On Tue, 9 Jul 2002, Jim wrote:

...
>I had been contracted at the time to install and
>tune a new ATU - Antenna Tuning Unit - at a 10 KW
>AM broadcast station.

Ok. 10kW is way out of my league ;-).

>As it was - I did get to borrow the GR bridge, but
>the inspired circuit *still* played a critical
>part - I had to re-tune the ATU and part of the
>process calls for finding the parallel resonant
>point ... my "R" bridge works optimally for this
>purpose - as one can easily see the phase change
>most radically with frequency as one tunes with a
>signal generator through the resonance point on a
>tuned circuit (the ATU in this case) - at this point
>the composite Z (R+jx) remains fairly constant and
>causes little change in the amplitude of a signal, but
>the phase stands out and identifies the resonant
>point like a sore thumb.

I aggree but as you would surely say, it takes cojones to do that with
what amounts to diy equipment and then turn on the juice (even slowly).
100W RF has pretty impressive built-in fault indicators afaik. 10kW from a
tube PA more so ...

>Whilst your suggestion is most likely valid - I doubt
>you can assure accurate numerical results without
>some difficulty (calibration charts etc) over several
>octaves (unto a decade) of bandwidth ...

Of course not. You need to spot-check the d.c. itself from time to time at
the working frequency. However the d.t. is a SWR meter and it takes a
couple of seconds (and four ganged good quality 100ohm cermet trimmers).
This is harder to explain than to do. I got the right kind of wire in 3
tries. The best kind of winding for wideband is with progressively more
spaced-out turns, but this does not work on a d.c. so I just put in the
turns (deliberately disordered winding).

Peter

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body