The CGC Communicator Newsletter Web Posting

Concerning finding the spurs on KKZZ(AM) -- good sleuthing! Now for a similar situation:

  I had an interesting case in Philadelphia in about 1993/4. This was the period of time when Cable TV was staring to clean up its act, and I have to say most systems have come a very long way since then. The goal at the time was to achieve "VHS quality" transmissions on cable.

  One of my clients was Garden State Cable, a very large system in southern New Jersey, just across the Delaware River from Philadelphia. They had just purchased (from me, I was a Tek Sales Engineer) a Tektronix 1450 demodulator and a VM700A Measurements set -- a far cry from the days the only tools cable had were a Signal Level Meter and maybe a sweep system.

  The cable system had received a few complaints about interference on KYW(TV), Channel 3. It was coherent interference, a "herringbone" pattern with evidence of modulation. I went to the cable headend and we looked at the signal on the system using the Tektronix equipment. (As many of you know, there is an FFT in the VM700 that allows you to look at spectral information in the baseband video.) There was an interfering signal 1.54 MHz above KYW's video carrier. We also knew that the interference was not always there, although it took a while to realize it was only present sunrise to sunset.

  We disconnected the headend receiver and connected the 1450 demod directly to the receiving antenna and observed the results on the VM700A.  The spur was still there, but we could not rule out the possibility that RF signals were mixing in the antenna itself and producing the spur.  Sometimes corroded antennas make excellent mixing pots.

  After running another test, we called the Chief Engineer at KYW. 'Not us,'he said. He was a good friend and I sort of set him up. I had the cable folks call him, and he was going to brush them off until they said "VM700 and 1450."  Ah, now we had his attention! ;-) He took a hard look at the TV in his office and told us, yes, he could see the interference in his over-the-air receiver. So, we knew the spur was originating at KYW. It was them, not us.

  Next the KYW Chief called Dave (N3CX, who has since passed away) at their transmitter plant, and Dave can see the spur too. He confirmed it to be 1.54 MHz above the video carrier.

  It turns out that the KYW transmitter is just a short distance from a 50 kW AM daytimer on 1540 kHz. Armed with that clue, Dave goes to the output of the STL microwave receiver and there's the spur. A day or two later, with some work on the microwave receiver, the problem is fixed.

  Those were fun times.

  Linc Reed-Nickerson
  President, KORC Radio, Inc.
  Waldport, Oregon

  linc45r-n (at) lincster.com
  January 13, 2010

Posted by Steve Blodgett
Earthsignals.com