Radio Frequency Interference

1. Radio Frequency Interference ARRL Book Section 3.5

2. Interference • Hearing something or signals getting where not suppose to • Things like hearing the radio on your telephone or telephone calls on your TV • Sometimes get complaints that radio is interfering with neighbors television • Since TV and amateur radio use different frequencies that should not happen • Can happen if your radio is putting out off frequency signals or TV has poor shielding and tuning and tunes in anything in the area • TV and telephone manufactures are notorious for this problem • Doppler shift does not cause interference problems (T5D02) • The major cause of telephone interference is that the phone was not built with proper interference protection (T3D03) • The most likely cause of telephone interference from a nearby transmitter is that telephones components are making it act like a radio receiver (T5D03)

3. Fundamental Overload • Quality of device design measured when very strong but off frequency signals in the area – proper shielding should block • Strong off frequency signals can interfere • If I can reject 99% of stray signal but the stray signal is 100 times stronger than what I’m listening to I will have interference • Called Front End Overload when interference results from a strong nearby source (T3D11) • Can also be called Fundamental Overload when one is talking about a receiving device getting interference from strong but off-frequency signals (T5D01)

4. Cable Television • Can get interference with cables with bad shielding • If there is a break in the cable Radio signals can be picked up by cable line or cable signals can radiate out and be picked up by Radio (T3D07) • Obviously the cable company needs to fix their cable • Interesting problem BPL – trying to send data over bare power lines • Lines radiate signals notorious for interfering with HF • Called unintentional radiator • FCC is responsible for seeing that people don’t put out signals on someone elses frequency • Bush administration was pushing BPL but FCC lost a case and was ordered to protect other peoples frequencies

5. What To Do If You Get a Complaint • Check you station to make sure it meets standards for good amateur practice (T5D08) • This would be checking for things like proper connections, grounding, antennas • Make sure your station is operating properly and that it does not cause interference with your own TV (T5D05) • If radio is not operating properly it can put out stray frequencies or harmonics • Your TV is probably much closer – if your radio is putting out TV signals you should have the problem • To stop spurious emissions you can install a filter at the transmitter (T5A06)

6. Types of Filters • High pass filters (block all frequencies below a certain frequency but let high frequencies pass) • I wonder whether that is why they are called high pass filters • Low pass filters (block frequencies higher than a certain frequency but let low frequencies pass) • Band Pass filter (allow only frequencies in a certain range to pass) • Notch Filter (opposite of a band pass filter – blocks only a certain frequency range) • Snap on Ferrite Choke • One of problems is that coax cable or a rain gutter can start acting like an antenna and send out harmonics of an intended signal • Chokes reduce or stop that tendency • All of these types of devices can be used to correct radio interference problems (T5D07)

7. If your neighbors TV is getting front end overload from your radio it is the TV sets owners responsibility to fix (T3D02) Often people don’t have a clue how to keep radio waves from getting into their equipment – the trick is filters that eliminate frequencies art suppose to be monitored If manufactures didn’t short cut they would have put them there in the first place Who Is Responsible to Fix?

8. Things to try • If a TV is getting front end overload from a 2 meter transceiver try a notch filter that blocks out 2 meter amateur band(T5A07) • If a telephone is having a radio interference problem install an RF filter on the telephone(T5D04)

9. What if Your Radio is Picking Up the Interference • Overload will often be bursts of tones or unwanted conversations suddenly popping in(T9B03) • FCC allows household devices to put out weak signals on other users frequencies provided they don’t cause interference • These are called part 15 devices • If you radio station is getting interference from a neighbor’s part 15 device, check to make sure your station meets good amateur radio design standards (if someone is using your frequency with an unlicensed device you can’t filter it)work with your neighbor to identify the device, if problem cannot be corrected politely inform him that FCC rules require him to stop using the device. (T5D09)

10. Other Amateurs Are A Great Resource in Helping You Track Down Problems • If another operator tells you that he hears a high pitched variable whine from your mobile the most likely cause is the radio’s wiring is picking up noise from the vehicle’s electrical system(T5D10) • One of the reasons Chris Pixton dislikes my plug the radio through the cigarette lighter trick.