28 April 2014

Kruger Park Satara, Pretoriuskop and Berg en Dal camps

Back from our morning drive near Satara on Saturday morning, I was on 40 meters at lunchtime with a 569 report from Barrie ZS6AJY. Band conditions were definitely down from the evening before. I then checked into the AWA (Antique Wireless Association net) and met Andy ZS6ADY and also a call from ZS6PTA  (Pretoria Radio club), Ray who was looking for points towards the QRP contest.

On Saturday evening we went for a drive towards Orpen Gate. Fantastic! We witnessed two young male giraffe 'necking' in the middle of the road. We watched them tangling with each other for quite some time competing for dominance. Thereafter we returned to camp for an evening braai under the stars. The  piri piri chicken was certainly spicy but went down well with a couple of glasses of red wine....

On Sunday morning we were up bright and early and heading along the S100. There they were! Two massive male lion walked out of the bush just in front of our car and crossed the road.

One of the large male lions crossing the road in front of our car.

We were treated to a wonderful private viewing. We switched off the car engine and watched them walking silently down to the river for a good drink.

At 9:30am I had a QSO with Monk ZS4SF. He was using a vertical half wave. His signal was definitely down (559). But he had worked some PY (Brazil) stations earlier. This is definitely a DX antenna.

We then headed for Pretoriuskop camp about 150 kms to the south, stopping for a pie and ginger beer lunch near Skukuza atop a granite outcrop 'called a dwala where we come from' with a 360 degree spectacular view of the veld.

Pretoriuskop and the surrounding bush has a much different feel to Satara. This being thick bush with fewer game. Ideal Rhino country. We checked into a rondavel with a great view of an open space area looking towards the swimming pool which is built into the side of a granite outcrop. Most picturesque.

My end fed antenna here was excellent. Completely in the clear. A sloper configuration. One end at 20ft and the other at 8ft. The band at 06:15 pm was in good shape with 569 reports received from Barrie, Bruce ZS6BK and Evert ZS6AQW (599). Evert  was using a fb older rig which I believe was a Halicrafters but not completely sure. QRM was high but the band was open for mid-distance 500Km QSO's with no problem.

Lamb Chops Sunday evening. Oh how we will miss South African lamb chops when we return to the USA!

On Monday April 21st I was up early, before the sun, listening to DX. Initially South America and then North America as the sun was rising. By 7 am the band was short enough that I was able to establish excellent contact with Peter ZS1JX in Cape Town (559/QSB), Barrie and Monk. We were surprised that the band was so good so early in the day.

End Fed Half wave wire at Pretoriuskop camp. In the clear and high
After an excellent hearty breakfast of roosterkoek, bacon and eggs at the outdoor bush restaurant I spent some time optimizing my antenna. I was able to get both ends up above 20 ft and completely in the clear. Judging by the 589 reports from Monk ZS4SF, this was a superior antenna. We spent about an hour comparing signal strength's from ZS4SF as he tried different lengths of end fed wire on 40m. The conclusion was that a true half wave of 66ft 4 inches yielded marginally better results when compared to a better balanced 61 ft antenna. This was an interesting set of experiments.

The view from our hut at Pretoriuskop

 At 11 am om Pierre ZS6A gave me a 579 report. It was nice to talk to Pierre again.

After a leisurely time spent swimming and exploring the camp I was QRV at 3pm. QSO's and good signal reports were made with Eddie ZS6BNE who was running 15 Watts, and another round with Monk. Thanks chaps!

Our evening game drive was fully rewarded when we came across two young male elephants fighting for dominance right in the middle of the road. We spent about 30 minutes watching them clash and push against each other in a show of strength. Eventually one of the young elephants stormed off into the bush and started to push over a number of small trees as if to emphasize his superior strength. At one point we were obliged to reverse the car to give then space. Certainly we had never experienced anything like this before.

That evening back at camp I enjoyed my contacts with Eddie, Evert and Monk before a dinner of braaied borewors and a glass (or two) of good red South African wine. Good night...

On Tuesday morning after a relaxed breakfast we set out for our final destination in the south of the park near Malelane Gate. A camp called Berg en Dal. We had about 60 kms to drive. Soon we were amongst a large group of elephant feeding on the lush vegetation.What a pleasure it was to watch these majestic creatures going about their daily routine in their own habitat.

