106.9MHz notch filter
Hi,
I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz The FM antenna has a "standard" 300ohm screw connection for the lead-in (to which I connect a 300/75 xfmr and use coax down). I would appreciate anyone's help in designing and implementing a notch filter that would suck out a major part of the interfering energy. It seemed to me that a reasonable attempt would be to use a piece of 300ohm flat-lead and short it at an appropriate distance from the screw-terminals of the antenna. None of my attempts have yielded any observable improvements. It may be that the filter must be "deeper" (higher Q?) than what my attempts provide. Any ideas? Thank you. ....Bryce |
On Fri, 10 Sep 2004 03:50:44 GMT, "A-Tech"
wrote: It seemed to me that a reasonable attempt would be to use a piece of 300ohm flat-lead and short it at an appropriate distance from the screw-terminals of the antenna. None of my attempts have yielded any observable improvements. It may be that the filter must be "deeper" (higher Q?) than what my attempts provide. Any ideas? Hi Bryce, Length can be very critical as you can imagine. Trying to find the sweet spot, over and over, can become taxing if not simply fruitless (undoubtedly your experience to this point). A simple method is to connect the twin lead in the usual manner. Measure out a little more than 1.5M to work with (this will be too long, but it is better than being too short). Take a razor blade (old style used for scraping paint) and working from the far end cut through the insulation of both leads at once to short them out. Repeat at 1/16" intervals (or 1mm intervals if we are sticking with metric) and note results. You don't want to cut through completely, just enough to nick the copper and make a good short with the blade. Keep in mind that while you are handling the line doing this, you are part of the tuned circuit (and possibly detuned circuit too). It will give you a quick ball-park and reduce the cut-and-try. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Fri, 10 Sep 2004 03:50:44 GMT, "A-Tech" wrote: It seemed to me that a reasonable attempt would be to use a piece of 300ohm flat-lead and short it at an appropriate distance from the screw-terminals of the antenna. None of my attempts have yielded any observable improvements. It may be that the filter must be "deeper" (higher Q?) than what my attempts provide. Any ideas? Hi Bryce, Length can be very critical as you can imagine. Trying to find the sweet spot, over and over, can become taxing if not simply fruitless (undoubtedly your experience to this point). A simple method is to connect the twin lead in the usual manner. Measure out a little more than 1.5M to work with (this will be too long, but it is better than being too short). Take a razor blade (old style used for scraping paint) and working from the far end cut through the insulation of both leads at once to short them out. Repeat at 1/16" intervals (or 1mm intervals if we are sticking with metric) and note results. You don't want to cut through completely, just enough to nick the copper and make a good short with the blade. Keep in mind that while you are handling the line doing this, you are part of the tuned circuit (and possibly detuned circuit too). It will give you a quick ball-park and reduce the cut-and-try. 73's Richard Clark, KB7QHC Unfortunately, you will find that the -3dB BW of a single stub is in the neighborhood of 5% or 5MHz at your frequency of interest. The discrimination between your desired and undsired channels will be negligible. We manufacture a line of small multiple cavity symmetrical and asymmetrical notch filters- getting under 0.5% BW with small topology filters is nearly impossible at VHF. Dale W4OP |
I agree with Dale that you're not going to get anywhere near sharp
enough a filter by making something out of twinlead. Since the signals are coming from different directions, it might be more practical to make a null in the antenna pattern in the direction of the interfering signal. This is most likely to work if the desired station is reasonably strong, so it doesn't have to be in the direction of the antenna's maximum response. A two element phased array can be designed to achieve a deep and fairly narrow null, and you get a deep null off the end of a very symmetrical dipole or the side of a Yagi. If the directions are fairly close, an antenna with wider beam, like a 2-element phased cardiod pattern array or a dipole, would give you more of the desired signal when the undesired one is in the null, than a Yagi would. You might have to use some sort of feedline decoupling (common mode choke, also known as a "current" or "choke" balun) to prevent the feedline from being part of the antenna and reducing the depth of the pattern null. Roy Lewallen, W7EL |
You could also put the filter not at the antenna, but back on the lead
