rw-lpfmguest.ls

RW July 5, 2000

Art: 4/c photo, file "rw-lpfmguestPHOTO"

Caption: Jeremy Lansman

Suggested RO:

Standing hed: GUEST COMMENTARY

JUMP HED:

By Jeremy Lansman

Regarding LPFM, maybe I have an "attitude". In the ’70s I encouraged many groups to start "community" FM stations, some as 10 Watt class D. However, I have also constructed and/or owned commerical and non commercial full power radio and TV outlets, as well as acted as consultant to broadcasters of all kinds.

The NAB asserts that Low Power FM "will create additional interference to hundreds of thousands of FM radio listeners." Yup. It’s true. Allowing LPFM equals more interference. Same for new regular FM stations, AM stations, TV stations, cell phones, or remote control car key locks.

Peek back to 1894. Marconi is testing the first spark gap transmitter. In spite of splattering garbage throughout the spectrum, he causes no interference. With one transmitter and receiver on Planet Earth, mutual interference is impossible. Only with a second spark transmitter can Marconi encounter interference. Building the second spark gap transmitter created the need for wavelength allocation!

Improvements since 1894 have reduced interference, but every radio transmitter still has the potential to interfere with reception of some other transmitter. Each new transmitter costs some degree of interference, however minute. Will a new station bring more benefit than cost? That is the regulatory conundrum.

The FCC ruled that LPFM stations may park .6 MHz from spectrum neighbors instead of the usual .8 MHz. Uusal, that is, with the exception of hundreds of class D, translator, grandfathered superpower, grandfathered shortspaced, and some TV 6 aural stations that cozy up at .4 MHz or less. Does .6 MHz flat out spacing permission given LPFM mean new interference? Sure. But how much?

Does reduced protection make sense? For some reason, I always just knew it did. As I tried to formulate arguments for LPFM support, I began to recall I had "been there, done that."

In the late ‘70s I had been part owner of KFAT a full power class B station in California. This was a powerhouse 1.15 KW ERP on 94.5 MHz transmitting from Mt. Loma Prieta, just south of San Jose. Though I had maximum class B power, the station was squeezed between superpower KPFA, 94.1, at 9.3 dB over the power limit, and KYLD at 94.9, with 5.6 dB over the power limit. Walking out the transmitter building door, 4,000 feet above San Jose, I could see Novato, some 90 miles distant. I could see up highway 101 past Palo Alto to San Francisco, or turning a bit to the right I could make out highway 17 to Berkeley. We know line of sight signal strength is a whole lot better than FCC predicted F50/50 predicted. All three stations had line-of-sight killer signals up the San Francisco peninsula and East Bay. If second adjacent slop-over caused trouble, I would know about it first hand..

KPFA-(FM), Berkeley, is a listener-supported radio station attracting a politically active and articulate audience. When we purchased KFAT, it was a Class A 250 watt ERP station, call sign KSND, on 94.3 MHz, transmitting from Mount Madonna. The Santa Cruz mountains blocked our signal toward San Jose, Berkeley and San Francisco, yet KPFA listeners had filled a good chunk of a file drawer with complaints that KSND had wrecked reception of a favorite station. We got FCC approval to move to the much higher Loma Prieta site, with higher ERP and a change of frequency to 94.5. If 2^nd adjacent protection were not adequate we and the FCC would have been deluged with complaints.

In the weeks following our move, the letters from KPFA listeners came in. To our surprise, they were letters of praise thanking us for eliminating interference to KPFA. In the years that followed, listeners and personal experience tested the limits of 2^nd adjacent FM protection.

So, why should you believe me, not NAB, CEA and CPB? Fair question. Broadcast radios are superheterodyne. That is, the signal is first amplified on its radio frequency (RF), then converted to an intermediate frequency (IF) before final conversion into audio and/or video. To convert the signal to IF, a local oscillator signal is mixed with the RF signal to produce the IF frequency of 10.7 MHz. The principal function of the RF front end is to amplify the fragile signals gathered by the antenna. Unfortunately, the front end also amplifies beafy superstrong signals. The front end is broad band, usualy several megahertz wide. Selectivity, the ability to reject stations close on the dial is produced by the IF section. IF selectivity is a function of IF filter quality. Since the 1950’s, filter technology has been vastly improved. IF filters have changed from little cans with slugs, wires and capacitors held together by parafin into nearly microscopic ceramic chips.

On the other hand, front ends got much worse with the invention of the transistor. Tubes anodes ran at 90 volts or more. Transistors get only a few volts. The result? Modern radio front ends are more likley to distort when confronted with strong signals. Strong signals from all over the dial. All at once. The invention of the transistor was a disaster, insofar as RF front end overload was concerned. But RF overload is not what 2^nd and 3^rd adjacent protection is about. Heck, a station half way to the other side of the FM dial can crunch reception of your favorite FM station due to overload. If FM radio teststers had tried to separate interference due to wide band front end overload from narrow band IF selectivity limitations they might have come to different conclusions.

The worst radio I encountered back at KFAT had a Lloyds label. It was a clock radio owned by a San Jose advertising client. It didn’t receive our station at all. But then it didn’t get KPFA or KSAN either, though it had no problem with competing San Jose stations. Must have been front end overload. I recall car radios with both aggressive Automatic Frequency Control and poor selectivity. Remember, these radios had slide rule dials, now an endangered species? Driving North, these radios wanted to switch to KSAN or KPFA. Remember FM adapters? You stuck this thing on the dash, plugged your AM antenna into the converter, and you heard FM on the AM car radio. Those adaptors all had aggressive AFC. Still, these screwy things lost our signal around Palo Alto, well outside our protected contour.

No one heard us in Berkeley, but we did OK in Oakland. When I visited my grandma in San Francisco, I remember tuning to our station on her very inexpensive phonograph-FM combo. The strong 2^nd adjacent signals never intruded into our signal.

Did our relatively low power FM receive Interference? Sure. There was co-channel garbage from Ukiah. In places a few miles from our transmitter, you could hear Ukiah clear as a bell. Where our signal was strong, Ukiah still caused co-channel noise. Then I remember STL co-channel noise and plenty of multipath. Hey, FM ain’t perfect!

I lived with 2^nd adjacent signals. Interference to your station from 3^rd adjacent LPFM won’t kill ya. That is, assuming those stations run clean transmitters. I can rant endlessly about poor engineering practice polluting the spectrum, and lack of FCC oversight but that is another story.

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