By lunchtime we had arrived at Berg en Dal. Here in this lovely wooded camp we had the pleasure of meeting up with my brother Ed and his wife Linda. We spent an enjoyable few hours catching up on news while we sat in their caravan camp overlooking a large river. 

I deployed my antenna into some nearby trees and reasonably in the clear. Up about 20 ft. and sloping down to 15 ft. I received a good signal report from Monk before we headed out for our evening game drive.

We could not believe it when a young Leopard casually walked out of the bush in front of us and ambled down the road to the drift crossing the river below. What a magnificent sight! We crept down the road to obtain a second glimpse before the leopard was gone. This was the first time Berry and I had seen a leopard in the wild. Memorable indeed.

On returning to camp I had a nice chat with Barrie and Monk at 6pm. Excellent band conditions once again.
After an enjoyable evening with Ed and Linda we crept into bed for our last night in the bush. Hyena could be heard throughout the night calling from somewhere deep in the bush. When would we hear that sound again?

I awoke early on Wednesday, our final morning. It was still pitch dark when I called CQ. To my surprise I was answered by Adrian ZS1TTZ at 05:45am (439). The band was wide open to Cape Town and Welkom! After an enjoyable chat with Adrian and Monk, Ed and I were at the gate by 6 am for another early morning drive. This time we were rewarded by the sight of three Rhino sleeping side by side sardine fashion and a Hyena standing quietly by the side of the road. Surely the litter must have been nearby. 
What a lovely end to a wonderful trip!

Hyena near Berg en Dal
Rhino sleeping 'sardine fashion' at Berg en Dal
Our hut at Pretoriuskop


The operating position at Berg en Dal
A kudu bull with a large pair!


The little table is perfect for QRP operations and can easily be moved around

My lovely wife Berry. How can I ever thank her for her tolerance of my hobby :)

QRP Field operations. Lessons learnt from the Kruger trip and many other field trips

Herewith in this blog are some of the practical lessons learnt on the Kruger trip and other field operations during my time in South Africa. These observations pertain to the deployment of a end-fed wire halfwave (#18 awg) and a counterpoise of 8ft 6 inches in length and the use of a tunable tank circuit as the coupler.

These comments relate to a simple operation where the aim is to get on the air easily in order to make local (South Africa wide) contacts and then to take down the system easily and quickly after a day or two of operations. It is assumed that a minimum system has been packed with very limited spares. In my case my whole station plus antenna and accessories easily fitted into a plastic container the size of a shoe box. Refer to my previous blog showing pics of the packed station.

