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Problem of nearby transmitter breakthrough
Sorry for the crossposting, but I'm looking for more expertise than I have.
I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. I've tried quick fix by attenuating the input signal by trimming (shortening) the antenna, but this doesn't really help. This was supposed to be a quick and dirty (gratis) job for the airport, and I've already spent more time and money on it than I wanted to. Any suggestions as to how I might fix this problem? Cheaply? Obviously a better receiver would work (my Yaesu aviation handheld works perfectly at the same location), but I have no other (free) receivers handy. I can move the receiver another 50 feet down the fence, which is my next option, but what if this doesn't work? I can't get it any farther away for several reasons. Anyone have a 120 MHz preselector they can give me? Any really steep (and cheap) 120.6 notch filter designs? Thanks for any help you can offer. -- Bob (Chief Pilot, White Knuckle Airways) |
Why anybody would want to sit and watch Aircraft sitting still,landing
or taking off,, anyway,,, let alone listen to stuff like that at Airports,,, is beyond me! cuhulin |
Bob Chilcoat wrote:
Sorry for the crossposting, but I'm looking for more expertise than I have. I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. I've tried quick fix by attenuating the input signal by trimming (shortening) the antenna, but this doesn't really help. This was supposed to be a quick and dirty (gratis) job for the airport, and I've already spent more time and money on it than I wanted to. Any suggestions as to how I might fix this problem? Cheaply? Obviously a better receiver would work (my Yaesu aviation handheld works perfectly at the same location), but I have no other (free) receivers handy. I can move the receiver another 50 feet down the fence, which is my next option, but what if this doesn't work? I can't get it any farther away for several reasons. Anyone have a 120 MHz preselector they can give me? Any really steep (and cheap) 120.6 notch filter designs? Thanks for any help you can offer. While this may not be exactly what you are looking for, it may help. http://www.radagast.org/~dplatt/hamr...IK-filters.pdf |
Are you absolutely sure the offending signal is coming in via the
antenna? Is it there if you disconnect the antenna or replace it with a dummy load (termination)? That strongly influences what you need to do to fix it. Roy Lewallen, W7EL Bob Chilcoat wrote: Sorry for the crossposting, but I'm looking for more expertise than I have. I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. I've tried quick fix by attenuating the input signal by trimming (shortening) the antenna, but this doesn't really help. This was supposed to be a quick and dirty (gratis) job for the airport, and I've already spent more time and money on it than I wanted to. Any suggestions as to how I might fix this problem? Cheaply? Obviously a better receiver would work (my Yaesu aviation handheld works perfectly at the same location), but I have no other (free) receivers handy. I can move the receiver another 50 feet down the fence, which is my next option, but what if this doesn't work? I can't get it any farther away for several reasons. Anyone have a 120 MHz preselector they can give me? Any really steep (and cheap) 120.6 notch filter designs? Thanks for any help you can offer. |
Put a tee connector on the receive line coax a length of coax on it with
a short at the far end. It must be cut to be exactly 1/4 wavelength (including connector spur.) This will appear t be an open circuit at the resonant frequency, but will severely attenuate your nearby unwanted signal. Failing that, a series LC network across the receive line will do a similar task. Try a 47pf trimmer cap in series with a coil made from 4 to 6 turns around a bic pen. (you can remove the pen after making the coil... :) Tune your receiver to the unwanted signal and tune the trimmer until you see the unwanted signal drop out. You may have to experiment with the coil turns and spacing, but this design will also help wipe out the unwanted station. Fortunately your undesired signal is likely a constant-on transmission so it's always there for you to play with. Secondly, you can make and tinker with all this at home (provided you live within receiving range of the airport AWOS) and bring the working product to the airport for installation. Good luck. B. Bob Chilcoat wrote: Sorry for the crossposting, but I'm looking for more expertise than I have. I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. I've tried quick fix by attenuating the input signal by trimming (shortening) the antenna, but this doesn't really help. This was supposed to be a quick and dirty (gratis) job for the airport, and I've already spent more time and money on it than I wanted to. Any suggestions as to how I might fix this problem? Cheaply? Obviously a better receiver would work (my Yaesu aviation handheld works perfectly at the same location), but I have no other (free) receivers handy. I can move the receiver another 50 feet down the fence, which is my next option, but what if this doesn't work? I can't get it any farther away for several reasons. Anyone have a 120 MHz preselector they can give me? Any really steep (and cheap) 120.6 notch filter designs? Thanks for any help you can offer. |
