30 June 2013

AWA CW net Saturday June 29th

The net on Saturday was particularly good. Band conditions were the best we have heard for some time. All station were 599+ including Derek ZS5DM who is usually an S point or two down. The exception was ZS1JX Peter who I copied 589. However given that he is in Cape Town this was a very good report. Peter also gave me a 579 on my QRP signal which was really pleasing. Peter came back for a round using 100mW QRPp. Amazingly Andy, Barrie and myself could hear Peter's signal just at noise level. He was a good 449. Helping Peter was the fact that he has a great antenna system. I copied a Vee Beam for his transmitter and beverages for his receiver. Very FB and I hope we will copy Peter again. Stations reporting into the net included:

ZS0AWA / ZS6ADY - Andy, Benoni net control
ZS1AJX - Peter, Worcester
ZS6AJY - Barrie, Benoni
ZS6JBJ - John, Witbank
ZS6RSH/QRP, Dick, Fourways
ZS5DM - Derek, Kloof, KZN

29 June 2013

WB7AEI Audio Filter Bandwidth tests

Test Objective: Test the WB7AEI Audio Filter bandwidth with it integrated into my W1FB DC Receiver. The filter is inserted between the audio pre-amp and the LM386 audio amp.

I used the K2 transmitter set at approx 100mW transmit on the test frequency of 7020KHz.
I leaked some signal through the connection to a TEE piece of a piece of wire approx 12inches long. The RX was connected to a 51 ohm load in order to ensure proper matching of the input stage.

Test information and process.

  1. Headphone DC resistance = 12.3ohms
  2. Signal strength adjusted in the receiver for normal listening levels using terminated K2 as the signal source.
  3. Vcc= 10.9v
  4. Connected 1X probe to the speaker connection and used Channel 1 set at 50mV per division.
  5. Adjusted volume control for a peak to peak signal of 8 divisions while peaking the audio filter and input filter. Adjusted the receiver to give an approx 700Hz tone.
  6. Recorded the peak audio frequency at about 677Hz.
  7. Then tweeked the receiver trimmer for a peak to peak signal of 4 Divisions on the scope.
  8. Recorded the audio frequency at approx 637Hz.
  9. This yields a -6dB point bandwidth of 2X(677-637) = 80Hz.
  10. The design spec of the filter quotes a calculated 3dB bandwidth of 32Hz. This I concluded that the filter is working within spec.
  1. The receiver oscillator is not stable enough to obtain better more accurate readings.
  2. The signal coupling should be through an attenuator to better be able to stabilize the input signal level.
  3. More work is needed to improve the VFO stability for sure.
W1FB Receiver showing connection of 51OHM resistor to the antenna input

Rear of the K2 showing TEE piece and dummy load for signal leak

WB7AEI Tunable Audio Filter. BW at -6dB ~ 80Hz. ie in spec.

28 June 2013

The effect of supply voltage on transmitter output power.

My friend Monk related an interesting experience where his battery charger has an intermittent fault. Meaning that the actual voltage being supplied to his QRP transmitter was varying. This was resulting in inexplicable  power output variations in his transmitter.

I calculated that in his case a variation in Vcc from 13.7volts down to 9.68volts (4.02volts) will in fact result in a 3db power output change from 6watts to 3watts!

The moral of the story. Measure all voltages all the time.

A Scope test of an NE602 Osc in my DC Receiver

I had to do some basic tests using my newly acquired antique test instruments! The pics below show the scope trace and frequency meter readings when looking directly at the oscillator pin6 of the NE602 on my DC receiver. As you can see, the trace shows a nice sine wave with no 'visible' distortion. How cool is that! I am now motivated to take many more measurements of the NE602 in my DC Receiver configuration.

You will also notice that the frequency of oscillation is 6961KHz. With no probe attached the resonant frequency is 7020KHz. Meaning that the effect of the 10X probe and scope input impedance is to add capacitance that will pull the frequency down by 59KHz. This is approximately an addition of 5pf in my oscillator configuration. Assuming the capacitance is 269pf at 7020KHz then this represents a 1.9%error. Acceptable?

The following results were recorded:

Test configuration. Scope connected to the NE602 Pin 6 (collector) on Channel 1 using a 10X probe and set at 20mV/Div. The frequency meter is connected to channel 1 of the scope via a BNC connector on the rear panel.

1) Voltage reading is 900mV (p-p)
2) Frequency reading is 6961.2Khz.

Observation: The spec calls for at least 200mV (p-p) on Pin 6. What would happen if I reduced the size of the feedback capacitors on the Colpitts configuration? In theory this should reduce the phase noise on the oscillator. The problem is that by doing that I would have to redesign the rest of the capacitive components in the circuit. No trivial task and only worth it if it would result in more oscillator stability and less noise.

Next I will use the sweep B function to see if I can see any phase noise on the oscillator signal.

