Efficiency not what I thought

[W2VW]

John,
 You can have the bragging rights of all time best efficiency because all I have is a BS-O-Meter referenced to another BS-O-Meter.

Hey Frank, Do you have the matching BS Step Attenuator?

[WA1GFZ]

Attenuators are the only accurate thing in my shack because I can bring them to work and check them against real test equipment with real cal stickers referenced to the NIST.
Even then I rarely trust my own dynamic range numbers unless I get the same number 3 times on different days.
As performance increases I trust measurments less.

The BS-O-Meter S meter is another good one. 

[k4kyv]

You just might be a redneck if...

http://amfone.net/Amforum/index.php?action=dlattach;topic=8910.0;id=1391;image

My dummy load consists of twelve 600-ohm glo-bar resistors in parallel.  Not sure of the wattage, but each one is about 18" long and 1" in diameter.  I acquired them new old stock from 1945, but the data sheet did not include power rating, but a similar size wirewourd resistor would be over 200 watts.  That would make the whole thing rated at 2400 watts, with each resistor mounted vertically to produce convection cooling through the hollow tubes.  I car run the BC1-T at full power for hours, and although it makes a good room heater, nothing appears to overheat.

I have noticed that the DC resistance, as measured with my Fluke DVM, changes when the resistors heat up, from 50.0 ohms to +/-  2 or 3 ohms (can't remember which).  Interestingly, the SWR reading stays exactly 1:1 as the resistors heat up.

My best estimate of power output is made using that dummy load, and a thermocouple rf ammeter.  I have several ammeters, so I use more than one, and go by the ones that give identical readings, on the assumption that they are not all off by exactly the same percentage.

My next step is to check the calibration of the kv meter and plate milliammeter in the Gates.  If the efficiency turns out to be close to the same as the homebrew rigs, that will tell me that my power readings are probably low.  If  it indeed gives a higher rf amps reading on the same frequency, into the same dummy load, at the same plate voltage/plate current as the other transmitters, that will tell me that the other rigs are running at lower efficiency.

I think the FCC calls this "indirect power measurement."

[Jim, W5JO]

Speaking of BC 1Ts, in the years I was in broadcasting, I worked at a couple of stations that used that transmitter.  Both had a board mounted on the left side of the cabinet that had big Globar resistors mounted in that fashion.  The board also had a ceramic light socket that a light bulb of about 40 watts painted red attached. 

I forget how the power was switched from the antenna connection to the dummy load, but when you transmitted into it, the light would illuminate.  I thought that was standard in that transmitter.

[k4kyv]

It was missing from mine.  Didn't see any imprints from removed components, so I'm not sure if the dummy load was removed, or if the transmitter never came with one.  As I recall, those transmitters used a dummy load made of  specially wound wirewound power resistors.  They work ok on the BC band, but get flaky at higher frequencies.  My load uses Glo-bar composition resistors.

[steve_qix]

You just might be a redneck if...

http://amfone.net/Amforum/index.php?action=dlattach;topic=8910.0;id=1391;image

My dummy load consists of twelve 600-ohm glo-bar resistors in parallel.  Not sure of the wattage, but each one is about 18" long and 1" in diameter.  I acquired them new old stock from 1945, but the data sheet did not include power rating, but a similar size wirewourd resistor would be over 200 watts.  That would make the whole thing rated at 2400 watts, with each resistor mounted vertically to produce convection cooling through the hollow tubes.  I car run the BC1-T at full power for hours, and although it makes a good room heater, nothing appears to overheat.

I have noticed that the DC resistance, as measured with my Fluke DVM, changes when the resistors heat up, from 50.0 ohms to +/-  2 or 3 ohms (can't remember which).  Interestingly, the SWR reading stays exactly 1:1 as the resistors heat up.

My best estimate of power output is made using that dummy load, and a thermocouple rf ammeter.  I have several ammeters, so I use more than one, and go by the ones that give identical readings, on the assumption that they are not all off by exactly the same percentage.

My next step is to check the calibration of the kv meter and plate milliammeter in the Gates.  If the efficiency turns out to be close to the same as the homebrew rigs, that will tell me that my power readings are probably low.  If  it indeed gives a higher rf amps reading on the same frequency, into the same dummy load, at the same plate voltage/plate current as the other transmitters, that will tell me that the other rigs are running at lower efficiency.

I think the FCC calls this "indirect power measurement."

Wow, PROBLEM !

If the load is increasing in value, of course the current will drop and the error will be MULTIPLIED (I ** 2) * R  And of course, the resistance may be changing more than you might be able to measure because you can't measure it "in circuit", so to speak.

As Frank, myself and others have experienced, it is VERY hard to ACCURATELY measure efficiency.  I've got a calibrated, verified RF amp meter around here (2 of them, actually), but I only trust it at the current at which it was calibrated.  I've found, particularly with thermocouple RF amp meters, the accuracy is not linear across the range of the meter.  I've even found this to be true of standard milliamp meters.  So, I calibrate them at the current I'm going to be actually measuring - at least then, I can trust the reading at the set operating point.

I am partial to using a 'scope [assuming the calibration has been verified !!] for calculating efficiency because you can see exact, and extremely small changes in the output voltage (across a known load).  Of course, this assumes a pure sine wave........ because the RMS calculations will be inaccurate for other waveforms.

Yikes !

Regards,

Steve

[WA1GFZ]

When I discovered class e (some years after Nathan) I wouldn't say a word about efficiency for about 6 years because I didn't think it was true. 90% just sounded like I was cheating.

Bottom line if you are still blowing out parts you don't have it right.
Layout is very critical before you can make claims of high efficiency.
You can't cheat the laws of physics........capt. 

[KF1Z]

Just to confirm.... Am I using the right formula to find my output power?

Using a scope, and dummy load...
Measuring the voltage at the load....

I've been doing this calculation....

Vpk-pk  X  0.35355 =  Vrms

(Vrms X Vrms)  /  Rload   =  Powatts


Obviously dependent on whether the scope is calibrated......
But, that is the correct method, right?

[steve_qix]

Just to confirm.... Am I using the right formula to find my output power?

Using a scope, and dummy load...
Measuring the voltage at the load....

I've been doing this calculation....

Vpk-pk  X  0.35355 =  Vrms

(Vrms X Vrms)  /  Rload   =  Powatts


Obviously dependent on whether the scope is calibrated......
But, that is the correct method, right?

That's one way to read it... or .7071 x the peak (not peak to peak), but either way, it's RMS assuming a true sine wave.

This is where the calibration becomes critical, becuase an inaccuracy in the measurement instrument is multiplied twice (squared !!).