25 December 2013

RF Power Meter Calibration reworked

I reworked the calibration procedures again for the 1N34A based power meter and the 1N4148 based power meters. The full calibration results are included on this Blog page.

For the 1N34A I used a DC calibration procedure but this time I used my commercial attenuators to set the input DC Voltage levels. I then compared the measured voltages with the expected value. The correlation as can be seen from the table below is exactly the same. In other words the measured values = the expected values over the measurement range.

For the 1N34A meter I also injected RF power from my K2 rig into the RF power meter at FSD. I then inserted the attenuators using the same procedure as the DC readings. The Meter readings for the DC input and for the RF input at 7020KHz were exactly the same. This increases my level of confidence that the DC calibration method is valid, at least to within a single decimal place of a dBm measurement.

I also reworked the 1N4148 calibration. I reworked these due to the upgrade of the terminator to a newly built dummy load using 9X 2 Watt 470 Ohm resistors and 1X 1000 Ohm 2 Watt resistor (refer to the pics below). This allowed me to increase the calibration range up to 25 VDC. The voltage ranges used were set by varying the input DC Voltage from the variable voltage power supply unit.

Thus the RF Power measurement range now attainable from this power meter is summarized as follows:

1N34A
Vpk (V)  0.3 - 3
P (mW)   0.92 - 93
P (dBm) 0 - 19

1N4148
Vpk (V) 1.5 - 10
P (mW)  20 - 960
P (dBm) 13.5 - 29

Vpk (V) 1.8 - 15
P (mW)  30 - 2280
P (dBm)  15 - 33

Vpk (V) 2.91 - 25
P (mW) 80 - 6240
P (dBm) 19 - 38

Refer to the calibration curves below







1
Heading
1N34A RF Power Meter Calibration
2
Label
RF Power Meter Calibration
3
Date
12/23/2013.
4
Acknowledgements
1.      Author : ZS6RSH.
2.      Reference: EMRFD Section 7 paragraph 7.3.
5
Revision
Rev 2
6
Revision History
This is a repeat of the previous measurement but aimed at improved accuracy.
7
Scope
Calibrate the 1N34A RF Power meter using measured input DC voltages that are derived using calibrated commercial attenuators.
8
History
Reference 6 above
9
Configuration
 Refer to Figure 2 below.
10
Test equipment specifications
1.      Keithley model voltmeter.
2.      Homebrew variable voltage current limited, power supply 2VDC-15VDC.
3.      3dB, 6dB, 10dB, 20dB attenuators.
4.      Connection leads.
11
DUT specifications
1. Homebrew RF Power meter using a 1N34A diode.
12
Workbench process
1.      Set the DC Voltage level and the variable pot to read 3VDC at FSD
2.      Insert the attenuators one at a time and record the DC input voltage and the Meter Reading.
13
Expected Results
1.    The recorded input voltages should be very close to the expected voltages when the attenuators are inserted.
14
Uncertainties
1. Keithly meter inaccuracies. No difference to 2 decimal points is seen between the Keithly and the Fluke voltmeters.
2. The RF meter may respond differently to RF input as opposed to DC input. This calibration method fundamentally assumes that at least at 7MHz, that there is no measurable difference between the DC and RF meter response. This was confirmed to 2 decimal places by measuring the same meter readings for RF power input against the expected attenuator values.
3. The commercial attenuators are assumed to be accurate. The theoretical voltage drops aligned with the measured voltage drops. Refer to the measurement table.
4. The termination load in the RF power meter is measured as 51.6 Ohms using the Keithly Ohmmeter
15
Preparation
Start with the DC voltage on the variable power supply set to minimum.
16
Perform validation measurements
Both DC and RF at 7020KHz were injected as separate measurements. Starting from a baseline of approximately 3VDC FSD and 3VAC FSD the meter readings were exactly the same between RF and DC for each inserted attenuator pad.
17
Perform the full measurement plan
Carried out as per 12 above down to approximately 0dBm
18
Observations
The results were consistent with the expectation.
19
Change Control
None
20
Computation
Calculate the RF Power for each meter reading as P = Vdc^2/2R. (where R = 50 Ohms)
21
Analysis
The plotted curve shows the transfer characteristic of the RF Power meter over the measurement range.
22
Conclusions
Using the calibration curve, the Power Meter will provide acceptable results. However this should ideally be confirmed against a calibrated RF signal generator in order to determine the accuracy with higher confidence. This procedure fundamentally assumes that a valid result can be obtained by using DC Voltage levels.
23
Documentation
Done as shown on the Blog.