Put a tee connector on the receive line coax a length of coax on it with
a short at the far end. It must be cut to be exactly 1/4 wavelength (including connector spur.) This will appear t be an open circuit at the resonant frequency, but will severely attenuate your nearby unwanted signal. Failing that, a series LC network across the receive line will do a similar task. Try a 47pf trimmer cap in series with a coil made from 4 to 6 turns around a bic pen. (you can remove the pen after making the coil... :) Tune your receiver to the unwanted signal and tune the trimmer until you see the unwanted signal drop out. You may have to experiment with the coil turns and spacing, but this design will also help wipe out the unwanted station. Fortunately your undesired signal is likely a constant-on transmission so it's always there for you to play with. Secondly, you can make and tinker with all this at home (provided you live within receiving range of the airport AWOS) and bring the working product to the airport for installation. Good luck. B. Bob Chilcoat wrote: Sorry for the crossposting, but I'm looking for more expertise than I have. I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. I've tried quick fix by attenuating the input signal by trimming (shortening) the antenna, but this doesn't really help. This was supposed to be a quick and dirty (gratis) job for the airport, and I've already spent more time and money on it than I wanted to. Any suggestions as to how I might fix this problem? Cheaply? Obviously a better receiver would work (my Yaesu aviation handheld works perfectly at the same location), but I have no other (free) receivers handy. I can move the receiver another 50 feet down the fence, which is my next option, but what if this doesn't work? I can't get it any farther away for several reasons. Anyone have a 120 MHz preselector they can give me? Any really steep (and cheap) 120.6 notch filter designs? Thanks for any help you can offer. |
I am making some assumtions here. You state that you only hear the AWOS
when a transmission occurs on the CTAF freq. I would think this means that the receiver is NOT receiving the AWOS signal all the time or it would trigger the system constantly. If you listen to the receiver output while at the airport does it contain the AWOS audio or just the CTAF audio? My inclination is that the AWOS sig is being picked up by the interface circuitry between your receiver and the 88.1 transmitter. If this is the case then more bypassing and rf filtering is needed on the interface. Then again I could be barking up the wrong tree here, only some suggestions, hope it helps. Cheers "Bob Chilcoat" wrote in message ... Sorry for the crossposting, but I'm looking for more expertise than I have. I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. I've tried quick fix by attenuating the input signal by trimming (shortening) the antenna, but this doesn't really help. This was supposed to be a quick and dirty (gratis) job for the airport, and I've already spent more time and money on it than I wanted to. Any suggestions as to how I might fix this problem? Cheaply? Obviously a better receiver would work (my Yaesu aviation handheld works perfectly at the same location), but I have no other (free) receivers handy. I can move the receiver another 50 feet down the fence, which is my next option, but what if this doesn't work? I can't get it any farther away for several reasons. Anyone have a 120 MHz preselector they can give me? Any really steep (and cheap) 120.6 notch filter designs? Thanks for any help you can offer. -- Bob (Chief Pilot, White Knuckle Airways) |
"Bob" wrote in message ... Put a tee connector on the receive line coax a length of coax on it with a short at the far end. It must be cut to be exactly 1/4 wavelength (including connector spur.) This will appear t be an open circuit at the resonant frequency, but will severely attenuate your nearby unwanted signal. Failing that, a series LC network across the receive line will do a similar task. Try a 47pf trimmer cap in series with a coil made from 4 to 6 turns around a bic pen. (you can remove the pen after making the coil... :) Tune your receiver to the unwanted signal and tune the trimmer until you see the unwanted signal drop out. You may have to experiment with the coil turns and spacing, but this design will also help wipe out the unwanted station. Fortunately your undesired signal is likely a constant-on transmission so it's always there for you to play with. Secondly, you can make and tinker with all this at home (provided you live within receiving range of the airport AWOS) and bring the working product to the airport for installation. Good luck. B. While this approach looks good on paper, it often fails badly when the desired frequency is so close in to the notch frequency. I just put a quarter wave stub on our VNA and found that while it does diminish the 123 signal -33dB, it also attenuates the 120.6 signal by a whopping -22dB. There is also an enormous VSWR upset -120:1 or so- this is perhaps not important in your receive only application. Each year we build hundreds of filters for this exact application- AWOS/UNICOM separation. Typical insertion loss is under 1dB while the notch is -40dB. The filter is about the size of a cigarette pack exclusive of the N connectors. W4OP |
To each his own. Sounds to me like a great innovative idea.
