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On 2008-02-13, RST Engineering wrote:
"noman" wrote in message ... Otherwise, range and clarity are fine. I just got to wondering if my homemade antenna or cable are a problem. To do the original install, I used an rf field strength meter and simply adjusted the antenna length for highest reading. That probably wasn't the cleverest way of doing it. Maybe not the cleverest way, but it's probably good enough that it's working fine. Describe your installation a little more, but I think I can guarantee you that it isn't the antenna. I agree. This sounds like a front end overload problem. No antenna in the world will fix that. (Well, at least not and still work for the intended purpose.) -- Jay Maynard, K5ZC http://www.conmicro.com http://jmaynard.livejournal.com http://www.tronguy.net http://www.hercules-390.org (Yes, that's me!) Buy Hercules stuff at http://www.cafepress.com/hercules-390 |
#2
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Jay Maynard wrote:
On 2008-02-13, RST Engineering wrote: "noman" wrote in message ... Otherwise, range and clarity are fine. I just got to wondering if my homemade antenna or cable are a problem. To do the original install, I used an rf field strength meter and simply adjusted the antenna length for highest reading. That probably wasn't the cleverest way of doing it. Maybe not the cleverest way, but it's probably good enough that it's working fine. I've had variable luck using that approach on other types of antennas. It's probably a better approach than the typical SWAG. Describe your installation a little more, but I think I can guarantee you that it isn't the antenna. I agree. This sounds like a front end overload problem. No antenna in the world will fix that. (Well, at least not and still work for the intended purpose.) I agree - the only way his antenna is likely to fix it is by removing it altogether (which, as you point out will not help the operation of the radio much). No doubt the problem is that (like Jim pointed out) the FM signal is mixing with some other signal. The question is whether that's happening inside the portable radio, or outside it. If it's inside the radio, reducing the amount of broadcast FM signal getting into the receiver's front end would help. Personally, I'd do this by putting a T connector in line with the antenna, and plugg in a shorted quarter-wavelength (at the offending FM frequency) chunk of shorted coax on the extra connector. The length of the stub depends on the coax you use, and the type of connector. If this doesn't help it's likely that the mixing is occurring somewhere else. If it's outside the aircraft, there's not a lot you can do (though that would mean that almost everyone would have the problem). If the mixing is happening in the aircraft, it'll be "more fun" to find. Someone suggested unplugging your ELT (a possibility, and easy enough to do - just don't schedule any crashes when doing this). ;-) I've seen all kinds of metal structures become effective RF mixers - so the problem could be almost anywhere in a typical aircraft (note: I don't recommend removing metal in the process of looking for the mixing source - you may really NEED that ELT if you do that). ;-) Mark Hickey |
#3
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![]() "Mark Hickey" wrote in message ... -----------------preceding posts snipped----------- If it's inside the radio, reducing the amount of broadcast FM signal getting into the receiver's front end would help. Personally, I'd do this by putting a T connector in line with the antenna, and plugg in a shorted quarter-wavelength (at the offending FM frequency) chunk of shorted coax on the extra connector. The length of the stub depends on the coax you use, and the type of connector. ----------------remainder snipped----------------- Mark Hickey This is the method which I have seen work most frequently, although that mainly involved listeners radios in their homes. It also has the advantage of being cheap. Peter |
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On Thu, 14 Feb 2008 19:16:09 -0700, Mark Hickey wrote:
Jay Maynard wrote: snip If this doesn't help it's likely that the mixing is occurring somewhere else. If it's outside the aircraft, there's not a lot you can do (though that would mean that almost everyone would have the problem). If the mixing is happening in the aircraft, it'll be "more fun" to find. snip Mark Hickey Also you might try putting a 20-30 db attenuator on your radio connector. If the signal remains then probably originating (ie mixing products) outside the radio. If the interference goes away it is most liking occuring inside the radio. John |
#5
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On 2008-02-17, Jumpin Jahosaphat wrote:
Also you might try putting a 20-30 db attenuator on your radio connector. If the signal remains then probably originating (ie mixing products) outside the radio. If the interference goes away it is most liking occuring inside the radio. Good idea. One note: DO NOT TRANSMIT WITH THE ATTENUATOR IN THE LINE. You WILL destroy it, and likely your transmitter's final amplifier as well if you transmit for too long after destroying the attenuator. -- Jay Maynard, K5ZC http://www.conmicro.com http://jmaynard.livejournal.com http://www.tronguy.net http://www.hercules-390.org (Yes, that's me!) Buy Hercules stuff at http://www.cafepress.com/hercules-390 |
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Jay Maynard wrote:
On 2008-02-17, Jumpin Jahosaphat wrote: Also you might try putting a 20-30 db attenuator on your radio connector. If the signal remains then probably originating (ie mixing products) outside the radio. If the interference goes away it is most liking occuring inside the radio. Good idea. One note: DO NOT TRANSMIT WITH THE ATTENUATOR IN THE LINE. You WILL destroy it, and likely your transmitter's final amplifier as well if you transmit for too long after destroying the attenuator. While it may cook the attenuator, provided he's move on from vacuum tubes, it's unlikely that a 12 or 24v driven transistor final is going to burn up even if it's a complete mismatch. |
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On 2008-02-26, Ron Natalie wrote:
Jay Maynard wrote: Good idea. One note: DO NOT TRANSMIT WITH THE ATTENUATOR IN THE LINE. You WILL destroy it, and likely your transmitter's final amplifier as well if you transmit for too long after destroying the attenuator. While it may cook the attenuator, provided he's move on from vacuum tubes, it's unlikely that a 12 or 24v driven transistor final is going to burn up even if it's a complete mismatch. Transmitting at full power into an open circuit - which is what you'd get after burning up the attenuator - is about the worst thing you can do to a final, especially a transistor one. Solid state finals are less forgiving than tube finals, and abuse that would just make the tubes run hotter than normal will burn up transistors quickly. -- Jay Maynard, K5ZC http://www.conmicro.com http://jmaynard.livejournal.com http://www.tronguy.net http://www.hercules-390.org (Yes, that's me!) Buy Hercules stuff at http://www.cafepress.com/hercules-390 |
#8
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Excuse me, sir, but most of us who design moderate power solid state
transmitters wouldn't THINK of a circuit that didn't have full VSWR protection, either by overdesign of the output devices or a VSWR loop to shut the finals down in the event of a situation that you describe. And no, they don't run hotter. The normal failure mode is secondary breakdown due to voltage spikes in the output matching circuitry. And, given a random length of coax between the transmitter and the open circuit, all you can guarantee is that you will be on the outer ring of the Smith chart somewhere between an open and a short ... a pure open is a very low probability. Jim Transmitting at full power into an open circuit - which is what you'd get after burning up the attenuator - is about the worst thing you can do to a final, especially a transistor one. Solid state finals are less forgiving than tube finals, and abuse that would just make the tubes run hotter than normal will burn up transistors quickly. |
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