For this type of deployment it is absolutely NOT necessary to try to get the wire up more than 15 - 20 ft in the air. Attempting to get it higher will almost guarantee difficulties in the form of multiple launch attempts, tangled cord and wire, frustration, and if it goes really badly, the loss of the whole antenna system.
  1. Be very patient when un-winding the cord and antenna wire. The total length of my 40m system is 66ft of wire with a 60 ft cord attached. If you let this get in a tangle then it can take another 20 minutes to un-tangle it. I wind this up over one hand and turning the wrap around every 10 winds or so to prevent it twisting. This can be easily un-wound by releasing each turn carefully. Once the wire and cord is lying on a nice bare piece of ground, make certain that the coils are laid so that the end of the cord is at the 'top of the pile.' It will then uncoil during the launch without incident. Use slippery plastic cord as found in the hardware store. 
  2. Make sure that the end of the wire that will connect to your radio is fixed before launching. Preferably tied to a convenient tree branch about 8ft above the operating position. Here I am assuming that the most likely deployment will be a sloper with one end up at 20ft and the other end up about 8 ft. Trust me it is no fun to watch the whole system end up in the tree. Been there done that!  
  3. If an error is made in the launch and you need to retrieve the cord then disconnect the weights before pulling the cord back through the tree. Do this every time and don't take a chance. The weights will hang.
  4. Consider a spare set of detachable weights. The chances of hanging in a tree are extremely high. The most likely is a situation where the weight and cord does not drop to the ground after the launch. Now you are forced to retrieve the cord by pulling it back through the tree. Very often the weights will snag in the 'Vee' of a branch joint making it impossible to retrieve. A good strong cord is an asset in this case because heavy pressure can be applied in an effort to try to dislodge the weight.  
  5. Spend time and be patient in carefully selecting the tree to launch over. Preferably launch over a tree as opposed to trying to select a specific branch. The tree is much easier to aim for. An ideal launch tree would be about 25 ft high. Select a tree that has smooth branches and few leaves if possible. Do not select an Acacia thorn tree! Optimize the tree selection taking into consideration the fact that you want to be QRV quickly, you want the weight to drop to the ground and you want to be able to easily recover the wire and cord after the operation.
  6. 1.3Ah battery easily lasts a week provided it is in good working order. I had many lengthy QSO's during this 5 day trip. This assumes the use of minimum power on 40m. In my experience it is easily possible to make regular QSO's throughout most of the day on +500km path lengths using 1.5Watts output power maximum. Refer to the battery power measurements below.
  7. Try to get as much of the antenna up in the air and horizontal to the ground. Ideally both ends at 20ft above ground between two trees. An inverted L with 46 ft horizontal and 20 ft vertical. It is amazing how much this configuration improves the signal strength. This is why I carry two cords. The same set of weights can be used to deploy each cord.
  8. Put some plastic putty over the joint between the cord and the antenna wire. This will allow it to glide through the tree easier.
  9. When laying the 8ft counterpoise along the ground be sure that it is not a safety hazzard. It is easy for people to trip over this wire.
  10. Pack away the radio and accessories each time after use. Leaving it set up is an invitation to the monkeys to pull the counterpoise and to try eating the battery.
  11. Take a small portable plastic table. One of those small green ones. They pack easily in the boot and allow the operating position to be set up comfortably under a tree. It can also be moved as the sun position changes.
  12. Consider using a length of coax between the radio and the tuner to avoid RFI issues. I certainly experienced RFI/common mode on this trip on one occassion when I had the tuner directly connected to the radio. I believe the end of the wire and tuner were too close to the radio.
  13. Build the coupler with a link switch so that you can connect the coax/ground to the counterpoise end of the secondary coil. Sometimes this will improve the match. Sometimes it will cause RFI.
  14. Make the coupler tunable with a small variable capacitor. The resonant point can change by up to 30pf depending on deployment conditions.
  15. If possible try to get the wire in the clear. However if it has to snake through the tree it will still work fb and no problem.  
  16. If possible have a variable power output rig so that when conditions are good you can reduce power and save battery. In South Africa the rig only needs to operate on a single frequency of  7020Khz CW. A simple DC Rx can be used with RIT and an attenuator. BCI is not an issue during daytime operations, however it is an issue during early evening unfortunately.   
  17. Always carry a Leatherman of Swiss Army type knife. Preferably with one of those spikes that used to be used to take stones out of horse's hooves. This spike can be used to loosen tight knots. The knife blade is useful for cutting off snake's heads and for general defense.
  18. Wear a hat and stay hydrated.
  19. Look after the XYL and the kids.
  20. Enjoy!

Audio Record by Jan ZS6BMN. Thanks Jan!

Audio Record by ZS6BMN.

Transcript of email received from Jan ZS6BMN.

"Hello Dick,
It was good to hear this morning that you were safely back in Fourways :-)
I have followed your Kruger Park trip/adventure all the way (with atllas and maps too) and it was obvious that you were enjoying the Park, the game and the QRP operation tremendously! Thank you for all the interesting CW operation from the Bush. I cannot recall when last there was such a lot of CW activity on 40m and also never realised how well the band was performing over these medium distances early in the evenings. Your 1.5 W Norcal did a sterling job and has often peaked at S9 on my simple setup with the small indoor whip antenna and TS-140S. Just a great pity that I was without an antenna as I have heard you calling CQ when nobody else was around and you thought that you were not getting out. Still, I have enjoyed listening and must thank you for again for providing such a lot of listening pleasure and for giving QRP operation so much positive exposure! You have even managed to get my old friend Evert to operate 40m after dark :-) BTW: I have listened on the little Rebel to his last QSO with you and then his signal was the first CW signal that could register a reading on that set's stingy S-meter - all of S2, but the reception was great and I could even hear the DX in the background. The Rebel was set up with the single band whip in the dining room while I also had the TS-140S running in the 'shack'.
I have a great recording of your first early evening's operation with Monk, Barrie and Dave all calling in. Later that same evening OM Monk, OM Peter, ZS1JX, and you were calling at different times on the same QRG and just missed each other. At that time OM Monk was putting in the strongest CW signal that I have heard to date and it has sounded like FM with 100% quieting! I have had a lucky break in that the noise was completely gone that day and the S-meter was idling at S1-S2 on background noise.
So all-in-all in the terminology of "1066 and all of that" your enjoyable weekend in the Park has been a 'good thing' for many others as well :-)
73 for now!"