in just before the receiver. That way any 106.9 being pick up in the feed line are also attenuated. Have you tried calling the 106.9 station? Years back, many stations freely (or very cheaply) provided filters to people experiencing interference from their signal. I remember back in 1987 I had a customer whose Muzak SCA reception was being trashed from a nearby broadcaster. I was expecting to pay for a filter, but the station engineer literally left one at their reception desk for pickup. I would not expect free, but you could hope for inexpensive. Never hurts to call. Good luck, B. |
Hello,
Firstly what does "Q" stand for? You could try and shield the antenna from the other transmitter depending on where everything is located. You might be better buying a decent tuner with a narrower filter built in, often Kenwood or Sony have this feature. I have a DENON and it suffers from the same problem of having WIDE filters so it can't select individual stations too well without getting hammered by the one next door. "A-Tech" wrote in message ... Hi, I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz The FM antenna has a "standard" 300ohm screw connection for the lead-in (to which I connect a 300/75 xfmr and use coax down). I would appreciate anyone's help in designing and implementing a notch filter that would suck out a major part of the interfering energy. It seemed to me that a reasonable attempt would be to use a piece of 300ohm flat-lead and short it at an appropriate distance from the screw-terminals of the antenna. None of my attempts have yielded any observable improvements. It may be that the filter must be "deeper" (higher Q?) than what my attempts provide. Any ideas? Thank you. ...Bryce --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.756 / Virus Database: 506 - Release Date: 09/09/2004 |
Many years back, either Radio Electronics or Radio and TV news published a how-to article which described using two
identical Yagis horizontally "stacked" and gave formulas or charts to determine the spacing for creating an off axis notch to do what Bryce needs to do. Otherwise, either Dale's filters or some from an outfit named Eagle may be the only answer. -- Crazy George Remove N O and S P A M imbedded in return address |
On Fri, 10 Sep 2004 03:50:44 GMT, A-Tech hath writ:
Hi, I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz The FM antenna has a "standard" 300ohm screw connection for the lead-in (to which I connect a 300/75 xfmr and use coax down). I would appreciate anyone's help in designing and implementing a notch filter that would suck out a major part of the interfering energy. It seemed to me that a reasonable attempt would be to use a piece of 300ohm flat-lead and short it at an appropriate distance from the screw-terminals of the antenna. None of my attempts have yielded any observable improvements. It may be that the filter must be "deeper" (higher Q?) than what my attempts provide. I think, as Roy suggested, that your best attack will be to aim the antenna to provide the deepest null to the "... off-axis stronger signal at 106.9MHz". Peaking the desired signal is only a secondary goal here. Don't spend a lot of Big Bucks trying to solve this problem. Marvin's Law (Murphy's brother) says that once you _have_ put considerable money, effort, and time into this, the station's (new) owner will change the format to Country or Jesus. (Now, How's *that* for provoking thread drift?) gl es 73 Jonesy -- | Marvin L Jones | jonz | W3DHJ | linux | Gunnison, Colorado | @ | Jonesy | OS/2 __ | 7,703' -- 2,345m | config.com | DM68mn SK |
On 10 Sep 2004 16:10:57 GMT, Allodoxaphobia wrote:
I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz Hello Bryce, I'm sure you're aware that these two frequencies are adjacent channels. The FCC allocated channels such that adjacent channels stations would not be located in the same geographical area, expressly fo the purpose of preventing such interference. Obviously the two channels involved are not in the same area, and it seems strange to me that you can even hear the station on 107.1 MHz. I believe that your only hope is to orient your antenna to place the null in the direction of the 106.9 station, as Roy suggested,, but I'm not sure that even that will reduce the 106.9 station sufficiently to allow acceptable reception of the 107.1 station. In addition, I firmly believe that attempting to filter out an adjacent channel will be futile, and not worth the effort. Walt, W2DU |