Not everyone fancies hunting through junk at the Goodwill all day long and making passes at the wimmins behind the counter. wrote in message ... Why anybody would want to sit and watch Aircraft sitting still,landing or taking off,, anyway,,, let alone listen to stuff like that at Airports,,, is beyond me! cuhulin |
Haw Haw Haw,,,, Boy Howdy,I could tell y'all stories about Goodwill and
them wimmins over there,, but I wont. www.johnweisman.com cuhulin |
I can easily listen to the Airports and Pilots around here,but 99
percent of it is dead silence,so why bother? cuhulin |
Dale Parfitt wrote:
While this approach looks good on paper, it often fails badly when the desired frequency is so close in to the notch frequency. I just put a quarter wave stub on our VNA and found that while it does diminish the 123 signal -33dB, it also attenuates the 120.6 signal by a whopping -22dB. There is also an enormous VSWR upset -120:1 or so- this is perhaps not important in your receive only application. Each year we build hundreds of filters for this exact application- AWOS/UNICOM separation. Typical insertion loss is under 1dB while the notch is -40dB. The filter is about the size of a cigarette pack exclusive of the N connectors. I think you get narrower selectivity if you use an odd integer multiple of quarter wavelengths for the stub. The longer the line, the greater the phase change with frequency. A 1/4 wave shorted stub goes from open to short in a 1:2 frequency ratio. A 3/4 wavelength stub goes from open to short in 3:4 ratio of frequency. Etc. I think. |
Maybe you need to buy a real good Marine (civilian radio) radio.They are
not cheap though. cuhulin |
|
Put a little almost one inch square magnet like the one on my keychain
on top of your tv set and see what happens.My magnet does nothing at all to tv sets. cuhulin |
The Death Of Global Communications Security. Middle isle at,
www.rense.com It isn'r secure anymore. cuhulin |
On Mon, 19 Sep 2005 18:07:25 -0400, "Bob Chilcoat"
wrote: Sorry for the crossposting, but I'm looking for more expertise than I have. I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), What is intermediate frequency (IF) of the receiver ? Some offending signal (or mixing product) could fall on the image frequency and get through that way. boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 Is the interface present when this transmitter is turned off and the signal is monitored through the speaker ? If the transmitter cables are disconnected, does this change anything ? to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. One thing to try is to rotate the antenna, so that the receiver antenna is in opposite polarisation than the transmitter position, one in vertical polarisation, the other horizontal polarisation. This will attenuate the offending signal by 10-20 dB. The null might be quite narrow, so you would have to slowly rotate your receiver antenna to find it. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. If you defeat the squelch, do you constantly hear the AWOS transmission ? Does it matter if the 88.1 MHz FM transmitter is on or not ? Any other VHF transmitters on site (VOR?) that might take part in the mixing process ? Do you hear any other background noises than the AWOS recording ? Paul OH3LWR |
Bob Chilcoat wrote:
Sorry for the crossposting, but I'm looking for more expertise than I have. I usually hang out in the aviation groups, but am an engineer by training. I have a radio problem: I have just completed a special rig for our local airport, but it has a problem. I took a rather old but serviceable Sony digital air band receiver (Air 8), boxed it up in a waterproof enclosure and piped the audio out to an FM microwatt transmitter. The idea of this is that visitors to our airport who like to sit in the parking lot and watch the airplanes can listen on their car radios on FM 88.1 to the radio traffic on our Common Traffic Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. I've tried quick fix by attenuating the input signal by trimming (shortening) the antenna, but this doesn't really help. This was supposed to be a quick and dirty (gratis) job for the airport, and I've already spent more time and money on it than I wanted to. Any suggestions as to how I might fix this problem? Cheaply? Obviously a better receiver would work (my Yaesu aviation handheld works perfectly at the same location), but I have no other (free) receivers handy. I can move the receiver another 50 feet down the fence, which is my next option, but what if this doesn't work? I can't get it any farther away for several reasons. Anyone have a 120 MHz preselector they can give me? Any really steep (and cheap) 120.6 notch filter designs? Thanks for any help you can offer. -- Bob (Chief Pilot, White Knuckle Airways) Rather than building a trap, why not use a narrow bandpass filter? Also, if you are that close to both transmitters, try attenuating the signal from the antenna to prevent overloading the receiver. One other idea. if the signal is from a transmitter at the airport, why not see if you can take the audio feed to that transmitter and feed the FM transmitter. -- ? Michael A. Terrell Central Florida |
BC,
|
I told the truth.The Goodwill store had a price of five dollars for the
