As Jim says: "The coax can be a resonant circuit as well. Being open at one end it will reflect the signals back to the antenna to be reradiated."
Maybe the problem would have gone away by simply terminating the open end? Jim Hulbert Pitney Bowes From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Jim Bacher Sent: Monday, July 11, 2011 8:10 PM To: ri...@ieee.org; emc-p...@ieee.org Subject: Re: Puzzle: Radiation from a disconnected TV antenna? Rich, there is at least one preamp that was recalled due to defects in manufacturing. The preamp would not only function as a preamp, it would reradiate RF at other frequencies. What happened was the input fet or transistor would oscillate due to poor placement of the active part. Dependent on temperature the frequency of the oscillation will shift during the day. The oscillation would mix with all the incoming signals the antenna was picking up thereby creating new "signals". So the issue would move up and down the frequencies. As it was tied to an antenna if was a very effect interference source. The only way to chase it is with spectrum analyzers or communication analyzers as it moved too fast to chase with programmable radios. The FCC has mandated recalls on at least one preamp. One of them was used in campers, which made finding them more difficult as they were moving some of the time. It some of the connections on the antenna are corroded, it can function as a diode mixer and reradiate. As it is an antenna it is effective at radiating what it is mixing. Although cleaning and resetting the elements likely would clear the issue, and as you have had a visit by the FAA, I would replace the antennas to show good faith in trying to resolve the issue. The coax can be a resonant circuit as well. Being open at one end it will reflect the signals back to the antenna to be reradiated. Jim ________________________________ From: "ri...@ieee.org" <ri...@ieee.org> To: emc-p...@ieee.org Sent: Mon, July 11, 2011 12:35:55 AM Subject: Puzzle: Radiation from a disconnected TV antenna? My normal subject is safety. However, I have an EMC situation that I would like to understand. When I built my Vancouver, Washington, house, I installed an FM antenna in one attic and a VHF-UHF antenna in the other attic. The line of sight from my house to the grouped Portland, Oregon TV stations is about 12 miles. The line of sight crosses the eastern edge of Portland International Airport, probably about 4-5 miles. The FM antenna points in the same direction, with about 12 miles to a group of FM transmitters. The house has been rented for a number of years. Last month, we moved back into the Vancouver house. When I hooked up the TV antenna, I was surprised to find no signal. In the attic, I found the coax had been disconnected from the antenna, and the F connector removed and discarded. I asked my former tenant about this. Here is what he said: "...the FAA from the Portland Airport showed up one day with a machine that said it was getting a signal from the house that could potentially interfere with airport signal, they walked around the house and eventually went into the attic and said it was coming from antenna in the attic, they said they unhooked it..." My tenant did not use the TV antenna. The coax from the antenna goes to a junction box. There was no connection in the junction box, so the coax was open-circuit. My question: How does a TV antenna with about 20 feet of coax (open at the far end) cause interference? My hypothesis: Any signal picked up by the antenna goes down the coax to the open circuit, then reflects back up the coax to the antenna where it then radiates back towards the source. Clearly, this is what happens to the TV signals. Sorta like a radar reflector. I would expect standing waves in the coax, depending on frequency. Even though the antenna is tuned to the VHF-UHF frequencies, it still picks up extraneous signals, which could include signals from Portland International Airport. Hence, potential interference. Can you explain how my passive and disconnected TV antenna causes interference? Can you tell me what equipment the FAA used to find my antenna? (I'm sure it wasn't a grid-dip meter!) Maybe a portable spectrum analyzer? Why wasn't the FM antenna also detected and disconnected? The FM antenna had the same situation: about 20 feet of coax and an open circuit at the end. I suppose it is due to the much narrower band of the antenna. Thanks for any explanations, and best regards, Rich - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com>