Good Will Instruments Oscilloscope Repairs

I was very fortunate to acquire this oscilloscope from ZS4SF om Monk along with some additional test instruments. The list being as follows:\

  1. Good Will Instrument Company 20MHz Dual Trace Oscilloscope Model GOS623
  2. Keithly Model 179 Digital Multimeter
  3. Good Will Instrument Company Function Generator Model GFG 8015G
  4. Tabor Electronics 100MHz Counter Timer Model 6003
These units are all of a high quality and are worth looking after. They were manufactured in the 1970's. I noted two issues with the scope. The first being Channel 1 not working correctly on the Vertical axis with noise and no ability to connect via the DC switch. Also an intermittent fault whereby the beam would fade after a period of operation of about 45minutes to an hour. The scope would then take about the same length of time to become operational. Both these faults turned out to be a challenge for me to troubleshoot. They both turned out to be dry solder joints which required a total strip down of the scope's sub assemblies to identify.
Since I could not find the Service Manual on the internet I called the GW office in Taiwan. They were very helpful and sent me the full service manual. Without that I would never have been able to fix the scope. Thanks also to Monk who provided input and suggestions throughout the process.

The attached diagrams and notes show the issues and solutions. I am putting this information on my blog for ease of future reference in the event that I have additional issues.

Channel 1 input to Attenuator

In Fault condition showing no HT Voltage

No Fault condition showing HV available. 

High Voltage Circuit for CRT supply showing the Oscillator and Step-Up transformer

Voltage Divider Calculations. How to measure 2100 volts with a 1000 volt DMM

13 June 2013

Elephant Speak. Infra-sound Detector

In a recent discussion with my brother-in-law he mentioned that it would be neat to be able to detect elephants communicating with one another. Apparently elephants use so called infra sound frequencies to achieve this interesting form of communication. This is a band of frequencies in the range of 4Hz - 30Hz. A band of frequencies lower than can be detected by the human ear. A search on the web found one particular paper written on the subject by some members of the University of Colombo School of Computing. This design uses a woofer speaker as the detecting device since a woofer will be more closely matched to the long wavelength infra-sound frequencies than a microphone which is of course optimized for the higher audio frequencies.

As a prototype I have built a DC coupled inverting amplifier and 2 pole butterworth low pass filter. See the details in the attached diagram below. I have set the amplifier gain at 270 and the filter cutoff frequency at  100Hz. The circuit drives a milliamp meter.

Since I don't have a woofer to hand I simply used a pair of coil movement headphones as the sensor. Initially I tried to couple the sensor into the amplifier using a power transformer. This was interesting. The transformer is meant for transforming 240VAC to 12VAC. This provides a 20:1 turns ratio. Thus I could achieve a 400 times impedance transformation. This higher input impedance to the amp meant that the amp was extremely quiet. No noise floor could be detected in some earplugs which I connected to the output of the amp. I was easily able to hear loud audio in the earplugs from the earphone 'microphone'. The circuit exhibits excellent sensitivity. The problem is that the high input impedance and the transformer picks up significant hum. So while the amp is 'noiseless' the hum is problematic. I tried putting the transformer in a metal container and screening the cable, however I was unable to eliminate the hum.

In the end I settled for connecting the mic directly to the amp via a 1000ohm input resistor. This circuit is definitely noisier than the transformer coupled circuit. However there is no hum and the circuit is well behaved. I set the gain to 270.

Then I built a 2 pole filter as per the diagram below. This appears to be working. There is no detectable noise and  the system appears quite sensitive to low frequencies. The lack of noise certainly points to the filter working. I need to look at the performance on an oscilloscope (which I don't have) to really understand how it is performing. I am able to drive a 500mA meter to FSD by scratching the earphone diaphram.

Next Steps:

  1. Build a 4Hz - 300Hz variable oscillator and amp so that I can generate a test signal and thus get some idea of selectivity, sensitivity and linearity using two woofers (one as the transmitter and one as the receiver) (my sister has two medium range speakers which I will try next).
  2. Find some elephants.
The Future
If the prototype works then there are many applications and advancements that can be implemented. The most value-add would be to interface the detector to an A/D converter so that digital signal processing can be applied. The Colombo University project utilized a 44KHz samples/second fourier transform technique which would yield the elephant language patterns. Now that would be exciting to research!

Prototype 1 _ Infrasound detector
Prototype 1 Infrasound detector

05 June 2013

A visit to ZS6AJY

I paid a most enjoyable visit to OM Barrie, ZS6AJY and his XYL, yesterday afternoon. Barrie is an avid and highly active radio ham, having received his licence 60 ago. I cannot even start to imagine the depth of knowledge  Barrie has when it comes to radio!

Barrie showed me around his well appointed shack and antenna farm. Naturally being a CW man (in fact at one time he was a professional telegrapher) he has Ten Tec vintage gear. Until recently he also operated a full station of home brewed equipment.

He also uses an old time bug and a South African post office key. I sent a few characters with this old key and immediately fell in love with it. Barrie has a collection of fine keys including a McElroy bug.