"Walter Maxwell" wrote in message ... On 10 Sep 2004 16:10:57 GMT, Allodoxaphobia wrote: I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz Hello Bryce, I'm sure you're aware that these two frequencies are adjacent channels. The FCC allocated channels such that adjacent channels stations would not be located in the same geographical area, expressly fo the purpose of preventing such interference. Obviously the two channels involved are not in the same area, and it seems strange to me that you can even hear the station on 107.1 MHz. ..... Walt, W2DU When you are BETWEEN markets, this is common. Or, if you want to listen to a distant station and the wrong one is close. I'm equi-distant from Chicago, Milwaukee and Rockford and have this problem hearing WDCB 90.9, west of CHi-town with WHAD 90.7, S-W of Milwaukee pounding in. They're in different markets. This was discussed not to long ago here. A notch in the antenna pattern is not too difficult. Two vertical dipoles spaced (about) a half wave and fed with the proper feed line so they are 180 degrees out from each other, can make a nice null - actually two. They are perpendicular to the plane of the antennas. I'd have to sit and tinker to remember the feed system, but there has to be 1/2 wave more feed line in one dipole, compared to the other. It is also possible to do the same thing and get a one notch pattern (cardioid). If I recall, the antennas are spaced 1/4 wave apart and the feed is 3/4 wave of feed line between them. Again, the receiver simply connects to one of them and the null is off one end. -- Steve N, K,9;d, c. i My email has no u's. |
On Fri, 10 Sep 2004 13:38:34 -0500, "Steve Nosko"
wrote: When you are BETWEEN markets, this is common. Or, if you want to listen to a distant station and the wrong one is close. I'm equi-distant from Chicago, Milwaukee and Rockford and have this problem hearing WDCB 90.9, west of CHi-town with WHAD 90.7, S-W of Milwaukee pounding in. They're in different markets. This was discussed not to long ago here. A notch in the antenna pattern is not too difficult. Two vertical dipoles spaced (about) a half wave and fed with the proper feed line so they are 180 degrees out from each other, can make a nice null - actually two. They are perpendicular to the plane of the antennas. I'd have to sit and tinker to remember the feed system, but there has to be 1/2 wave more feed line in one dipole, compared to the other. It is also possible to do the same thing and get a one notch pattern (cardioid). If I recall, the antennas are spaced 1/4 wave apart and the feed is 3/4 wave of feed line between them. Again, the receiver simply connects to one of them and the null is off one end. Yeah, Steve, but isn't FM broadcast wirh horizontal polarfization? With vertical dipoles the only signal you'll receive is from reflect;ions off objects such as buildings. What kind of reflection coefficient would one expect ? Enough for a good signal? Walt |
{original msg at end}
Firstly, let me thank you one and all for your responses!! Also, I did not clearly define the antenna which is a multi-element yagi of Winegard design - a "standard" consumer product - and roof mounted. In addition, I have MapQuested the two transmitters and find that the interfering radiation comes from behind, about 25degrees off axis. This signal originates about 30miles behind me and the desired signal is about 55miles in front. Richard Clark took me along the method that I had tried - and that I have had no success with. Dale W4OP points out that this type filter, at these frequencies, have almost no effect - boo! Roy Lewallen, W7EL, concurred with Dale and suggested a "phased array" design. Unfortunately, this is beyond my capabilities. Roy and Walt, W2DU, both agreed that using "null tuning" might help. Roy also suggested a "choke" for the feedline to reduce feedline pickup. - But it seems to me that 95% shielded 75ohm coax should not be a contributing factor. Bob suggested that I call the offending station - I did that but I have to wait until Monday to speak with their engineer - boo. Crazy George alluded to an article about constructing a "stacked yagi" - interesting and within reasonable economics - but no details or source for details. Ah well. He also brought back into my memory EagleFilters which I have used in a cable television distribution setting. I'm looking them up on the Internet now. "jqo" suggested shielding from the bad signal, a strategy I have successfully used in C-Band-satellite-receiving-antenna placements but....on my rooftop? Also, he suggested a tuner with better selectivity. I agree and am on eBay looking for a better unit than the Cadet AM/FM Radio card that is installed in my PC and connected to the outdoor antenna. BTW, "jqo", Q, in a filter, means "quality" but is really a figure of merit that refers to width of the 3db down points (in frequency) and the total energy removed. High Q filters have narrow -3db points and suck out LOTS of energy. Jonesy suggests that if I just wait, like the weather, the station will change format and I'll no longer care to solve this!! Thanks all - any additional comments are most welcome. ....Bryce "A-Tech" wrote in message ... Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz The FM antenna has a "standard" 300ohm screw connection for the lead-in (to which I connect a 300/75 xfmr and use coax down). I would appreciate anyone's help in designing and implementing a notch filter that would suck out a major part of the interfering energy. It seemed to me that a reasonable attempt would be to use a piece of 300ohm flat-lead and short it at an appropriate distance from the screw-terminals of the antenna. None of my attempts have yielded any observable improvements. It may be that the filter must be "deeper" (higher Q?) than what my attempts provide. |
