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Author Topic: Accuracy of SB-1000 meter in Power setting?  (Read 323 times)

N3NUE

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Accuracy of SB-1000 meter in Power setting?
« on: March 30, 2023, 05:21:15 PM »

Was just curious as to the accuracy of the meter in the Power ouput setting. Maybe someone knows...can't find any published specs....

Thanks,

John
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N5XJT

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #1 on: March 30, 2023, 06:06:09 PM »

The SB-1000 is a derivative of the pre-MFJ Ameritron Al-80A, both are good amps.  The SB was a kit and like the AL-80A the output meter was not especially accurate, generally tending toward a optimistic reading.  For accuracy use a good external watt meter.
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W1ITT

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #2 on: March 30, 2023, 06:42:26 PM »

Even the much vaunted Bird Model 43 only claims an accuracy of 10% of the full scale reading of the particular slug in use.  The accuracy of the bridge and detector network used in amateur amplifiers can be fairly good, if care is taken during construction and setup.  But, without calibration against known standards, all of the common ham power meters fall short of mil-spec.  The point to remember is that the difference  between 1000 watts and either 900 or 1100 watts is very difficult to discern in actual on the air practice.  In my lab days, I have spent the most part of the morning optimizing couplers with a network analyzer on the bench, and it gets more difficult if the coupler has to have the same coupling factor and directivity over a wide frequency span such as the HF spectrum.  If you keep a technical log, it makes sense to record maximum output power into a dummy load on each band using the onboard meter.  Years later, if you suspect the final amplifier tubes are going soft, repeat the test and draw conclusions.   But don't expect the onboard meter to be lab standard.
73 de Norm W1ITT
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W1QJ

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #3 on: March 31, 2023, 04:00:53 AM »

The meter is NOT a watt meter at all.  It’s simply an RF “ sniffer”.  The RF gets rectified to a voltage, as the RF output increases so does that voltage.  You can calibrate the meter into a 50 ohm load and on the frequency it’s calibrated it will display a fairly  accurate reading into that 50 ohm load after calibrated.  Change frequency and load and all bets off on accuracy.  It’s only real worth is to show output as a relative amount which can be used to indicate a peak in output or to indicate a problem with output if it reads out of the norm.  You really need a directional coupled circuit for accuracy which this is not.
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N3NUE

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #4 on: March 31, 2023, 04:29:29 AM »

Thank for the replies gents, much appreciated. Just curious about that circuit and how well it worked.

Best, 73,

John
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W9IQ

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #5 on: March 31, 2023, 04:42:22 AM »

As Lou pointed out, the meter is simply an RF voltmeter with a scale that is calibrated in watts with a presumed 50 ohm load.

The challenge is that if the load is not 50 ohms (an SWR50 of 1), then the meter reading will be inaccurate. For example, with 1000 watts output into an SWR50 of 1.5, the meter will read somewhere between 660 watts and 1500 watts. That is quite a bit of inaccuracy.

The meter can be helpful for tuning and monitoring purposes. If you need a more accurate output power indicator, use an external directional power meter/SWR bridge.
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N3NUE

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #6 on: March 31, 2023, 04:55:13 AM »

Thanks Glenn...
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W1VT

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #7 on: March 31, 2023, 05:22:43 AM »

A directional coupler measures forward or reverse power.  So, we are a step closer to accurately measuring power.
But, directional couplers are frequency sensitive, some designs more than others. 
It is now possible to measure frequency and correct for that with a lookup table if an accurate reference standard is available.
It is also possible to subtract out the reverse power as well.
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VE7RF

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #8 on: March 31, 2023, 08:56:34 AM »