MEASUREMENT                            START OF TRIP                     END OF TRIP (after 5 days operations)        
Battery Voltage Radio OFF                   13.01V                                        12.3V
Battery Voltage Radio ON                     12.99V                                        12.22V 
Battery Voltage KEYDOWN                  12.75V                                        11.92V
Current RX                                           19.9mA                                       19.9mA 
Current KEYDOWN                                                                                 190mA
Power Output                                       1.58W                                          1.4W

NOTES: Measurements taken at 7020KHz with Keithley multimeter and AD8307 Power meter using a 36dB attenuator.

19 April 2014

Kruger Park Satara Camp

Here we are at the lovely Satara camp. The 569Km drive here from Joburg was quite uneventful in spite of the heavy traffic and packed facilities. This being Easter weekend. Our self catering bungalow is basic but very clean and pleasant. We are in a circle of rondavel huts numbering some 25 with a nice green space in the center with many trees.

Of course I had to deploy my antenna upon arrival. A single launch had the EFHW over  a strong branch about 20ft high. A non-thorn tree! Soon I was QRV using my Norcal 40A at 1.5 Watts. The following in my log:

17:08         599         599        ZS4SF  Monk in Welkom indicating some QRM but a FB signal
17:20         599         599        ZS6AZP  Dave in Centurion indicating rising QRM
17:30         549         599        ZS6AJY   Barrie in Benoni / QRM

The band was definitely going long and an hour later I was unable to make a QSO as I heard european stations coming in. The reports received were very satisfying, convincing me that the system was working ok.

After a delicious braai of short ribs and salad we retired for a good night's sleep.

This morning we were up bright and early before sunup and joining the queue of vehicles waiting for the gates to open at 6am. We headed for the famous S100 road where there are purported to be a large pride of lions in residence. Yes we were lucky! Right there walking alongside the road were a beautiful lion and lioness. This was certainly the highlight of the morning for us. What a memorable sight to behold in the golden morning sunlight.

We stopped at a nice picnic spot where we rented a 'skottle' with which to cook our breakfast consisting of eggs. tomato, fried banana and tea.

What a great start to our Kruger trip..

Picnic spot near Satara camp and overlooking Mozambique

17 April 2014

Kruger Park Easter April 17th - 23rd

This coming weekend we are excited to be going to the Kruger Park.

We will be in the park from the afternoon of Friday April 18th through the morning of Wednesday 23rd. We will stay in self catering accommodation at Satara, Pretoriuskop and Berg en Dal. This should be fun!

I am taking along my QRP 40m station as stored in a plastic food container, including my end fed halfwave system. I do hope that I will be able to make a few CW QSO's with my CW friends.

Thank you in advance for trying to work ZS6RSH.

I will take my laptop along and thus I plan to post blogs covering our experiences at the park.

Details of my working conditions as follows:

QTH's:      April 18th evening - April 20th morning SITARA
                 April 20th afternoon - April 22nd morning  PRETORIUSKOP
                 April 22nd afternoon - April 23rd Morning BERG EN DAL
OPERATING TIMES  Noon 12:00-1:30,  Early evening 18:15-19:00, Other casual times.
QRG: 7020KHz
RIG: NORCAL 40A, 1.53 Watts
ANTENNA SYSTEM: End Fed Half Wave (66' 4") + Tank coupler + 8" 6" Counterpoise
PADDLES: Palm field model
POWER: 12V, 1.3Ah SLA, Car Battery power as backup

List of Equipment:

