[BC] Barry asked for my long winded response on 12 tower metering.

[Harold Hallikainen]

>The system was originally considered a "critrical array".  The required a Precision Phase Monitor. Vitro (Nems-Clarke) had manufactured a model to meet the FCC requirements. They went out of business and David Harry, Bob Ellenberger and Cliff Hall formerly of Vitro, I believe formed Potomac Instruments.  
>
>For many years their brochures had photographs of our 12 tower AM-19D  to read the phases and ratios to 0.1% and 0.1 degree and the PMA-19 Precision Monitor where we read the deviation from ratio digitally.
>
>Ralph Dippell and I tried to convince the VP of engineering to get an auto-logging system but the cost was pretty extreme.  It was a small percentage as compared to the overall cost of 160 acres, 12 towers,  8 years and three or four pattern designs at two sites with consultants and lawyer to get to get the FCC to accept for filing.
>
>Dave Hultsman

Great story! I also worked with Ralph Dippell on a much smaller station (a
class 4 AM and a class B FM).

On another station (mid-1970s), we built a 3 tower DA-N that was also a
critical array. We used the Potmomac PMA-19, which directly displayed
deviation from licensed ratio and deviation from licensed phase.

We connected the PMA-19 to a Moseley TRC-15A with one of my digital
adapters. The PMA-19 had an op amp based differential amplifier that gave
a single ended output of the deviations. However, I don't think the
differential amplifiers had good CMRR. The output would change depending
on what the tower current was, even if the ratio was the same. The
differential amplifier was a two op amp circuit. A voltage follower drove
the inverting input of a single op amp diff amp to raise the input
resistance. I changed the circuit to be just a pair of voltage followers
giving us a differential output instead of a single ended output. Since
the sample inputs on the TRC-15A were floating, they handled this fine. We
then got our remote display to agree with the display on the PMA-19, no
matter what the actual tower current was.

Interesting stuff. The station is now long gone. It was bought out by a
larger co-channel station and shut down.

Harold