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.

IMG_0716_w800

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

image

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
aat.exe

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:

image

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.

 Dx_wire_loss_100m

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.

Is lead based solder banned for all electronic purposes?

KESTER SOLDER24-6040-0066Many sources on the internet seems to indicate that lead based solder is no longer possible to purchase and is in fact banned for use in electronics. However, this is not the case. It is correct that the EU has passed a regulative that prohibits the use of lead based solder in new consumer electronic products. However, the use of lead based solder for repair of older equipment is still perfectly OK as far as I know. Also, new military electronics is ok to manufacture with lead based solder. I was starting to worry about soldering problems that may affect many amateur radio projects like soldering PL259 coax connectors, after my supply of solder went out. With leadfree solder, a much higher temperature is often necessary to use. The center of the non teflon PL259 connectors then melts and several other problems occur. The leadfree solder doesn’t flow as well as lead based. I us the 60 Sn / 40Pb variant that has been the standard for decades. Farnell sells it and has it in stock.  I have replenished the stock to last for several years in different thicknesses so i have for SMD, hole mounted, plugs and larger devices.

Here are Farnell’s ordering codes for good old lead based solder:

1610446 SOLDER, 40/60 2.36MM 453G;
419310 SOLDER WIRE, 60/40, 1.63MM, 500G;
453614 SOLDER WIRE, 60/40, 1.0MM, 500G;
5090787 SOLDER WIRE, 60/40, 0.5MM, 250G;
5090830 SOLDER WIRE, 60/40, 0.7MM, 500G;

Just go ahead and order so you have solder supplies for hobby use for 30 years. Not easy to know what the bureaucrats in EU will think up next!

Analysis of a tank circuit and notch circuit with the DG8SAQ VNA

image series lc dip IMG_0459

Experiment 1: I did an S11 analysis with my VNA on a 500pF transmitting capacitor in parallel with an airwound copper coil of good cross section. The question is how to interpret the first picture. The voltage over the parallel circuit is in phase with the drive voltage and almost as high in amplitude as the drive voltage when there is a real component present. (Blue trace, left image). The drive voltage cannot drive a current when the circuit is in steady state oscillation since the circuit has almost the same voltage over it as the drive voltage itself. Low voltage differential, results in low current. This is the definition of high impedance. Low in frequency the resultant voltage is not in phase however the voltage amplitude close to the resonance is almost as high as the drive voltage. The circuit presents a inductive load. Above the frequency there is a change from inductive to capacitive load. (Red trace). It looks like the phase changes abruptly, but I interpret it as a high impedance that occurs only due to a change from high inductive to high capacitive voltage. The phase goes “over the top” but only since someone defined capacitive reactance as minus. The impedance is still high and almost real close to resonance. |Z| represents the resistance to AC current in this context. It is the length of the R+jX vector.

Experiment 2: after that I did a S21 sweep of the same components in a series notch configuration as per VE2AZX’s paper http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb_2012/QEX_1_12_Audet.pdf  to try to determine the Q (of the coil primarily – I assumed the Q of the capacitor very high compared to the coils Q).

The second picture gives the notch depth so the Q can be calculated as per VE2AZX’s QEX paper http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb_2012/QEX_1_12_Audet.pdf

Note1: the picture to the right is of the S21 test, not of the parallel circuit and the frequency may have changed slightly as the coil were changed between the two experiments if I recall correctly.

Note2: don’t worry about the long leads. This is an exercise in understanding the design and operation of antenna trap circuits and loss in reap circuits due to resistive losses in the inductor primarily. The leads would be almost as long in a real trap.

Antenna isolation calculator for colocated contest / DXpedition antennas

 image

I have made some quick isolation calculations (dB) for the scenario of two rigs with 100W output power collocated using two dipoles mounted some meters apart on the same support but with separate feedlines.

A Preliminary conclusion is that a BPF + notch is likely necessary for interference free operation. With good filters and notch, the second harmonic will be at a  7uV level and the hash will be at a S1 level on the antenna connector of RIGB which is equivalent to a moderately strong signal on 20m with low noise.  Without filters, the second harmonic will be S9+50dB. The phase noise "hash" will likely be at S3. This will likely be a significant problem.

Download the calculator here: isolation_calculator_nine_islands. The calculator is posted under a Free Beer license. You owe me a beer if you use it! Please give feedback and peer review the calculations.

Pictures from OH8X WPX SSB 2012

Here are some pictures from the WPX SSB 2012 at Radio Arcala. Enjoy!

DSC_5491_w800_txtDSC_5510_w800_txtDSC_5437_cpd_w800_lb3hc_v2_txt DSC_5535_w800_txtDSC_5519_w800_txtDSC_5534_w800_txtDSC_5470_w800_txtDSC_5462_w800_txtDSC_5274_w800_txtDSC_5283_w800_txtDSC_5351_w800
DSC_5342_w800_txtDSC_5418_w800_txtDSC_5277_w800_txtDSC_5306_w800_txt

                   CQWW WPX Contest, SSB

Call: OH8X
Operator(s): LB3HC, LA7JO, CU2DX, CU2CE
Station: OH8X

Class: M/S HP
QTH: Arcala
Operating Time (hrs): 48

Summary:
 Band  QSOs
------------
  160:   36
   80:  106
   40:  478
   20: 2259
   15: 1082
   10:  178
------------
Total: 4139  Prefixes = 1379  Total Score = 11,906,286

Club: Contest Club Finland

Comments:

Great to be back operating from Arcala now that the SSN is higher! All
operators: LB3HC, LA7JO, CU2DX, CU2CE had a great time and a lot of fun (as
usual from OH8X - Arcala). 

Thanks to OH2BH, OH8NC and OH6KN for hosting us! Also thanks to the rest of the
Arcala team for making this possible!

This time we also had time to take some HD video and wide angle pictures of the
station.

Plusses:
=========
This time we had relatively good conditions up here at 65 degrees north
latitude in the ice and snow.

No significant technical issues were experienced.

This station station is professionally built by the extremely skilled guys in
the Arcala team. Kudos!

The Arcala antenna park is nothing less than extreme.

It was nice running with the new Yaesu FT DX 5000. Yaesu did it again! 


Minuses:
=========
We were likely affected by aurora at several time periods during the contest
that affected rates. We were unable to achieve runs at 40 and 80 to DX
locations and that resulted in that the DX runs was worked on 20 and 15. That
cost us 6-3=3 points per DX QSO and our average QSO points were below target. 

Our target was to beat OH10X from 2011 and we wkd more mults, but somewhat less
QSOs corrected by the QSOs/36*48 factor due to condx and latitude (claimed
score). In fact 40 meter did not give good DX propagation at all.  OH10X is
approx 500 km to the south. 

We lost power due to a power company outage / spike at night and lost control
over all tower rotators and the voice keyer. It took several hours to correct
this.

Summary:
=========
WOW! What a station and what a team. Contesting has got a new meaning. 
Updated pictures will be posted at http://www.lb3hc.net

On behalf of the team 
LB3HC
Marius

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.)

DSC_5192DSC_5189
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).  
singleturn_thru_corepulsed_short_2t_choke_current_trafosingleturn_NO_core
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.