turn loop). The green conductor is the secondary winding. The load could made by paralleling several resistors in series with the secondary (the blob on the left side, right picture). Note that the measurements below were done with a one turn loop and a short – no resistor. The measurement device is a VNA from DG8SAQ calibrated by O S L  references in the S11 measuring plane (the SMA in the end of the coax from the TX port).  
The left picture shows measurement 1) The mid picture shows measurement 2) the right picture shows measurement 3)

Edited: What is interesting to see is that the image to the left shows that this core has some resistive loss as can be seen on the blue trace. The Q is quite low. When there is a secondary winding present, the loss is shorted out but the inductancechanges since the inductance of the secondary is reflected into the S11 measurement plane. On the right image it can be seen that there is no resistive loss and a linear inductive reactance caused by the air core inductor (no appreciable drop off or frequency dependent effects, in the measuring frequency range)

What this tells me is that this core setup probably is not too well suited as a high current measuring setup for frequencies above 160m because it will affect the measuring circuit too much. Not in terms of the R element but because of the +jX element. The R is low to the RF current passing thru the core (in the common mode), but the +jX element will present a reactance to the RF current and thereby giving you lower current than should be expected without the core.  My analysis says that another core type should be selected or that a frequency compensation technique should be used. Alternatively that a lower turns ratio should be tried. However looking at the plot to the left, it can be seen that below 30 Mc*s^-1 the +jX component is too high. Perhaps this core has a too large permeability and that a lower permeability core should be used. The primary turn with secondary loading should give a low reactance on the primary. The voltage given over Rt would then be lower but the gain of the detector could be adjusted. (I may be wrong). Please comment if you have comments or suggestions.

Collinear airband antenna design

I wanted to test a simple airband antenna design since I had a roll of balanced feedline laying around in my shack doing nothing useful. First I had to measure the velocity constant of the feedline with my MFJ-259 to be able to come to an estimate of the required length of the matching section. I could also have used my vector network analyzer (VNA) to do that by the way.  To measure without interference from coupling to adjacent objects I did the measurement with the cable hanging out from my balcony as you can see in the left picture. The MFJ-259 was connected in the end and held by hand (that is a benefit of the battery operated MFJ 259 even if the instrument is not of the most accurate on the market). I wanted to make two antenna segments folded over each other. Therefore the top of the feedline is shorted and the currents will be in phase if the antenna is of a proper length. The matching section is a shorted line section that is tapped by the transmission line. The coil on the coax is a choke (I haven’t done any measurements on that choke yet by the way).

DSC_5052DSC_5054DSC_5074  DSC_5076DSC_5064DSC_5050

The procedure I used to find the velocity factor of the balanced transmission line was to first measure a length of the feedline with a tape measure. Then I connected the MFJ-259 and found the frequency where the lowest reactance could be measured. (See pic two from left on the upper row above. You can see that the X is very low). This was done in the mode of the MFJ-259 where it is possible to measure both R and X. This is the quarter wave frequency of the line when the wave propagates in the line – not in the free air (“ether”). Then I calculated that frequency back to the wavelength with  y=300/f. I then divided the tape measure length by the calculated length and came to a velocity factor of 0,89. This is the ratio of the wavelength in free air and the wavelength in the transmission line. This is directly related to the propagation speed of the line when it operates in transmission line mode. From that I calculated the required length of the matching transformer and the approximate tapping point on that transformer to reach 50 ohms. Please note that you cannot use the velocity factor of the transmission line to calculate the required length of the antenna (only the matching section), since the RF currents on the two folded legs on the antenna are in phase and therefore the one lead is coupled to the ether and not to the other lead.


The picture above shows the SWR as measured with my vector network analyzer from DG8SAQ. The markers on the right side shows a 1:2 SWR bandwidth of 118,5 to 128,6 Mc/s which is OK. The reference level is 1:1 SWR. This level is lifted one division for clarity. (I think the Mc/s  is a cool way to express frequency by the way.)


Here my DG8SAQ1 kc/s to 1,3 Gc/s VNA is shown. It is connected to my PC via USB.

Conclusion: a combination of the MFJ-259, the DG8SAQ vector network analyzer, some balanced line and some coax can be used to make a good collinear airband antenna in less than one our at a cost of a few dollars. The antenna was screwed to a wooden section of my roof by a small screw by the way and can be removed in approx 2 minutes.