"Walter Maxwell" wrote in message ... On Fri, 10 Sep 2004 13:38:34 -0500, "Steve Nosko" wrote: When you are BETWEEN markets, this is common. Or, if you want to listen to a distant station and the wrong one is close. I'm equi-distant from Chicago, Milwaukee and Rockford and have this problem hearing WDCB 90.9, west of CHi-town with WHAD 90.7, S-W of Milwaukee pounding in. They're in different markets. This was discussed not to long ago here. A notch in the antenna pattern is not too difficult. Two vertical dipoles spaced (about) a half wave and fed with the proper feed line so they are 180 degrees out from each other, can make a nice null - actually two. They are perpendicular to the plane of the antennas. I'd have to sit and tinker to remember the feed system, but there has to be 1/2 wave more feed line in one dipole, compared to the other. It is also possible to do the same thing and get a one notch pattern (cardioid). If I recall, the antennas are spaced 1/4 wave apart and the feed is 3/4 wave of feed line between them. Again, the receiver simply connects to one of them and the null is off one end. Yeah, Steve, but isn't FM broadcast wirh horizontal polarfization? With vertical dipoles the only signal you'll receive is from reflect;ions off objects such as buildings. What kind of reflection coefficient would one expect ? Enough for a good signal? Walt To the best of my knowledge- commercial FM broadcast has transmitted on both V and H for many years- how else would a vertical whip on an automobile receive FM? Dale W4OP |
On Fri, 10 Sep 2004 23:18:13 GMT, "Dale Parfitt" wrote:
"Walter Maxwell" wrote in message .. . On Fri, 10 Sep 2004 13:38:34 -0500, "Steve Nosko" wrote: When you are BETWEEN markets, this is common. Or, if you want to listen to a distant station and the wrong one is close. I'm equi-distant from Chicago, Milwaukee and Rockford and have this problem hearing WDCB 90.9, west of CHi-town with WHAD 90.7, S-W of Milwaukee pounding in. They're in different markets. This was discussed not to long ago here. A notch in the antenna pattern is not too difficult. Two vertical dipoles spaced (about) a half wave and fed with the proper feed line so they are 180 degrees out from each other, can make a nice null - actually two. They are perpendicular to the plane of the antennas. I'd have to sit and tinker to remember the feed system, but there has to be 1/2 wave more feed line in one dipole, compared to the other. It is also possible to do the same thing and get a one notch pattern (cardioid). If I recall, the antennas are spaced 1/4 wave apart and the feed is 3/4 wave of feed line between them. Again, the receiver simply connects to one of them and the null is off one end. Yeah, Steve, but isn't FM broadcast wirh horizontal polarfization? With vertical dipoles the only signal you'll receive is from reflect;ions off objects such as buildings. What kind of reflection coefficient would one expect ? Enough for a good signal? Walt To the best of my knowledge- commercial FM broadcast has transmitted on both V and H for many years- how else would a vertical whip on an automobile receive FM? Dale W4OP You're right, Dale, my age and ignorance are showing. Walt |
A-Tech wrote:
. . . Roy Lewallen, W7EL, concurred with Dale and suggested a "phased array" design. Unfortunately, this is beyond my capabilities. Roy and Walt, W2DU, both agreed that using "null tuning" might help. Roy also suggested a "choke" for the feedline to reduce feedline pickup. - But it seems to me that 95% shielded 75ohm coax should not be a contributing factor. . . . With the added knowledge of your antenna and your technical capabilities, the approach I'd encourage you to try is to simply turn your antenna while listening. It might not take much turning to get the interfering signal into a null, leaving the desired signal close enough to the peak of the front directional lobe to be strong enough. The problem with feedline pickup has nothing to do with the shielding, it's caused by current on the outside of the shield. But it's not likely to be a concern with your commercial antenna and coax because of the way these antennas are typically fed. Roy Lewallen, W7EL |
A company in FL - well, they might have been blown away by now - called
