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

[Burt I. Weiner]

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

>From: "Ronald J. Dot'o Sr." <ron.doto at comcast.net>
>
>I've followed this thread with interest as I maintain four BE AM-1A's
>and a Gates 1.
>
>Only one station (a directional) has the BE output network.
>
>The output network is like adding tuning and loading controls to the
>AM-1A and is NOT a substitute for an ATU.
>
>I have a simple method for adjusting the ATU used with an AM-1A.  I
>start out at low power, say 50 watts with no modulation and adjust the
>coil taps one turn at a time for the lowest reflected power.
>
>It's important to do the adjustments without modulation so that the
>tuning is centered on the carrier frequency.
>When you add modulation the sidebands will raise the reflected power
>slightly.  So far the sidebands seem centered during our annual proof
>for occupied bandwidth with this method.
>
>In the past I've adjusted the ATU with a bridge to a 50 ohm, 0J match
>but the TX didn't like it when I applied power, as I guess things
>change under full power, so I adopted this method which doesn't
>require the test equipment and works for me.