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POWER CALIBRATION

A load manufactured by Bird; 50kW

In many cases an engineer can not simple read the power of a transmitter on a load. In the case of a NTSC visual television there is a conversion factor because of the specific AM modulation. The FCC wants visual power read as "peak" power: the power of the sync pulses. Loads can not tell the kind of power; they just get hot.

Here is a Load by Bird. It is double heat exchanged:
First by water, and second to air. Many oil-to-air heat exchangers can not dissipate heat as well as water based coolants.

A small inline load on top; 25kW. A bigger load on the bottom; 60KW This load served four large TT-10 transmitters. Tripple conversion: oil (sealed), to water-coolant (non contained), to air (fans). I could not run the big load very long as it produced much humidity in the rooms.

Broadcast dummy loads dissipate heat; necessarily showing this RF heat in average power. When calibrating power levels from Visual NTSC transmitters the average power is multiplied by 1.68 to obtain the peak power.


The direviation of this factor is straight forward...





PreEqualizing lines (labeled 1,2,3)
Equalizing lines = 6 x 1/2 Hlines = 3 lines
Equalizing pulse is 2.3uS at 100%
Equalizing blanking is 29.47uS at 75%
Equalizing average power is (2.3uSx(100%^2) + 29.47uSx(75%^2)) / 31.78uS = 0.594

syncronizing lines (labeled 4,5,6)
Syncronizing lines = 6 x 1/2 Hlines = 3 lines
Syncronizing pulse is 27.1uS at 100%
Syncronizing blanking is 4.7uS at 75%
Syncronizing average power = (27.1uSx(100%^2) + 4.7uSx(75%^2)) / 31.78uS = 0.936

PostEqualizing lines (labeled 7,8,9)
Equalizing lines = 6 x 1/2 Hlines = 3 lines
Equalizing pulse is 2.3uS at 100%
Equalizing blanking is 29.47uS at 75%
Equalizing average power is (2.3uSx(100%^2) + 29.47uSx(75%^2)) / 31.78uS = 0.594

The rest of VertBlanking (labeled 10-20) and Active lines (labeled 21-262)
Horizontal lines 525/2 each of two Fields - 9 in the vertical sync = 253.5 lines normal
Horizontal line is 4.7uS at 100%
Horizontal remainder is 58.86uS at 75%
Horizontal average power = (4.7uSx(100%^2) + 58.86uSx(75%^2)) / 63.56uS = 0.595

the average of:
3 pre-equalization lines, 3 syncronizing lines, 3 post equalization lines, and 253.5 normal lines
(3 x 0.594 + 3x 0.936 + 3x 0.594 + 253.5x.595) / 262.5 = 0.599 average power
If average is read on the load then peak power = 1/0.599 = 1.67 (official FCC amount is 1.68)
When I calibrate visual transmitters I maintain vertical sync integrity. And, of course, remove the burst and any setup.
This procedure can be accomplished with just the twist of a couple of pots. Time should never be wasted on video and timing setups. Time will be needed in the transmitter.

Analog NTSC transmitters are like old steam engines: Powerful, but maintenance intensive.

Time will always be needed for re-broadbandings and, at high power levels between 100% and 110%, sync stretch and linearity adjustments. There is another law: "Nothing is ever simple." To power-cal a transmitter may take as little as a half hour, or may take several nights, or possibly can not be done with present old tubes - at all!

Power tubes cost, in the range of, $23,000; and klystrons cost approximately $35,000. After those frustrating nights, where I have lost the battle, and have had to resort to replacing a tube, I have felt the financial loss; not to mention, in each case, a couple of more nights of re-broadbanding. An engineer is hired by a broadcast station, but is licenced by the FCC. Instant conflict of interest. In many aspects, especially financial, a seemingly loyal engineer is obligated to enforce unpleasant news.