[HCDX]: Re: [AmFmTvDx] Re: tuning the

["Sheldon Daitch" <Sheldon_Daitch@xxxxxxxxxxxx>]

Jerry Monroe wrote:
> 
> I wonder if one use a small tweeker stick (like a mini insulated
> screwdriver) to keep unintended capacitive effects to a minimum  or
> something much larger so as not to get fried ??
> Maybe this is done at low power  ??

No, this is high power stuff.  A 5000W transmitter into 50 ohms is 500V,
enough to kill you.  This is done in an interlocked cabinet with large
knobs that have numeric indicators so you can record your settings.  At
least this is how it was done in the 60's.  The inductors/capacitors are
big to handle the voltages.

Maybe it is computer controlled in this day and age.  I don't really
know

-Allan-


         Adding to Allan's two informative posts on tuning a MW
         array, I agree there is a lot of art to it.  Especially if
         you have more than one directional pattern.

         I am not aware of any computer controlled arrays, in the
         sense other than from a logging standpoint.

         New, state of the art high power MW tranmsitters from
         Harris, do have processors that run things....even my
         combiner, that takes the power from 5 power blocks, to one
         output, has a processor, to realign the system when one
         power block drops and it goes to a mnus 1 situation.  The
         reject load will handle the unbalance for only a few
         moments, otherwise the system will shut down.

         At the real high power levels, the inductors are generally
         fixed and the capacitors are the tuning devices.  I have a
         MW directonal array with four patterns, at the megawatt
         level, and all of the capacitors in the central tuning house
         and the four tower tuning houses are gas-filled Lapp (R)
         capacitors that can be tuned, but require a screwdriver to
         turn the movable plates.  This is a situation, where you
         turn off the transmitter, adjust the cap and turn it back
         on, then using the antenna monitor to see what happened.
         The coils are also adjustable (sometimes), but at the high
         power level, the adjustable tap has a secure mechanical
         connection to reduce resistance losses, or damaging arcing.

         The consultants who do directional array work have porgrams
         that give them the calculated vales based on the proposed
         directional pattern.  It used to be done by hand, whiz
         wheels and slide rules, no less.  Carl Smith, a name well
         known in broadcast circles, is probably the father of
         directional array theory.  The calculated values are then
         adjusted on the coils and caps and then tested on the air,
         to see how it really works, and then tweaked for final
         adjustment.

         Another consideration for most DA arrays is that the
         radiation pattern is generally adjusted to protect another
         station on the frequency and the nulls are generally more
         important than gain in the major lobe(s).

         And with all that, there are consultants who have forgotten
         more about directional arrays than I'll ever know.

         Sheldon
         WA4MZZ
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