MGA/Mitsubishi tv set because sometimes it has some shadowy lines moving up from the bottom of the screen to the top of the screen.I paid the lady at the checkout counter five dollars for the tv set.I am watching Katrina: American Catastrophe on one of the History channels on the tv set right now. cuhulin |
You are nothing but one of them Chicken Yellow Belly Coward
Canadians.You don't amount to anything and you never will either.You are just like all of those other Chicken Yellow Belly Coward Canadians. cuhulin |
|
Never even once.It is a trick I read about in a magazine over fifty
years ago.You would have to use a big powerfull magnet to make it work anyway.I carry my little bitty magnet on my keychain for when or if I want to check metal for steel or brass. cuhulin |
Think about the many different kinds of things that may be sitting on
top of tv sets all over the World.Everything from A to Z.Many of those things are probally magnetized to some extent and there are bound to be many other folks around the World who have magnets on their keychains sitting on top of their tv sets.A lot of folks who work in scrap metal yards carry keychain magnets on their keychains or in their pockets for checking metal. cuhulin |
"m II" wrote in message news:hg5Ye.240462$9A2.197177@edtnps89... wrote: I told the truth.The Goodwill store had a price of five dollars for the MGA/Mitsubishi tv set because sometimes it has some shadowy lines moving up from the bottom of the screen to the top of the screen.I paid the lady at the checkout counter five dollars for the tv set.I am watching Katrina: American Catastrophe on one of the History channels on the tv set right now. cuhulin So, it was the *MGA/Mitsubishi* tv which you put the magnet on in the Goodwill store..ok..It would appear Honus apologized to you in error. The *Sony* that you put the magnet on was in a DIFFERENT store. Now I get it. Out of curiosity, just how many times have you played this magnet trick and in how many different stores? Crap. Just when I think I can bow out gracefully.... |
White Knuckle Airways,eh?
cuhulin |
In article , "Bob Chilcoat"
wrote: Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. Bob- By now you have probably solved your problem. If not, someone else suggested that you insert attenuation in the Sony's antenna lead. I think that approach is most likely to produce the results you want. If you were to replace the Sony's antenna with a dummy load, there may still be sufficient signal bleeding into the radio to make your system work. The interfering signal would also bleed into the radio, but at such a low level that the Sony's tuned circuits ought to be able to handle it. If even this does not solve the interference, you may find the problem to be audio rectification inside the 88.1 equipment, perhaps in the oscillator stage! Please keep us informed of what it takes to make it work. 73, Fred, K4DII |
I'm guessing that because of all the new B.S. concerning Homeland
Security, the airport probably has a fence around it with a security coded gate control so that non-pilots can't get onto the field... Scott John S. wrote: Rather than re-broadcasting the signal why not try something simpler. Just set up an Airport Traffic room within the airport and pipe the audio in. Hang some old pictures and other memorabilia of the airport on the walls to make it interesting. |
Scott wrote: I'm guessing that because of all the new B.S. concerning Homeland Security, the airport probably has a fence around it with a security coded gate control so that non-pilots can't get onto the field... Scott John S. wrote: Rather than re-broadcasting the signal why not try something simpler. Just set up an Airport Traffic room within the airport and pipe the audio in. Hang some old pictures and other memorabilia of the airport on the walls to make it interesting. Most airports have a lounge of some kind.... |
All,
Thanks for all the suggestions, advice and information. I learned a lot about VHF that I didn't know. Should have stayed with it way back when I almost finished my "Novice" amateur license (couldn't manage the Morse code part -- I guess that gives away my age). Fred nailed it, although by the time I saw his note, I'd solved the problem. After relocating the receiver as far away as I could (60 feet, given the limitation of where the units needed to be and sources of power) which didn't really help, I tried some simple filters, etc. During the course of this, someone keyed a mike while I had the antenna disconnected, and it worked fine with no antenna connected! A bit more experimentation indicated that the 120.6 interference dropped out completely while the desired 123 signal could still be picked up from five miles away if I left the short coax jumper inside the box from the receiver to the antenna disconnected at the antenna end. I'm not sure I fully understand why I still get a pretty robust signal this way, but I guess enough 123 MHz RF is leaking in from the bare antenna or all the other wiring. Since the radio traffic of interest is mostly local planes near the airport or in the traffic pattern, a five mile radius is probably more than adequate, and this is certainly the cheapest solution. I will also look into the regulations about rebroadcasting non-commercial radio signals. The transmitter satisfies the part 15(IIRC?) FCC reg about a 100-foot reception limit, but I thought the rules about rebroadcasting were to prevent commercial rebroadcasting. The 100 foot limitation will keep all