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: http://pa0fri.home.xs4all.nl/Lineairs/Frinear150/fri150eng.htm

SMD resistor lab kit


Finally Elfa has increased their range of SMD lab kits. It is somewhat difficult to select the kits from their webpage (that detoriated after they started to use SAP). The manufacturer Nova has a website with better information. You can check out the resistor kit pictured above here http://www.nova-elektronik.de/en/compcards/chip0805.php 


Nova also has capacitor kits. Their SMC-36 kit contains 6030 pcs. SMD ceramic capacitors in size 0603. (6 mil x 3 mil). The range is E6 to 4,7pF with CØG dielectricum. Then they have a 6,6 pF to cover the gap and after that the kits includes the E12 series up to 680 pF. This also CØG dielectricum. Wikipedia has some info about C0G diectricum here: http://en.wikipedia.org/wiki/Ceramic_capacitor You can probably use < pF values up to approx 1400 Mc/s (Megacycles per second = 1/p = Megahertz, p= period) before hitting the self resonant frequency.

By the way the information in Elfas catalog is inaccurate in a lot of areas so make sure to do research before you order from them. For example they stated that the above resistors can dissipate 1W. The manufacturers datasheet says 0,1W. Only a factor of 10 wrong. (Probably due to that incompetent spotty teenagers are making their catalogs these days, instead of engineers?)

Sending hellschreiber from an Arduino –> Helldunio!


Mark over at www.brainwagon.org has posted some interesting info about sending hellschreiber from an Arduino (see Marks image above). He has used KD1JVs oscillator circuit that was originally used for the wireless morse code thermometer project. Mark has written code for the Atmel Tiny 13A uC for Hellschreiber transmission. You can find code on his website. Below you can study the schematic designed by KD1JV ( you can find more info over at KD1JVs site http://kd1jv.qrpradio.com/temp2morse/temp2morse.htm).



The way this circuit works is that the XTAL oscillator (left) is powered on and off via PB3 output from the Atmel uC. The emitter of the 2N3904 has been tapped with a 6” antenna. Here a DS18B20 Maxim temp chip is used for accurate temp sensing. The temp is sent out via Hellschreiber together with the callsign. The circuit could be expanded with a buffer tapping the signal from the emitter of Q1. That buffer could feed a power amplifier (well filtered of course). Then a Hellschreiber telemetry beacon could be set up. Cool idea!

HF converter for SDR Funcube dongle


Tony CT1FFU and Diogo CT2IRW has released a converter for the Fun Cube dongle. The design is a based on the old NE602 workhorse. One new approach is to use a wideband MMIC amp in the front end. The unit is powered via a USB connector. Looks like this may be a good add on if you want to do some basic HF monitoring with the Fun Cube. You can find more information here: http://www.ct1ffu.com/site/index.php?option=com_content&view=article&id=178&Itemid=104

Selected photos from OH8X, the megastation in Finland

By popular demand I have posted some pictures from OH8X, Radio Arcala. Enjoy!


This picture (above) gives a good overview over the antennas at OH8X. You can see the M7 and the M1 towers stand out. Notice how small the M6 rotatable tower looks. The M6 tower in not small in real life its 32m high.


This is how a real stack should look (above). Notice the icy elements. The orange cables inside the tower is for operating the ice knockers that keeps the elements free from ice and snow. (Snow turns to ice etc). The tower is fully rotatable.


This is the 5 el yagi on 80, 3 el yagi on 160 and 4 over 4 on 40 (above). The tower is rotatable. It weighs approx. 40 tonnes. The rotor sits in the bottom of the tower and the rotator gearbox is BIG!


This is the correspondent LB3HC calling in to the shack (via cellphone and not VHF for the occasion) to ask for a rotator turning operation to check proper rotation of the tower before the ARRL CW contest. The tower in my view that is… (behind the camera). The other towers speaks for themselves in the background.

The guys that built this station are extremely skilled. Kudos and congratulations to the Arcala team!
You can find more information here: www.radioarcala.com

PowerSDR and Flex-Radio offerings becoming prime contest tools


FlexRadio systems is launching new products and new improved software all the time. I have been following the SDR scene for several years now and experimenting with the Softrock 40 and softrock RX-TX as well as some earlier VHF SDRs has been fun.

