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

[Ronald J. Dot'o Sr.]

Hi Burt,

Thanks for the input.

No directionals or common points are involved, just non directional 
single antennas.  There is one directional station that I repaired the 
BE TX that was dead but I didn't mess with the phasor or any of the 
settings when I got it going again.  As Dirty Harry says: "A man has 
got to know his limitations".

The station with the AM-1a and Gates 1 backup has the ATU in the shack 
about four feet from the  BE and about two feet from Gates so I can 
see the reflected power meters of both TX's as I adjust the ATU.  The 
feed from the ATU to the tower is copper tubing through the wall (with 
insulator) with a one turn loop and braised to the tower.

Both TX output coaxes go to a rocker type antenna switch on the wall 
with the switch output coax going to the ATU input.

The ATU is an old Gates "T" configuration.  The coil on one of the 
legs has a vernier adjustment disk mounted on a threaded rod which 
extends through the bottom of the cabinet with a knob on it.  If I 
recall it's in the output lag so controls the "R".  The disk moves 
inside of the coil to increase or decrease inductance for fine tuning.

I would assume that the reflected power meter on the BE and the Gates 
is looking down the tunnel at everything downrange from the type N 
output connector.  Therefore when I tune the ATU for minimum reflected 
power in the CW mode I would think that I got the best match possible.

As you say, with a 50J0 input at the ATU input the TX may not be 
seeing that match at its output terminal as it doesn't take into 
account the values seen through the coax and antenna switch, so that 
would explain why I have to retune the ATU from a 50J0 to what 
actually works.

After I got everything adjusted it has been working fine for several 
years and several annual occupied bandwidth proofs.

I have another station that is operating from a temporary location 
with a long (actually rather short) wire "T" antenna and a home brew 
ATU  while we locate a new tower site as we lost the lease on the old 
site.  I used the same method to match the TX to the antenna and got 
things down to a low enough reflected power that the AM-1a will 
tolerate.

As for losing the null at higher power; no I keep the null but on 
higher power the BE won't tolerate the higher reflected power so I 
have to start out on low power and work my way up to the sweet spot. 
What I meant to convey is that when I set the ATU input to 50J0 that I 
don't have as good of a match as I do when I adjust the ATU for 
minimum reflected.  I would think that using this method would take 
into account all the reactance's etc. in the tunnel as you suggest.

In any event it's all working so I ain't gonna fix it any more than it 
already is. :-)!

Ron D

----- Original Message ----- 
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
>