Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!netsys!vector!telecom-gateway From: heller@lewis.crd.ge.com (Aaron Heller) Newsgroups: comp.dcom.telecom Subject: Re: Praise the Lord and Pass the RF Filters Message-ID: Date: 27 Jun 89 22:59:00 GMT Sender: news@vector.Dallas.TX.US Lines: 79 Approved: telecom-request@vector.dallas.tx.us X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@vector.dallas.tx.us X-TELECOM-Digest: volume 9, issue 216, message 1 of 6 Fred R. Goldstein writes: >That's a great story, but either it's a put-on, or the facts don't sound >quite right. ... Had WYCA been an AM station, then it would have been >perfectly believable. ... But an FM station? Not so fast. FM >transmitters maintain a constant amplitude, so if you "detect" them with >a simple rectifier, you won't get anything but perhaps a steady DC >voltage. Actually it is possible for an FM station's signal to be AM'ed as well, if the transmitter is mis-aligned. I have seen it happen and I think the circumstances may lend some insight to what may be the problem with WYCA. About 12 years ago, I was chief engineer of my college's radio station (WRPI, Troy, NY, 91.5 MHz, 10,000 watts ERP, 470 ft. HAAT). We would get about 2 or 3 RFI complaints a year almost invariably from someone who lived within a mile or so of our transmitter. These could always be handled by properly orienting their antenna or installing an inexpensive tunable RF trap. According to FCC R&Rs, an FM station can measure its power output in either of two ways: The direct method -- a watt meter installed in the transmission line as it leaves the transmitter; or the indirect method -- multiplying the input power to the final amplifier by an efficiency factor. Even though we determined the power output by the direct method I would still calculate the efficiency factor to monitor the aging of the final amplifier tube. This was usually about 65 percent (i.e. 65 percent of the power going into the final amplifier was coming out of the transmitter as RF and 35 percent as heat). Once, while studying the FCC R&Rs, I noticed that they said that the efficiency factor for indirect power calculation was a number determined by the transmitter's manufacturer. I looked it up for our transmitter, at that time a Gates FM-5H, and the manual said 50 percent. This meant that if we switched to the indirect method and used Gates' efficiency factor of 50 percent we would be legally underestimating our power output and actually running at 13 kW ERP. The next morning before sign-on I went out to the transmitter installed our newest final amplifier tube and spent about an hour adjusting the tuning of the transmitter to the highest efficiency I could and achieved an efficiency of 78 percent. Add to that the fact that you are actually allowed to operate at up to 105 percent of licensed power output and you get 16.4 kW -- more than a 2dB increase in signal strength. I patted myself on the back for improving our reception in fringe areas and headed back for campus. Later that afternoon, I stopped by the station and was told by the station manager that we had received about a dozen RFI reports since we had signed on. People were getting us all over the FM band, on their telephones and TV sets (this is before the days of home VCRs and video games). I drove back out to the transmitter site and retuned it to the old settings and reset the (true) power output to 10kW. I called as many of the people complaining of RFI that I could reach and confirmed that things were back to normal. The following Saturday morning I fed a tone into the control room board and headed out to the transmitter with a spectrum analyzer and 100MHz 'scope to see what was actually going on that would cause all of this interference. I returned the transmitter settings to those of earlier in the week and hooked everything up. On the 'scope I saw that the signal was being AM'ed at about 20 percent modulation with the tone from the studio as well as another 1 MHz wave. The spectrum analyzer showed spurious emissions up and down the band at 1 MHz spacing. I realized that in tuning the final stage for such a high efficiency also caused it to have a very narrow peaked response instead of the broader and flatter response it was supposed to have. This caused the AM'ing. I also figured that some instability caused that 1 MHz oscillation. The symptoms described in the WYCA case sound very similar to those I observed here. It would seem that their signal could use some serious scrutiny with an oscilloscope and spectrum analyzer. This could all be done without their cooperation from any site near their transmitter. If the AM'ing is severe enough you can actually see the signal strength meter on a receiver move in time to the audio. Aaron Heller (heller@crd.ge.com uunet!crdgw1!heller 518-387-5542)