1
Heading
1N4148 RF Power Meter Calibration
2
Label
RF Power Meter Calibration
3
Date
12/23/2013.
4
Acknowledgements
1.      Author : ZS6RSH.
2.      Reference: EMRFD Section 7 paragraph 7.3.
5
Revision
Rev 2
6
Revision History
This is a repeat of the previous measurement but aimed at improved accuracy. The 50 Ohm load termination was changed by using 2 Watt carbon resistors. This gives a 20 Watt power dissipation capability.
7
Scope
Calibrate the 1N4148 RF Power meter using measured input DC voltages. Calibrate the meter using 2 FSD values of 25VDC, 15VDC, 10VDC
8
History
Reference 6 above
9
Configuration
 Refer to Figure 2 below.
10
Test equipment specifications
1.      Keithley model voltmeter.
2.      Homebrew variable voltage current limited, power supply 2VDC-15VDC.
3.      Connection leads.
11
DUT specifications
1. Homebrew RF Power meter using a 1N4148 diode and a 20 Watt 50 Ohm Terminator.
12
Workbench process
1.      Set the DC Voltage level and the variable pot to read 25VDC, 15VDC & 10VDC at FSD (Full Scale Deflection)
2.      Reduce the input DC voltage and take an input voltage reading for every 0.1 Ma meter reading as close as possible. Ie aim for 10 readings.
13
Expected Results
1.    The results should show a ‘fairly’ linear transfer characteristic.
14
Uncertainties
1. Keithly meter inaccuracies. No difference to 2 decimal points is seen between the Keithly and the Fluke voltmeters.
2. The RF meter may respond differently to RF input as opposed to DC input. This calibration method fundamentally assumes that at least at 7MHz, that there is no measurable difference between the DC and RF meter response.
3. Parallax error in reading the analog power meter
4. The termination load in the RF power meter is measured as 51.6 Ohms using the Keithly Ohmmeter
5. Termination load (dummy load) heating resulting in resistance variances.
15
Preparation
Start with the DC voltage on the variable power supply set to minimum.
16
Perform validation measurements
At the FSD levels the RF Power shown on the K2 Power Meter was the same as that derived from the RF Power Meter.
17
Perform the full measurement plan
Carried out as per 12 above between the levels shown below.
18
Observations
The results were consistent with the expectation.
19
Change Control
The 10VDC FSD was not possible to obtain with the variable pot and a 10K Ohm resistor in series. A ‘FSD’ level as shown in the table was used instead.
20
Computation
Calculate the RF Power for each meter reading as P = Vdc^2/2R. (where R = 50 Ohms)
21
Analysis
The plotted curve shows the transfer characteristic of the RF Power meter over the measurement range.
22
Conclusions
Using the calibration curve, the Power Meter will provide acceptable results. However this should ideally be confirmed against a calibrated RF signal generator in order to determine the accuracy with higher confidence. This procedure fundamentally assumes that a valid result can be obtained by using DC Voltage levels.
23
Documentation
Done as shown on the Blog.

24 December 2013

QRV in Cary North Carolina

We are really having a ball here in Cary North Carolina! We are staying in a friend's home while we are visiting our hometown and spending time with the kids. What a pleasure it is to be with them once again!

The WX has been incredibly mild the past few days but has now become more wintry for sure. The temperature now this morning having dropped to 43 F. However due to the rain, everything is wet. Lets hope this does not turn to ice over the next few days!

I have my QRP station now QRV on 40m. The band appears to be in excellent shape. My modest EFHW and 2Watt signal are giving me some decent reports up and down the east coast. I am hanging out on the QRP frequency of 7030KHz. What a pleasure it is to hear so many fantastic CW signals on the band!