  1. Plastic Food storage container
  2. Norcal 40 A, 1.53 Watts output, QRP, 40m Rig. 
  3. Granadila Tin homebrew end fed half wave coupler
  4. LED SWR Meter
  5. Palm Paddles
  6. 66ft 4inches, #16 AWG, 40m Halfwave antenna, with launch cord attached.
  7. 8ft 6inch Counterpoise
  8. Launch cord with 3 fishing weights attached
  9. Spare cord (20ft)
  10. 2 X BNC barrel connectors
  11. SLA Battery, 12V, 1.3Ah
  12. SLA power cable and 2A fuse
  13. Car Battery extension power cable
  14. 2 X 2A fuses
  15. Keyline
  16. Earphones
  17. Leatherman
  18. Mini Multimeter, Radio Shack
  19. Plastic tuning screw driver
  20. Pen
  21. Paper
  22. Operator ZS6RSH 
Some interesting links:

Kruger Game Park.  http://www.krugerpark.com/

Satara Camp http://www.krugerpark.com/self-catering/satara-restcamp/
           GPS :-24.3976800 (latitude), 31.7777800 (longitude)

Pretoriuskop Camp http://www.krugerpark.com/self-catering/pretoriuskop-restcamp/
           GPS: -25.1722 (latitude), 31.2692 (longitude)

Berg En Dal Camp http://www.krugerpark.com/self-catering/berg-en-dal-restcamp/  
           GPS: -25.4200(latitude), 31.4512 (longitude)

15 April 2014

JFET measurements continued

Since I am now getting ready to build the second oscillator for the RF signal source (10MHz - 45MHz) I thought it would be instructive to test a few more MPF102's for this oscillator. The same test fixture was used as previous (see Label JFET Experiments). Results as follows:

Vd-d = 10.04V

SAMPLE     Idss (mA)    Vp (Volts)
1B                  9.8              -2.75
2B                  7.3              -2.83    
3B                  9.98            -3.51
4B                  9.2              -2.4

Sample 4B was a re-measure of the previous Sample 1 and yielding similar results. Tests were carried out quicker this time to minimize the effects of sample heating with the smaller resistance range.

Sample 3B has the widest dynamic range between Vp and Idss. Start with that sample in the oscillator and then measure the output when compared to sample 4B which is the most linear.

Sample 1B
Sample 2B
Sample 3B

Sample 4B

12 April 2014

End Fed Halfwave SWR test

In preparation for our up coming trip to Kruger National Park over the Easter weekend, I performed some more tests on my EFHW Granadilla tin tuner to see if I could improve the SWR from the previously measured 1.7:1. The plan for this trip is to take along the Norcal 40A 2Watt 40m rig with a minimalist station with all station components stored in a plastic food container. The 40 A was used for these tests.

Based on further reading on the EFHW I removed the grounded link between the secondary and link turns. This to reduce the chances of common mode RF flowing along the coax cable. The BNC connector was replaced with an insulated version.

I depoyed the EFHW just behind our apartment in the park area with an 8.5ft counterpoise lying under the antenna. I found that 3 Turns on the primary is ideal and results in a perfect 1:1 match. The Tx was connected via a 1 foot coax to the LED SWR meter which was connected to the tuner via a barrel connector.

Interestingly the tuner tuned to resonance indicating some capacitive reactance (about 13pF) at the wire feed point. After reducing the length of the wire by about 2 feet no change in the feed point reactance was noted. Could this reactance be predominantly the effects of stray capacitance? The tuner may exhibit less capacitive reactance if housed in a wood or plastic enclosure as opposed to the steel food tin?

Reports received from Monk ZS4SF were 549 at 1:15pm. The antenna was deployed as an almost horizontal wire about 10 feet - 8 feet above the ground.

The end of the antenna was then raised to about 20 feet and sloping down to about 8 feet to a branch above the operating position. Now I received a consistent 559 report from ZS4SF clearly showing the benefit of height.

Many thanks to ZS4SF for hanging in there as usual during this field test.

11 April 2014

Rf signal source. Return Loss tests

Following up from my previous blog I was puzzled as to why the level had dropped from the initial tests of 16dBm to around 7dBm. On further inspection I discovered that the 220 Ohm resistor was still in place across the buffer amplifier broadband transformer!

The lesson learnt here being.....if there is an unexplained test then believe the test and investigate...as to why!

On removing the resistor the Return Loss was re-measured. Refer to lab notes for details. I was very pleased to see that the RL now was measured at 21dB. Definitely a good news story!.

Output power was measured as follows:

at f= 6651KHz = 15.36dBm.

I perceived that this was off the scale of the power meter so a 20dB attenuator was added. This derived a power output of 16.2dBm. Indeed, the above measurement was off the scale of the Power Meter.