something like Pizio Technologies makes VHF bandpass filters from "crystal" elements. Such filters are capable of rejecting adjacent channels in the 150 MHz range and are needed in some cities if you have to put a receiver downtown. As Dale points out, it takes a serious filter indeed to perform what you wish such a filter to perform. On the other hand, I have deliberately tuned some higher end FM broadcast receivers to a weak adjacent station in the presence of a fairly strong station to find a bit better than mediocre performance. Some receivers have much better filtering than other receivers. So, as another summery: rotate antenna for best SNR; get a receiver with better filtering; and consider a special-made crystal filter if you still have a problem. 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Dale Parfitt" wrote in message news:_X90d.8722$Va5.8116@trnddc01... Unfortunately, you will find that the -3dB BW of a single stub is in the neighborhood of 5% or 5MHz at your frequency of interest. The discrimination between your desired and undsired channels will be negligible. We manufacture a line of small multiple cavity symmetrical and asymmetrical notch filters- getting under 0.5% BW with small topology filters is nearly impossible at VHF. Dale W4OP |
"J. Mc Laughlin" wrote in message ... A company in FL - well, they might have been blown away by now - called something like Pizio Technologies makes VHF bandpass filters from "crystal" elements. Such filters are capable of rejecting adjacent channels in the 150 MHz range and are needed in some cities if you have to put a receiver downtown. As Dale points out, it takes a serious filter indeed to perform what you wish such a filter to perform. On the other hand, I have deliberately tuned some higher end FM broadcast receivers to a weak adjacent station in the presence of a fairly strong station to find a bit better than mediocre performance. Some receivers have much better filtering than other receivers. So, as another summery: rotate antenna for best SNR; get a receiver with better filtering; and consider a special-made crystal filter if you still have a problem. 73 Mac N8TT PTI is definitely still in business and are very capable of building such a filter- unfortunately in small quantities they are cost prohibitive. Dale W4OP |
"A-Tech" wrote in message ... Hi, I am a newbie here and have just dropped in to ask a single technical question. Try a receiver with decent IF filters. Unfortunately, this might be hard to do in this day and age, but at one time companies were using crystal filters in the IF stages that offered some decent adjacent channel rejection. Pete |
" Uncle Peter" wrote in message news:CbE0d.138$iK2.93@lakeread08... " Try a receiver with decent IF filters. Unfortunately, this might be hard to do in this day and age, but at one time companies were using crystal filters in the IF stages that offered some decent adjacent channel rejection. Pete I should add other than phased stacked antennas to null the unwanted station, there is really no practical filter you can use to notch an adjacent FM channel. A custom crystal filter might do it, but that's many hundreds of dollars. It's far easier and cheaper to do this sort of filtering at the first IF. It might be easier to retrofit the receiver with a good IF filter as compared to trying to notch the undesired signal or bandpass the desired signals at the operating frequencies. Pete |
"A-Tech" wrote in message ... Hi, I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz Hi Bryce, you can try phase cancellation technique as described at http://www.geocities.com/toddemslie/...ncellation.htm Good luck! Mario 9A4DE |
"Yeah" wrote in message ... "A-Tech" wrote in message ... Hi, I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz Hi Bryce, you can try phase cancellation technique as described at http://www.geocities.com/toddemslie/...ncellation.htm Good luck! Mario 9A4DE This *IS* simply another way of describing an antenna with a null. -- Steve N, K,9;d, c. i My email has no u's. |
Hi, Mario
Thank you for this response. Of all the replies that I received, you came closest to the information I was looking for. Even those recommending "phased arrays" failed to point me toward practical information. I have the tech-savy to build the circuits shown in your link and to understand the theory. Thanks again. ....Bryce "A-Tech" wrote in message I am a newbie here and have just dropped in to ask a single technical question. Living as I do in a suburban area, I use an FM antenna to draw the stations located in a certain vector from me. A station of interest is located at 107.1MHz but is interfered with by an off-axis stronger signal at 106.9MHz Hi Bryce, you can try phase cancellation technique as described at http://www.geocities.com/toddemslie/...ncellation.htm Good luck! Mario 9A4DE |
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