I am thinking of our friends back in South Africa and the totally different weather and life style. Thunderstorms every afternoon ho.  May you all have a safe and very very Merry Christmas!




23 December 2013

A tiny glimpse into South Africa's rich radio history

SA RADIO HISTORY (ZS6RSH)

I recently received an interesting email from OM Jan ZS6BMN. Jan related a few pieces of the radio history of South Africa, which I found very interesting. Jan kindly gave me permission to record these valuable pieces of history on this blog. Perhaps someday someone will take on the challenge of making an in-depth study of the rich history of radio in South Africa.

Jan did sound a warning that the accuracy of some of the information may have "suffered as a result of the passage of time." All information is posted here in good faith and in a positive spirit. No offense is ever intended.

With kind permission from Jan ZS6BMN

On the Barlow Wadley and Racal RA17 Radios. (slightly edited)
Great to learn that you own one of those very FB RA-17 receivers! Trevor Wadley designed the loop and it was first used by Racal. It was news to me to learn very recently that he had been contracted by National Co. as consultant way back then and was involved with the HRO-500 project! He designed the portable (Barlow) Wadley as a low-cost unit, but ran into legal problems with Racal. They were initially built by Barlows at their New Germany plant in Natal (KZN now), but due to the ongoing legal battle the manufacturing was moved to Taiwan. I have owned a number of these through the years and now have one of the latest (Taiwanese) versions. The first units came with a chromium-plate front panel, then came the grey ones and finally those from Taiwan were sporting military green panels. Performance wise I found the last ones to be the best, but all were good performers. The block diagram shows that the BW XCR-30 is in concept virtually identical to the RA17. 

Derdepoort
At the start of my career in electronics I did my practical part of training at Derdepoort Radio Station in 1965/1966. I have finished school in 1963. Went to college in 1964, turned 18 and could at long last write the Amateur Radio Examination in November 1964. Those days one could not apply for a Ham licence in South Africa before the age of 18! At Derdepoort we had a pair of RA-17s with a Plessey diversity FSK unit on a trolley and this system was used for monitoring and as the receiving setup when running minor pres or diplomatic news services. At that time I still had my listener's call sign: ZS6-237 and was trying to get to grips with CW - nothing much has changed :-) I have also volunteered for night-shift (19:00 to 07:00) and then when everybody went to sleep used the Racals on the trolley and turned the Collins log-p to the States to decode RTTY. The log-p was part of the Bapsfontein USA system and was used with the latest Collins phase-lock loop receivers. These were easy to tune, but sometimes only locked on the correct frequency after numerous loop resets. On Amateur Radio side only ZS6UR was a regular on RTTY in South Africa so it was still a very rare mode. I still have all those printouts! I have really enjoyed the time at Derdepoort a lot! As the only trainee interested in Ham Radio I had privileges like access to the library (CQ, QST and all of that) and the laboratories and was permitted to use all of the test equipment too! Great training that was!

Those masts/towers in the Derdepoort photo were 110 feet high and were all over the place supporting rhombic antennas in groups of three pointed at London, NY, Sydney, Leopoldville. The three rhombics in a group were a mile apart. The large receivers, made by STC, were triple diversity units (used on London and Tangiers) while most of the others, Marconi, Philips and Mullard were double diversity. The Marconis were for FSK only, while the others were all independent sideband units for voice and high-speed telegraphy. The London service carried 16 telegraphy channels on the one sideband and 2 voice channels on on the other and operated in duplex for the overseas telephone communications. Some of the telegraphy high-speed telegraph units used one sideband for traffic and the other for quarter speed repeats. They were operating also in the TOR mode in full duplex. Pity that everything got demolished and this is the only photo I could get of the station in an old magazine. No photos of the inside exist as everything was wrapped in security. Even in the history write-up only a single small paragraph mentions the Derdepoort receiving and Olifantsfontein transmitting stations. I believe a similar fate hit the AT&T radio stations in the USA, but hopefully some there were preserved.... In an article that I read on the fate of some of the well-known USA stations I got the impression that real estate there too has triumphed over sentimentality as stations were demolished to make place for expansion. That is called progress in the Western World!