Next inserted a 30dB pad and the output was derived at 16.1dBm. This output is 9dB higher than expected. Why? The output amp does run hot. Do calcs for the output amp to check the collector current.

A quick check of linearity. (Using a 30dB pad).

Fmin = -15.6dBm
Fmax = -13.95Bm
Change over the range = 1.65dB. This is good.

Next complete the inclusion of the bandspread caps and check frequency stability of the oscillator.

10 April 2014

RF Signal Source. Amplifier stage 1

I now build the amplifier stage for my homebrew RF signal source as per EMRFD Fig 7.27. The schematic is shown in the lab notes below. This stage uses a 2N3866 transistor as the amplifier with a standard 4:1 broadband transformer in the collector. A 3dB pad is inserted in the output to improve Return Loss. Some signal is fed off to a separate port for connection to a frequency counter. The build came together easily after about 45 minutes construction time UGLY style! No issues with getting the stage to work.

Initial quick tests showed an output of 12.3dBm. However subsequent more careful tests showed the output as follows which are more in line with expectations.

Fmax at 1381KHz output = 7.4 dBm
Fmin at 2783KHz output = 5.67 dBm

Why is the output higher for the higher frequencies than the lower frequencies? Investigate further.. Is 2 dB difference over the frequency range in line with expectations?

The oscillator tank circuit was shorted out to test the total system RL which = 18dB. This is a satisfactory result although not stellar.

The RL was then measured with the tank circuit running at an offset frequency (3243KHz) as opposed to the test frequency of 7022KHz. This did not work indicating a negative RL.

The amplifier stage was then disconnected from the buffer and terminated in 50 Ohms across the base 330 Ohm resistor. This did not work as the RL was low.

The 50 Ohm termination was then moved to the input of the 3dB pad. This worked and resulted in a RL of 18dB.

Referring to the previous blog on buffer stage testing, this result was revealing. The buffer RL tests showed a poor 5dB RL. However when the buffer stage is connected to the amplifier the RL output is an acceptable 18dB. This RL figure is exactly the same as that obtained when the amplifier stage was tested in isolation with it's input terminated in 50 Ohms. Why is this? Perhaps there is an error in the procedure used to measure the RL of the buffer stage in isolation?

RF Signal source. Buffer stage measurements 2

Following on from my last blog I next worked to try to measure the Return Loss, RL, of the buffer stage. This was not so easy. Some notes as follows.

Refer to the below schematic of the test configuration using the MFJ259B as the signal source into the Return Loss Bridge with a 6dB PAD. The objective was to measure the output Return Loss of the buffer amplifier.

With the oscillator connected to the buffer stage and the oscillator tank circuit short circuited (to disable the oscillator), the Return Loss (RL) was measured at 5.48dB which is very low. The target being to achieve better than at least 18dB. This being an SWR of 1.2:1.

Next the input to the buffer was terminated in 50 Ohms with the oscillator disconnected. The RL measured remained unchanged. Turning the stage power on/off had no effect. Shorting the input to the buffer had no effect.

The 3dB Pad was disconnected from the broadband transformer and terminated in 50 Ohms. This gave a better RL of 26dB. Indicating that the 3dB pad is working.

The 4:1 broadband transformer was reconnected to the Pad but disconnected from the JFET collector. RL= 5dB. The transformer was then terminated in a 200 Ohm resistor. RL = 26dB, Good! This showed that the broadband transformer was working at the test frequency of 7020KHz. The RL remained at 26dB when the transformer was connected back in the collector circuit with the 200 Ohm resistor in place. RL remained unaffected by the stage being turned on. The output however dropped from -5.7dBm with no 200 ohm resistor to -11.45dBm with the resistor in place.

The output RL was completely unaffected by any change to the terminated input impedance. Perhaps this comes as no surprise because the input impedance into the emitter is very low at a calculated value of 2.3 Ohms. Thus to get an input match would be very difficult.

In order to get an acceptable stage output RL the broadband 4:1 transformer must be terminated in a 200 Ohm resistor on the primary side, however at the expense of a loss of output signal level. This since half the power developed in the collector is being dissipated in the 200 Ohm resistor.

