[Elecraft] high-power tuner

[Fred Jensen]

"The only thing that shows gross lack of knowledge is not asking the 
question for which you need the answer."  [Cal Poly Physics Professor 
whose name escapes me but whose class I remember as a personal struggle].

To know the net rate of heat input [i.e. generation within the tuner] 
from the temperature change, you need to know the mass of the tuner. 
Weigh it and a little arithmetic and you have the mass.  The RF that 
doesn't heat the tuner must go up the coax.  Weighing has nothing to do 
with the efficiency itself, it is part of the 
not-exactly-precise-measurement-method I suggested but don't recommend.

For the FM transmitter, we had temperature sensors in the grid 
compartment on the bottom where the refrigerated air entered, and just 
far enough above the chimneys where it came out hot.  There was a water 
manometer to measure the pressure difference between the air inlet and 
the vent that took the hot air outside.  The pressure differential was 
very low so I assumed that the air got hot at a constant pressure.  This 
was an important assumption because at that time I didn't know enough 
calculus for the alternative.   I probably don't remember now either.

I got up very early one morning [was 17 and living at the TX], and 
turned on the filaments.  When the temperatures had stabilized, and 
knowing the filaments were turning 600W of electricity into almost 600W 
of heat, I could calculate the mass rate of the air flowing through the 
chimneys.  Sign on came, I let the temps stabilize at 5.3 KV and 3.5A 
plate current, and calculated the heat input to give those temps.  That 
power blew out the vent, the rest went up the 3 1/8" rigid coax to the 
antenna, I assumed.  Turned out we were a little overpower, almost 12KW 
for about 70% efficiency and the 4-1000 plates were fairly bright.  OK, 
really bright.

For the case of the tuner, it's not so simple and I skillfully tried to 
skate past the effect of radiation from the "black body" [i.e. the 
"black" tuner which, while black, probably isn't a real black body] as 
the RF heats it.   While I know the first six digits of Boltzmann's 
Constant [138065 -- it used to be the master unlock password for a UHF 
repeater I maintain], I've forgotten some of the physics and math I used 
to know and it's guaranteed that, were I to attempt that calculation on 
this list, a countable infinity of people would correct me and then each 
other ... likely forever or until Eric stepped in.

It was a e-nerd semi-joke, which I've found are not often funny except 
to other e-nerds.  Undaunted however, I continued.  Cookie's comment 
about measuring the efficiency of tuners being hard is very true, and 
you can't do it with a Micronta SWR meter from Radio Shack.  It will 
involve some math, maybe even calculus, and quite a bit of expensive 
test equipment.

I would trust ARRL's numbers, and those of a few others like Sherwood, 
I'll remain a little skeptical of the Mfr's numbers, if they even 
publish them.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2012 Cal QSO Party 6-7 Oct 2012
- www.cqp.org

On 3/9/2012 3:30 PM, Ian Kahn - Ham wrote:
 > This probably shows my gross lack of knowledge of the physics involved
 > here, but what does the weight of the tuner have to do with its 
efficiency?