reception on the airport grounds, but I will do some more research on this. Thanks. As far as an area inside the building where people can listen to the radio is concerned, we already have that. This project is for the people who sit in their cars in the parking lot and watch the planes on their lunch hour, etc. We get quite a few of these. Anything that improves relations with the general public should help alleviate some of the problems we have with the neighbors. I still can't understand how anyone can buy a house near an airport, and then feel that they have a right to complain about airplane noise. The airport has been there since 1946, but that's another issue. Again, thanks for all the help. -- Bob (Chief Pilot, White Knuckle Airways) "Fred McKenzie" wrote in message ... In article , "Bob Chilcoat" wrote: Unfortunately, while this setup worked perfectly at home well away from the airport, we have an Automatic Weather Observation Station (AWOS) transmitting continuously on 120.60 MHz only 50-60 feet from the place I need to site the receiver. Even though this is only a 5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. Bob- By now you have probably solved your problem. If not, someone else suggested that you insert attenuation in the Sony's antenna lead. I think that approach is most likely to produce the results you want. If you were to replace the Sony's antenna with a dummy load, there may still be sufficient signal bleeding into the radio to make your system work. The interfering signal would also bleed into the radio, but at such a low level that the Sony's tuned circuits ought to be able to handle it. If even this does not solve the interference, you may find the problem to be audio rectification inside the 88.1 equipment, perhaps in the oscillator stage! Please keep us informed of what it takes to make it work. 73, Fred, K4DII |
"Bob Chilcoat" wrote in message ... ... a rather old but serviceable Sony digital air band receiver ...piped the audio out to an FM microwatt transmitter. ... visitors ...can listen on their car radios on .... Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM). ...5 Watt transmitter, it overloads the front end of the receiver. As soon as anyone keys on 123.00 and the automatic squelch is triggered, all you hear is the AWOS recording. Bob (Chief Pilot, White Knuckle Airways) My turn... Lots of really good ideas here. Make sure it is the Sony antenna that is picking it up... 1- The stub-trap idea may work with some large number of odd-quarter waves. As you stack more odd multiples of quarter waves on (extend the shunt trap length), the dips get closer together and as a result the peak-to-null distance shrinks. A stub that is 21 quarter waves long has peaks and nulls very close together (you could calculate this). Of course the dips will be less deep due to cable loss. This may make it easier to get the desired "Pass / Null" response. You need to make sure the signal is getting into the antenna and not the receiver itself. The 2.4 Mc spacing is quite close. NEW IDEA: 2- Try tuning it on the image (for your 123 Unicom frequency). You will probably need to do this away from the airport . You should hear the Unicom in two places on the dial spaced _TWICE_ the IF frequency away. These radios usually have pretty wide front ends There may be a schematic inside the radio to help determine this. If the IF is 0.455, the image is 0.910 away. This radio probably uses low side injection meaning the LO is below the 123 by 0.455. Therefore you must tune the radio 0.910 HIGHER than 123. This places the on channel further away from the 120.6 AWOS. Not knowing the IF, you have to hunt. -Maybe doing this _AT_ the airport will yield a tuning that fixes it. Just try "tuning around". Related idea, you could also figure out where the RF stage stuff is in the radio and de-tune it upward to get more attenuation at 120.6, but if that works, then just some attenuation in the antenna line may work as well and be easier. 3- Put the radio FAR away, but run the audio in some small coax to the 88.1 closer to the visitors. 4- I like the "cross Polarization" idea. Do a light saber thing with the Sony antenna. There may be a sweat spot that just might get the levels down and fix the IM or whatever is going on.. Mike T. Use the airport receiver... Sounds good, but they probably don't want you messing with it.... and what about the airport's transmissions... 5- HOWEVER, how about simply a microphone right next to this receiver and within earshot of the airport's microphone---ON the 88.1 Tx ?? You may hear other stuff in the FBO, but it may be ok too... Related... I have a little thing I plug into my dual band rig speaker output and it transmits the audio to my car FM radio. They're made for listening to battery operated CD players in your car. 73, Steve, K;9.D,C'I |
Around here, most "lounges" or "Terminal buildings" are inside the
fence, so the original problem remains that limits non-pilots (or pilots who happen to be driving by a strange airport) from gaining access to the field. Scott John S. wrote: Scott wrote: I'm guessing that because of all the new B.S. concerning Homeland Security, the airport probably has a fence around it with a security coded gate control so that non-pilots can't get onto the field... Most airports have a lounge of some kind.... |
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