At the end of the day perhaps it does not matter if the RL is high from the buffer since the objective is to ensure high Reverse Isolation and not amplification. But what about harmonic distortion introduced in the stage? Is that of concern? The stage appears to be working correctly in spite of the high return loss.

In the lab notes below the RL is shown as dBm. This is incorrect. RL is expressed in dB.


09 April 2014

RF signal source. Buffer stage measurements

Making measurements on the RF signal source buffer stage would be a good learning opportunity. The RF signal source as per EMRFD Fig 7.27 deploys a Common Base Amplifier using a 2N3904 transistor and a 4:1 broadband transformer in the collector. Refer to the schematic below. The output is then terminated in a 3dB pad before connecting to the power amplifier stage. The function of the stage is to create high reverse isolation so that the oscillator frequency will not 'pull' based on changes to the output load.

I analyzed the circuit using equations presented in Chapter 2 of EMRFD. Essentially the stage has unity current gain. The input to the stage is derived as a current source since the emitter input impedance is very low. This being the reason for the high reverse isolation characteristics. Thus it is a challenge to drive power into the stage. Using a theoretical Voltage gain of 200, I was unable to measure that much voltage gain. However using a unity current gain model, the measured and calculated output power correlated.

The open circuit oscillator output voltage was measured using a 10X scope probe. The stage Vin was then measured with the buffer connected. Using a simple small signal model to derive the stage input impedance (222 Ohms), the oscillator source impedance of 25.9 Ohms  was derived. Should this in fact be 50 Ohms (to be investigated). I assume that this is not critical since the oscillator stage is lightly coupled to the buffer stage through a 220 Ohm series resistor.

Some results are presented as follows:

Vcc = 12.17V
Ftest = 2825.09KHz
Ie (emitter quiescent current) calculated = 11mA (I used the long equation 2.11 EMRFD for this calc)
Ve (emitter quiescent voltage) calculated = 2.97V, measured = 2.8V
Ve (base quiescent voltage) calculated = 3.57V, measured = 3.45V
gm (transconductance) calculated = 0.4231, Rin (emitter) = 1/gm = 2.36 Ohms
Rl (collector load resistance) calculated = 200 Ohms.
Vosc open circuit = 960mVp-p
Vin = 860mVp-p
Rin (stage) calculated = 222.4 Ohms.
Rsource oscillator calculated = 25.9 Ohms.
Ib (base current) = 3.86mA
Power output in collector calculated based on voltage gain = 2mW (+3dBm)
Power output in collector calculated using unity current gain = 0.38mW (-4dBm)
Power output in collector measured (v*v/8R) = 0.38mW (-4dBm)
Power output after 3dB pad = -7dBm

All 'in situ' measurements were made using a 10X scope probe. Output power measurments at 50 Ohms were made using the RF power meter or by using the scope connected through a 50 Ohm thru terminator.

Next the stage input and output Return Loss, Power Gain and Reverse Isolation will be measured.

03 April 2014

RF signal source. Prototype build and test 2.

I have now rerouted the main variable capacitor wire away from the bandspread capacitor wire. This issue is now fixed. Next I replaced the 10pf blocking capacitor with a 6.8pf capacitor. No real difference seen in performance.

The inductor was also rewound using much tighter turns in an effort to try to improve stability. When I reinserted it back in the circuit the drift was far worse than the more loosely wound inductor. After doing some reading it was noted that often it takes time for coils to settle down due to stress in the wire caused by tight winding. I then boiled the inductor in boiling water for a few minutes. This appeared to have stabilized the coil.

I then changed out the JFET MPF102 for a J310. This resulted in a very distorted waveform (see pic). My assumption was that this was caused by some parasitic oscillation. I noted that the output voltage was in fact similar to the MPF102. Upon changing back to the MPF102 the waveform was once again clean. No further investigation carried out.

Next I made some measurements of Time, vs Temperature, Vdd and Frequency. I left the circuit powered on for a few minutes before beginning the tests so as to not measure warm up drift. The shack temperature varied between 23.5C and 23.4C with a stable Vd-d = 12.12V. The frequency varied between 7007430Hz and 7007404Hz according to my frequency counter. A variation of 26Hz. More tests over a wider temperature range needed once the buffer and amplifier stages are built.

Next steps: Build the buffer stage.
Waveform showing distortion when a J310 was deployed. Is this parasitic oscillation occurring?

