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"Dale Parfitt" ) writes: "Tom" wrote in message news:YBvjd.2272$bH2.1471@trnddc09... Don't know where you can buy a passband filter. Maybe be able to find some cavity type filters for 50 MHz, but probably only surplus. Building from scratch is easy but takes a network analyzer to tune. A coax stub can be either open or shorted, your preference. Cheapest way is to start with an open stub slightly too long and trim it to length. If you can borrow a signal generator and power meter, then you can tune it that way. It's practical to get about 25 dB of loss with a stub, better is difficult due to losses in the conectors, cables, etc. You may achieve -25dB loss at CH2 video, but unfortunately you will also have high loss at 50MHz, and a VSWR that is out of sight. Build one and see for yourself. Dale W4OP So you get a crystal cut for the channel 2 carrier frequency, and find a place to connect it in the receiver chain. This sort of thing was done apparently in repeaters forty years ago, to keep the transmitter out of the receiver. I do think one would have to modify the receiver because the crystal might not be able to handle the transmitter signal. Michael VE2BVW |
"Dale Parfitt" ) writes: "Tom" wrote in message news:YBvjd.2272$bH2.1471@trnddc09... Don't know where you can buy a passband filter. Maybe be able to find some cavity type filters for 50 MHz, but probably only surplus. Building from scratch is easy but takes a network analyzer to tune. A coax stub can be either open or shorted, your preference. Cheapest way is to start with an open stub slightly too long and trim it to length. If you can borrow a signal generator and power meter, then you can tune it that way. It's practical to get about 25 dB of loss with a stub, better is difficult due to losses in the conectors, cables, etc. You may achieve -25dB loss at CH2 video, but unfortunately you will also have high loss at 50MHz, and a VSWR that is out of sight. Build one and see for yourself. Dale W4OP So you get a crystal cut for the channel 2 carrier frequency, and find a place to connect it in the receiver chain. This sort of thing was done apparently in repeaters forty years ago, to keep the transmitter out of the receiver. I do think one would have to modify the receiver because the crystal might not be able to handle the transmitter signal. Michael VE2BVW |
OK. got access to the spreadsheet. Setting the stub+short+connector loss
to 0.25 dB, and cutting the stub for 55.25 gives about 15 dB loss at 50 MHz, and about 30 dB loss at 55.25. This probably is good enough to either prove or disprove the hypothesis about what is causing the problem (video carrier overloading the RF amp), and an inexpensive way to find out. As a long-term solution, a bandpass filter would be easier provided it had loss low and SWR (so it could be left in-line for transmitting). -- Tom "Dale Parfitt" wrote in message news:jZAjd.2538$bH2.1351@trnddc09... "Tom" wrote in message news:YBvjd.2272$bH2.1471@trnddc09... Don't know where you can buy a passband filter. Maybe be able to find some cavity type filters for 50 MHz, but probably only surplus. Building from scratch is easy but takes a network analyzer to tune. A coax stub can be either open or shorted, your preference. Cheapest way is to start with an open stub slightly too long and trim it to length. If you can borrow a signal generator and power meter, then you can tune it that way. It's practical to get about 25 dB of loss with a stub, better is difficult due to losses in the conectors, cables, etc. You may achieve -25dB loss at CH2 video, but unfortunately you will also have high loss at 50MHz, and a VSWR that is out of sight. Build one and see for yourself. Dale W4OP |
OK. got access to the spreadsheet. Setting the stub+short+connector loss
to 0.25 dB, and cutting the stub for 55.25 gives about 15 dB loss at 50 MHz, and about 30 dB loss at 55.25. This probably is good enough to either prove or disprove the hypothesis about what is causing the problem (video carrier overloading the RF amp), and an inexpensive way to find out. As a long-term solution, a bandpass filter would be easier provided it had loss low and SWR (so it could be left in-line for transmitting). -- Tom "Dale Parfitt" wrote in message news:jZAjd.2538$bH2.1351@trnddc09... "Tom" wrote in message news:YBvjd.2272$bH2.1471@trnddc09... Don't know where you can buy a passband filter. Maybe be able to find some cavity type filters for 50 MHz, but probably only surplus. Building from scratch is easy but takes a network analyzer to tune. A coax stub can be either open or shorted, your preference. Cheapest way is to start with an open stub slightly too long and trim it to length. If you can borrow a signal generator and power meter, then you can tune it that way. It's practical to get about 25 dB of loss with a stub, better is difficult due to losses in the conectors, cables, etc. You may achieve -25dB loss at CH2 video, but unfortunately you will also have high loss at 50MHz, and a VSWR that is out of sight. Build one and see for yourself. Dale W4OP |