Dr Trevor Wadley
Dr Trevor Wadley was a research scientist at the CSIR and developed the original Wadley-loop prototype receiver in 1948. One of these as well as the much later prototype of the BW portable is currently on display at the SAIEE museum in Johannesburg.

ZS6AKO
One of my practical tasks in ‘65 was to re-assemble and align the RA17 that was previously taken apart to serve as example when some of the Hams at Derdepoort Radio have embarked on an optimistic project to construct a number of these receivers. My 'Elmer', Gijs, ZS6AKO (SK), was a master die-maker and made the die and mold for the chassis and faceplate bezel. The end-result was a number of FB clones that possibly ran circles around the standard unit on SSB as they have added a product detector and a few other smaller refinements. The S-meters were calibrated in microvolts – rare for that time! He went one step further in having all of the controls on his receiver labeled in Afrikaans! Gijs has also constructed a matching exciter and a legal-limit linear amplifier using three TT21 tubes. His antenna was a cubical quad. I have spent many many hours at the QTH of OM Gijs just to monitor him working DX! My own station at the time consisted of his old transmitter and an old HRO-MX. When he passed away in 2008 I did not want to ask about the whereabouts of his FB station and only much later heard there were no takers and that receiver was sold for R 600.00 - a crying shame and I still feel very upset that I have missed out on the opportunity of owning that specific set. Great mentor and good friend OM Gijs was to me...... In later years he concentrated on his other hobby - building model steam locomotives.
BTW: When the undersea cable was commissioned in the late sixties followed by the commercial satellite service the demise of point-to-point radio stations was almost certain. Derdepoort Radio was closed down and all the radio equipment destroyed for security reasons. What a waste! The buildings were used as laboratories for a short while, but together with the large property surrounding the station (a game reserve in the old days) everything was eventually sold to property developers. The building was demolished and now there are a number of townhouse complexes. The location of the station was between Sinoville, Wonderboom Airport and the Kameeldrift agricultural holdings.

Photo credits
1. Derdepoort Radio Station: Suid-Afrikaanse Panorama, 1964
2. Wadley prototype: SAIEE Museum Webpages.
3. Barlow Wadley prototype: ‘AWA Newsletter’, 12/2013, taken at the SAIEE Museum.
4. The other photos came from the private collection of ZS6BMN.



Compiled b ZS6RSH (2013-12-12)

Edited by ZS6BMN (2013-12-13)




Wadley Prototypes 1948





BW XCR-30 block diagram
BW XCR-30 inside
ZS6BMN BW XCR-30
ZS6BMN 1966 HRO_ZS6AKO TX
HRO-MX ZS6BMN
HRO-MX inside bottom
HRO-MX inside top
Ham RTTY 1965 at Deerdepoort








11 December 2013

Simple test results of a 7MHz Halfwave Power Output Filter

As shown in the attached schematic and test results. Using my MFJ259B antenna analyzer I was able to obtain a view of the VSWR of this filter over the HF spectrum.

At the very least this illustrated 2 observations to me.

1) A reasonable match to 50 Ohms is obtained over the 40m band.

2) A mismatch exists for frequencies other than the 40m band. Both above and below this band.

The obvious benefit, and the whole point of the filter, is the fact that harmonics are attenuated by the filter. This is all good.

The more interesting Questions to me are :-

1) What is the effect of the reflected harmonic energy when it is absorbed by the Class C Non Linear Power amplifier?

2) Is the Return Loss of the harmonic energy so great (>80 dB) that it is insignificant or does the reflected energy generate significant IMD?

3) Why are diplexers mostly implemented on Mixer outputs but not on output filter circuits?

All good :)




NB6M Miniboots, ZS6RSH version

Since I did not have the exact components for the NB6M Miniboots QRP version, I tried adapting the circuit in both the input attenuator and the output filter. I also added a 1N4148 diode to the PA input to prevent negative pulses from possible destruction of the IRF510 MOSFET. I also added a ferrite bead to the Gate input (see comment below).