Thermal stability tests. A wider range in temperature is needed to measure frequency stability correctly.

02 April 2014

RF signal source. Oscillator prototype build & test

I am in the process of building an RF Signal Source as per EMRFD Fig 2.27. This is a Hartley design and incorporates 2 oscillators. The first covering a nominal 2-10MHz and the second covering a nominal 10-45MHz.

The Hartley topology is chosen for two reasons. a) The capacitor in the resonator can be fully variable and thus allows a wide range of frequency coverage unlike the Colpitts where, by definition, the feedback capacitors are fixed. b) The oscillator exhibits a reasonably flat output power over a wide frequency range.

Having built the chassis, installed the variable capacitors and cut the panels for the enclosure, I was now in a position to build a rough prototype of one of the oscillators. I chose the lower frequency oscillator. This took me a few hours to build. Since I don't have any of the specified 2N4416 JFETS, I decided to start with an MPF102. I selected sample 2 (Ref JFET experiments blog)  since it had a lower pinch-off voltage and higher Idss than sample 1, although not as good in performance as the tested J310. I decided to start with just the 400pf variable capacitor. This should allow easier troubleshooting since there are fewer parts. (refer to the schematic attached).

The oscillator powered up first time with no issues. The oscillator turn on voltage Vdd was 1.96V. Maximum output was obtained with a Vdd of 7.58V. Thus there was an excess of current flowing in the drain as Vdd was increased to 12V. The scope output waveform 'looks' clean. I have no way of measuring the harmonic distortion. Could increasing the rail voltage beyond 7.58V result in an increase of harmonic distortion?

The 3 turn link output was first terminated in a 50 ohm feed thru connector which was connected to the scope channel 1. This channel is also fed to the frequency counter. Then the output was connected to the Power Meter.

The oscillator is reasonably stable and certainly acceptable for general measurements and can be improved upon with more careful construction. Measuring at a frequency of 7030.44kHz and starting at 08:30 am the oscillator drifted upwards to 7030.46KHz after 23 minutes (10Hz). The shack door was closed. No thermometer is available. Then with the shack door opened and after another 10 minutes the oscillator was at 7030.73KHz. It then started to rain which presumably lowered the temperature the frequency measured was 7031.02KHz (562Hz). On blowing on the parts with a straw the indications were that the 10pf capacitor was the biggest drift contributor. As the temperature decreases the oscillator frequency increases.  Neither the JFET or the inductor seemed to be significant contributors. No vibration tests were carried out, however it was easy to tune the large capacitor to within 1KHz of the desired frequency. More precise tests needed once the buffer is built.

An error was discovered in the design. The Large 400pF variable capacitor wire runs parallel to the 30pf bandspread capacitor for about 4 inches. Although this capacitor was not connected it changed the oscillator frequency when it's capacitance was varied. Action here is to re-route the large capacitor wire in a different direction.

Tests with the Power meter showed an output variation of just over 1dB from an Fmin output of -1.33dBm at 2810KHz to -0.27dBm (frequency not measured but around 6000KHz). The output power at Fmax of 13663KHz = -0.75dBm. These measurements are in line with  those stated in EMRFD and very satisfactory.

Scope measurements
Fmax = 13663KHz (this frequency will decrease as the bandspread capacitors are added)
Fmin = 2810KHz (This frequency will increase as the bandspread capacitors are added)
Fmin Vpk-pk = 6div*0.1 = 0.6V (~0.5dBm)
Fmax Vp-p = 5.5div*0.1 = 0.55V
F(7175KHz) = 0.6V
Power Meter measurements
Fmin = DVM = 1770mV, thus Power = -86.29+(0.048*1770) = -1.33dBm
Fmax = DVM = 1782mV thus Power = -86.29+(0.048*1782) = -0.75dBm
Fpk = DVM = 1792mV thus Power = -86.29+(0.048*1792) = -0.27dBm

Next steps
Reroute the capacitor wiring.
Try different 10pf capacitors to try to improve thermal stability
Try a J310 JFET.

Prototype Hartley oscillator 2.8MHz - 13.7MHz

Pic showing the large variable and bandspread variable and oscillator circuit with terminations.

Oscillator set at 7030.64KHz with scope waveform showing no 'visible distortion'. Harmonic level not measured. The feedthrough 50 Ohm terminator on channel 1 can be seen.