His main antenna consists of an Inverted Vee 40m, and 80m loaded antenna, plus a fan dipole for 20m and 15m. I was also interested in the installed field strength meter near the antennas.

Barrie was, until recently, a keen home brewer. He still has a number of rigs in his possession including DC Receivers, Transmitters and Control circuits plus an excellent range of homebrewed test equipment. He truly embodies the spirit of QRP home brew and what ham radio is all about. Barrie kindly passed on a number of valuable parts and books to me during my visit. These will help me greatly in my own home brew projects. Many thanks Barrie and I look forward to many more FB CW QSO's with ZS6AJY.

ZS6AJY OM Barrie
ZS6AJY's Ten Tec vintage Station

Dullstroom N Drakensberg

We were invited by our friends to accompany them to their fishing syndicate near Dullstroom. Great fun was had by all including an opportunity to try out hand at some fly fishing in the beautiful Lunsklip river. We also enjoyed some walks in the vicinity including a visit to a waterfall where the Lunsklip cascades down the escarpment to the valley below.

I deployed my usual field station including my K2. I was able to erect my EFHW from the farm house verandah to some eucalyptus trees. The wire was in the clear and up about 25ft in the center.

Condx on 40m were not as good as they have been. I received mainly 559 reports except from ZS6JBJ OM John in Witbank. I was able to copy all stations on the AWA net including the two ZS5 stations. All 599+.  I also had three SSB QSO's  including ZS4SF and ZS6AJY which was most enjoyable as a change.

Lunsklip river

Pete and Berry

Looking down at least 1000ft at the escarpment far below

Farmhouse. Over 100yrs old. Ghosts there were!

W1FB DC Receiver experiments 7

I reworked the Oscillator components in an effort to try to:

  1. Improve the stability
  2. Improve the bandspread
Both objectives were achieved.

This was an interesting learning experience. I started out with 2x150pf CAPS in the circuit but the drift was still unacceptable. I narrowed the culprits down to these two caps using my 'straw blowing' method which is extremely effective at least in identifying the 'drift monsters'. I then spent about 5 hours trying different combinations. I had decided to try to eliminate the band edge trimmer cap in order to ensure that there was one less drift contributor. This made the fixed cap selection very challenging. The lesson learnt here was that the inductor should not have cement applied to it before the oscillator is trimmed. This allows a compressing and widening of turns possibility. To prove the point I ended up with a combination of 12 capacitors to achieve the result. These all appear to be COG/NPO Caps although they are not marked as such. I noted that it appeared that the bigger CAP values appeared to drift more than the smaller CAPS. This could be supported by the fact that the bigger caps would be handling more current and thus the heating effect may be greater. Especially as the bigger value caps are in fact not bigger in physical size.

The system is definitely now stable enough for use in my 7020Khz field system. I left the rig on in the sun for a couple of hours this morning with little noticeable drift. I have not measured the frequency with a frequency counter. The trimmer will allow tuning from 7006KHz - 7043Khz which is satisfactory. I would like to lower the band by 6KHz in order to allow the option to listen to DX when the band is open. Perhaps this will be a tweek that I can do in the future. I am not exactly sure how I would achieve this in practice since a 6Khz change constitutes a fraction of a picofarad change. Still even this narrow band is not easy to tune with a half turn trimmer. The good news is that this RX will be very compact on its small board. It will be fine to tune the rig with a small trimmer.


10, 12, 18, 22, 27, 56, 68, 100, 150, 220, 270, 330, 820.

04 June 2013

Impedance Matching using L networks

Attached example transformations are included for future easy reference.

The L network is essential as a tool for interstage matching.

02 June 2013

W1FB DC Receiver experiments 6

Last week I made good progress towards a reasonably stable oscillator circuit. First I was able to procure an amount of small value Ceramic Caps from a discount store in Pretoria. I also bought a number of small value trimmer Caps. I changed out the existing circuit for the new components and tweeked the values in line with what I had. Upon firing up the receiver I was very disappointed in the result. The oscillator drifted wildly both up and then down in frequency. The caps that I acquired were clearly not COG/NPO and almost definitely not even ClassII. They certainly look like ceramics and will probably be ok for non frequency sensitive applications.

I was able to confirm that the CAPS were the contributors by blowing on each CAP with a WD40 thin straw. This was the perfect tool for this test. I can direct hot air directly at even the smallest CAP. When I did this I was able to generate a frequency shift of at least 2Khz.

I went back to a previous store and was able to buy discontinued COG hole through caps. I also bought some COG SMT caps to experiment with. I have read that these SMT caps do not have acceptable Q. To test this I will first have to build a signal generator. I replaced all CAPS except the feedback CAPS in the Colpitts circuit. The result was much improved oscillator. Next I will replace the feedback CAPS because I can see that they are still contributing.

I will also try to locate a small air variable in order to improve the ease of tuning. Bearing in mind that the goal of this RX is to primarily receive a single frequency on 7020Khz.