OK. got access to the spreadsheet. Setting the stub+short+connector loss
to 0.25 dB, and cutting the stub for 55.25 gives about 15 dB loss at 50 MHz, and about 30 dB loss at 55.25. This probably is good enough to either prove or disprove the hypothesis about what is causing the problem (video carrier overloading the RF amp), and an inexpensive way to find out. As a long-term solution, a bandpass filter would be easier provided it had loss low and SWR (so it could be left in-line for transmitting). -- Tom "Dale Parfitt" wrote in message news:jZAjd.2538$bH2.1351@trnddc09... "Tom" wrote in message news:YBvjd.2272$bH2.1471@trnddc09... Don't know where you can buy a passband filter. Maybe be able to find some cavity type filters for 50 MHz, but probably only surplus. Building from scratch is easy but takes a network analyzer to tune. A coax stub can be either open or shorted, your preference. Cheapest way is to start with an open stub slightly too long and trim it to length. If you can borrow a signal generator and power meter, then you can tune it that way. It's practical to get about 25 dB of loss with a stub, better is difficult due to losses in the conectors, cables, etc. You may achieve -25dB loss at CH2 video, but unfortunately you will also have high loss at 50MHz, and a VSWR that is out of sight. Build one and see for yourself. Dale W4OP |
On Sun, 07 Nov 2004 16:47:18 GMT, GeorgeF
wrote: Tom wrote: The most likely cuase is that your receiver front-end is broad enough to pass the TV2 signal through the RF amplifier, and modulate everything else being received. It's difficult to get a low-pass or high-pass filter will a sharp enough cut-off to do much good. Better solutions are to build a notch filter for the TV2 video carrier at 55.25 MHz (using a T-connector and a length of coax trimmed to length, or to buy or build a passband filter for 50-52 MHz. I had this problem with channel 11 and a 220 MHz receiver, and ended up building a bandpass filter with a pair of coupled LC tanks, tapped at 50 ohms. Totally eliminated the TV cross-modulation problems. 1) Where can one buy a passband filter for 50-52MHz? Try http://www.dci.ca/ they make passband filters for six meters, tho' they're a little pricey. Bob k5qwg 2) When talking about coax trimmed to length, are you talking about a coax stub? Such as cut to 1/4 wavelength of the offending freq? If so would be ends be shorted or let open? never worked with stubs yet.... TNX George |
On Sun, 07 Nov 2004 16:47:18 GMT, GeorgeF
wrote: Tom wrote: The most likely cuase is that your receiver front-end is broad enough to pass the TV2 signal through the RF amplifier, and modulate everything else being received. It's difficult to get a low-pass or high-pass filter will a sharp enough cut-off to do much good. Better solutions are to build a notch filter for the TV2 video carrier at 55.25 MHz (using a T-connector and a length of coax trimmed to length, or to buy or build a passband filter for 50-52 MHz. I had this problem with channel 11 and a 220 MHz receiver, and ended up building a bandpass filter with a pair of coupled LC tanks, tapped at 50 ohms. Totally eliminated the TV cross-modulation problems. 1) Where can one buy a passband filter for 50-52MHz? Try http://www.dci.ca/ they make passband filters for six meters, tho' they're a little pricey. Bob k5qwg 2) When talking about coax trimmed to length, are you talking about a coax stub? Such as cut to 1/4 wavelength of the offending freq? If so would be ends be shorted or let open? never worked with stubs yet.... TNX George |
On Sun, 07 Nov 2004 16:47:18 GMT, GeorgeF
wrote: Tom wrote: The most likely cuase is that your receiver front-end is broad enough to pass the TV2 signal through the RF amplifier, and modulate everything else being received. It's difficult to get a low-pass or high-pass filter will a sharp enough cut-off to do much good. Better solutions are to build a notch filter for the TV2 video carrier at 55.25 MHz (using a T-connector and a length of coax trimmed to length, or to buy or build a passband filter for 50-52 MHz. I had this problem with channel 11 and a 220 MHz receiver, and ended up building a bandpass filter with a pair of coupled LC tanks, tapped at 50 ohms. Totally eliminated the TV cross-modulation problems. 1) Where can one buy a passband filter for 50-52MHz? Try http://www.dci.ca/ they make passband filters for six meters, tho' they're a little pricey. Bob k5qwg 2) When talking about coax trimmed to length, are you talking about a coax stub? Such as cut to 1/4 wavelength of the offending freq? If so would be ends be shorted or let open? never worked with stubs yet.... TNX George |
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