[BC] B.E. tuner and such...

[Dana Puopolo]

Also remember that a transmission line can also match some impedances. I once
had to bypass an ATU in an emergency so I connected the transmission line
right to DC blocking cap before the tower figuring that I might be able to run
500 watts that way (it was a 10 kW station that had 1 5/8 line out to this
tower). Lo and behold, the reflected power on the transmitter read almost
zero, so I cautiously brought it up to 1 kw. Same thing, so I dragged out the
OIB and measured 51 + j8 at the transmitter output. Then I went out to the
tower and calculated the power. I was getting 990 watts, so I knew that 10
watts max. was being lost in the line. Finally, I brought it up to 5 kW and
left it there. The reflected power barely moved on the pin and I figure I was
losing only about 30 watts in the line.
I ran that way for a week before the repair parts arrived. This was a tower
just a bit short of a quarter wave, near the center of the band.

-D

From: "Burt I. Weiner" <biwa at att.net>

Ron,

The intent is to get 50j0 at the transmitters antenna terminal.  If 
you set the ATU out at the tower to 50j0 it may be 51+j10 (or 
thereabouts) at the transmitter's antenna terminal.  I don't recall 
if the B.E. tuner is ordered by frequency and working impedances on 
both ends, although it should.  A lot of the tuners I've seen are 
pretty generic and will tune anywhere by simply adjusting taps.  Not 
a good way to go.  See Phil's recent articles in Radio Guide on that subject.

It's usually pretty simple to get the transmitter to see 50j0 at 
carrier, but there's a lot more to it than that.  The total signal, 
including sidebands are looking down a tunnel consisting of 
everything between the connection at the output of the transmitter to 
the tower itself.  The idea is to get all of the "tunnel" centered 
and wide enough around the sidebands to not make them 
ferkrimpt.  Accomplishing that is where the art of it all comes in.

Typically I will set the Common Point to 50 and somewhere around 
-j10, depending on what makes the transmitter the happiest at 
carrier.  To accomplish the same thing but including the sidebands 
can be a bit more involved.  In a few cases, where there's a Common 
Point Bridge, I've done that and then tweaked the reactance a wee bit 
to get minimal wiggle in the null on the bridge under high audio 
frequency modulation.

Is it possible that the reason you're seeing a shift in the impedance 
when you go to higher power is that at 50 Watts you cannot see the 
null as well?

Burt

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