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.

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!

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