The NB6M design also calls for a 13 V gate protection Zener. This implementation used a 15 V Zener since that was what I had in stock. This puts the MOSFET in danger of destruction from voltage spikes.

I used combinations of 1/4 watt resistors for the 3 dB attenuator. This is a PI network using 1/4 Watt 10% tolerance carbon resistors with a calculated power dissipation of 744 mW.  I then adjusted the variable pot until I measured a 1:1 VSWR on the input to the Miniboots. The input power was approximately 2 Watts which provided 1 Watt output into the 50 Ohm matched input divider circuit to the PA. (Refer to the schematic below).

A useful further adaptation would be to design a purpose built attenuator with a 33 Ohm output impedance and a 50 Ohm input impedance with attenuation (to be calculated). Further to this thought...the drive power currrently supplied to the PA input (including the 33 Ohm stabilization resistor is approximately 1Watt). With a 12 Volt supply this 'easily' gives +5 Watts output power. For these tests I did not go further than building a standard 3 dB  attenuator. I then adjusted the variable pot to yield a 1:1 VSWR on the input. With this drive level it was clear that the PA was subjected to some unmeasured level of gain compression. When I adjusted the pot for max drive without gain compression, the input VSWR rose to 1.5:1.  I did not want to operate my driver QRP rig into this mismatch for too long since I am still unsure as to how robust it is (this being my W7ZOI universal QRP rig). An option would be an optimized attenuator. Another option would be to add a small amount of regeneration into the emitter of the QRP rig and find other ways to reduce the drive power so that it nicely matches the Mini boots. This would now lead to a new design and thus the Miniboots would no longer be an outboard universal Miniboots...So what is the design objective here at the end of the day? Max efficiency or max utility? It is not possible to have it both ways... Interestingly the approach taken in EMRFD chapter 1 towards building up a transmitter, as a beginner, is to start with an oscillator and then build and match subsequent stages until the desired output power is reached. I have read that in the 'old days' the Old Times would build tube rigs in exactly the opposite direction! This is interesting.

Since I did not have the components in stock to build the specified output filter/matching section I instead built a 'standard' halfwave 7 MHz filter. This has a 50 Ohm input at 7 MHz and 50 Ohm output impedance.

The drain of the IRF510 expects to see an impedance of 12.5 Ohms (P=V^2/2R) for a 12Vdd rail for 5 Watts output power. Thus a 4:1 broadband UNUN transformer was built to perform the impedance transform. This UNUN was tested and confirmed to indeed be providing a 4:1 transformation at 7MHz. See a previous blog write up Label Broadband Transformers.  A ferrite core was used.

The system appears to deliver a clean 5 Watts output however a 'kink' was seen on the gate input and on the drain using my GOS 635 20MHz scope. It is unclear what is producing this kink. Is it the limited bandwidth of the scope or is this symptomatic of some stability issue. Further experimentation required. Clearly the limitation of my 20MHz scope creates some challenges here. A higher speed scope may well uncover further irregularities in the input/output waveforms since it would afford increased X axis resolution. (Refer to the attached scope pics).

According to EMRFD. The output power should increase smoothly as the Vdd is slowly increased. If there are any sudden irregular changes or jumps in output power then this is an indication of instability. An adjustable power supply is required for this test.

It was noticed that the waveform distortion was 'significantly' less after a ferrite bead was added to the gate lead.  Further experimentation needed.

The heat sink consisting of a piece of 2 inch X 1 inch aluminium becomes warm to the touch during normal CW operations but never becomes too hot to touch.

I have made at least 3 dozen QSO's of the rag chewing type with excellent reports received over the past month. One station reported a slow fading of the received signal. However this could have been due to QSB. To be further tested.

Efficiency and more precise measurements still need to be made on this adapted NB6M Miniboots, full gallon QRP Power Amplifier.


Gate +6.8Vp-p -0.8Vp-p

Drain +27Vp-p

Output into 50Ohm terminator. +50Vp-p