Even the much vaunted Bird Model 43 only claims an accuracy of 10% of the full scale reading of the particular slug in use.  The accuracy of the bridge and detector network used in amateur amplifiers can be fairly good, if care is taken during construction and setup.  But, without calibration against known standards, all of the common ham power meters fall short of mil-spec.  The point to remember is that the difference  between 1000 watts and either 900 or 1100 watts is very difficult to discern in actual on the air practice.  In my lab days, I have spent the most part of the morning optimizing couplers with a network analyzer on the bench, and it gets more difficult if the coupler has to have the same coupling factor and directivity over a wide frequency span such as the HF spectrum.  If you keep a technical log, it makes sense to record maximum output power into a dummy load on each band using the onboard meter.  Years later, if you suspect the final amplifier tubes are going soft, repeat the test and draw conclusions.   But don't expect the onboard meter to be lab standard.
73 de Norm W1ITT

Nope.  The bird + CD slugs are +/-5% of full scale....anywhere on the scale.  (with the pep kit, on the CD meter, it's then +/- 7% of full scale...anywhere on the scale.   With the pep kit on the bird meter, it's then +/-8% of full scale...anywhere on the scale. 

Bird / CD meter's are typ calibrated at 21 mhz, when their 2-30 mhz slugs are used.

The problem with the bird / cd meter's is..... you need to use the smallest slug for the job.

IE: +/- 5% of a 1000 watt slug (with meter displaying 1000 watts) is  +/- 50 watts.   IE: when meter reads...'1000 watts', the real power could be anywhere from 950 watts..to as high as 1050 watts.   If meter reads ...'500watts', real power could be 450-550 watts (now as much as +/-10% outa whack).

IF the same 1kw slug used to read just say  100 watts, the  real power could be anywhere from  50-150 watts. (now as much as +/- 50% outa whack).

On my L4B, in low power CW mode, with a 5 kw slug it reads 700 watts.   With a 2.5 kw slug, it reads 650 watts.  With a 1kw slug, it reads 625 watts.   When testing using a calibrated scope across the 50 ohm DL, it works out to 620 watts.

On  CW/ SSB / AM, without the pep kit, they just show quasi average PO.

Both (now discontinued)  Alpha wattmeter's, they used a freq counter...and a look up table, for a separate calibration for each band.  It would sense RF, and switch to correct calibration for that band..and within just 1 dot at 60 wpm if on CW. On
 SSB, cough into the VOX, and bam...new calibration point in use.

On my Array solutions 'power master'  (pep only at all times) wattmeter's, there is an option to 'auto subtract' the RVS power from the forward power...to display...'net power'.  There is also software controlled separate calibration  adjustments for frwd power...and also rvs power.  Both frwd + rvs calibration points can be adjusted in 1% increments...  from -15% up to  +15%.  That is done on the display unit, and no pots are used anywhere.   Each HF coupler is hand calibrated to nist standards, and the frwd + rvs calibration points are marked on each coupler...then programmed into the mating display unit. 

Back then, most of the amps had a ......  'relative PO' meter, used for tweaking the tune + load for max PO, that's it.  IE:  SB-200/220/230 /  TL-922, FL-2100B, etc
« Last Edit: March 31, 2023, 09:01:12 AM by VE7RF »
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W3SLK

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #9 on: April 01, 2023, 05:07:14 AM »

With the Bird watt meter not only do you have the 'meter accuracy' but you also have to account for parallax in taking the reading since neither uses a mirror behind the indicator, (Coaxial Dynamics does have this!). Most digital watt meters are usually as accurate as their least significant digit. In all the electronic indicators I have used, (especially in weights and measures), the LSD is rounded up based on 50% or more equaling that digit's value. I was always told that the most accurate way to measure RF power was to use a thermocouple ammeter into, (in this case) a 50 ohm load. And even there its not super accurate.
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W1VT

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #10 on: April 01, 2023, 05:59:28 AM »

An IR camera watching a dummy load heating up can yield a tremendous amount of data that can be used to calculate power.
The camera can spot "power leaks" like a connector or cable heating up excessively.  It would be interesting to see whether the heat map changes with frequency.
With old thermal devices you often have to make some assumptions on how the heat is distributed.
« Last Edit: April 01, 2023, 06:02:35 AM by W1VT »
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N3NUE

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Re: Accuracy of SB-1000 meter in Power setting?
« Reply #11 on: Yesterday at 04:25:44 AM »

Thanks again gents, always good to learn...

73,

John
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