I must say that the offerings from flex-radio is now becoming more tempting than many offerings from traditional suppliers like Yaesu and Icom. My FT-2000 with the latest software is surely a great radio (the best I have had so far) but I think a FLEX-5000A with diversity RX and the newest PowerSDR software must soon be tried in a serious contest effort. The filters, diversity functions and FFT bandscope functions are simply great. One thing that lacks are the feel of real buttons and controls. But that has also been taken care of (read below)

Here you can see a video from Flex-Radio where they take the new tracking notch filters (TNF-RF)  for a spin:

In case you lack the feeling of a real radio, DH1TW is here demonstrating the use of a DJ controller for adjusting VFO and other radio functions:

Surely great stuff! Stay tuned!

How to upgrade the firmware of your Funcube dongle

I had some problems of getting my Funcube dongle (purchased dec 2011) to work with SDR-RADIO version 1.5 build 879 (beta). The reason was that the funcube dongle didn’t have the latest firmware.

The www.funcubedongle.com webpage is a bit unclear about how to do the upgrade, so here I have written up how to upgrade the software.


1 ) Goto www.funcubedongle.com

2 ) Select downloads

3 ) Download Windows fully functional front end

4 ) Download Boot loader with source code

5 ) Unzip those two archives in a suitable directory. (If you need a free zip tool, you can use the free WinRar software that you can download from Rarlabs homepage http://www.rarlab.com/ )

6 ) Make sure you close all other SDR programs that may acess the funcube dongle. This is important so that the funcube is not “occupied” by another program

7 ) Plug in your funcube dongle to one of your USB ports

8 ) Wait 10 secs and listen for the “bling” sound that windows sends to signal that a new USB unit was detected

9 ) Start the FCHID.exe file (the front end)

You should now see something like this –>


10 ) Click the read device. You should now NOT see any error message. In case the read went ok, you now have a connection to your funcube dongle

11 ) You now want to set the funcube dongle in the bootloader mode. This means that the funcube is ready to boot from new software.

Here comes the confusing part: you must start another piece of software to upload the new firmware to the funcube. You cannot do that from the FCHID.exe file. The software you use for uploading the firmware is FCHIDBL.exe

12) Start FCHIDBL.exe (from the CHIDBL___Win32_Debug subdirectory where you unzipped the Boot loader with source code ) file from point 4) above

13) Download the latest firmware from Firmware v18i – Save it to the same directory as you saved the other files.  NOTE per Jan 2012 this was the latest software but this may have changed when you read this !!! Check what is the latest file before you upgrade!

14) Now start the FCHIDBL.exe file. You should see something like this:


15) Click the Open file button

16) Select the *.bin file you saved earlier (the *.bin file contains the firmware)


17 ) At this point -> if you are on a laptop make sure your laptop battery is OK or that the laptop is plugged into a AC adapter, so that you don’t risk that your laptop shuts down while you flash the firmware to the Funcube dongle.

18 ) Click WRITE FIRMWARE button. Now click RESET TO APP to get out of boot loader mode (Thanks to Jeff Murri for this tip).

19 ) Close both programs you opened earlier

20 ) Wait 10 secs (just in case)

21 ) Plug out the funcube dongle  (Probably overkill, but hey…)

22 ) Wait 10 secs (to let windows have time to unload the driver. Not sure if this is really necessary. But hey …..)

23 ) Plug in the funcube dongle again

24 ) Congratulations, you have upgraded to the latest firmware !

Receiving wideband FM with the Funcube dongle


I have recently acquired a Funcube dongle software defined radio. The Funcube dongle is a small USB unit that contains a E4000 Silicon Tuner (radio on a chip), a TLV320AIC3104 Audio Codec and a Microchip 24FJ32GB002 16bit Microcontroller. This small USB device gives me coverage from 64 to 1700 Mhz with some small gaps according to the manufacturers data. You can download more information here: http://www.funcubedongle.com

As a windows SDR RX I am at the moment experimenting with the sdr-radio program that you can download here: http://www.sdr-radio.com

The picture above shows a FM broadcast transmitter in the Oslo area. You can also see the pilot carrier that is used to encode the stereo information if you look carefully