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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Car radio antenna has an inline .85mfd capacitor, why?
I have an 87 Nissan van with an antenna wire that has a 0.85 mfd
capacitor soldered between the center wire and the center pin of the antenna connector, on the end that plugs into the radio. This vehicle actually has two antenna wires that connect side by side at the radio. One antenna is printed onto the centerline of the windshield, and the other is a conventional whip antenna mounted at the drivers side of the roof. The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. -- Bill Freeman |
#2
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Car radio antenna has an inline .85mfd capacitor, why?
In article ,
Meat Plow wrote: On Fri, 28 Sep 2007 20:13:10 -0400, Bill Freeman wrote: I have an 87 Nissan van with an antenna wire that has a 0.85 mfd capacitor soldered between the center wire and the center pin of the antenna connector, on the end that plugs into the radio. This vehicle actually has two antenna wires that connect side by side at the radio. One antenna is printed onto the centerline of the windshield, and the other is a conventional whip antenna mounted at the drivers side of the roof. The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? Reduces engine noise on AM. Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. Some technical advantage, maybe tuning the antenna circuit for AM. Although I think Arfa recently met an audiophile who could supply the OP with an $1800 coax, whose center conductor is made of 300 strands of 52AWG gold wire, specially braided to optimize the FM band. |
#3
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Car radio antenna has an inline .85mfd capacitor, why?
On Sep 28, 8:13 pm, Bill Freeman wrote:
I have an 87 Nissan van with an antenna wire that has a 0.85 mfd capacitor soldered between the center wire and the center pin of the antenna connector, on the end that plugs into the radio. This vehicle actually has two antenna wires that connect side by side at the radio. One antenna is printed onto the centerline of the windshield, and the other is a conventional whip antenna mounted at the drivers side of the roof. The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. -- Bill Freeman |
#4
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Car radio antenna has an inline .85mfd capacitor, why?
On the windscreen, in the winter dry snow will be brushing against the
windscreen, and in the summer during a rain storm the windscreen can act in a way they could build up some DC static type voltages. The cap will block this effect. The cap basicaly acts as a protection against any DC from going in to the radio's front end. Jerry G. -- On Sep 28, 8:13 pm, Bill Freeman wrote: I have an 87 Nissan van with an antenna wire that has a 0.85 mfd capacitor soldered between the center wire and the center pin of the antenna connector, on the end that plugs into the radio. This vehicle actually has two antenna wires that connect side by side at the radio. One antenna is printed onto the centerline of the windshield, and the other is a conventional whip antenna mounted at the drivers side of the roof. The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. -- Bill Freeman |
#5
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Car radio antenna has an inline .85mfd capacitor, why?
In article ,
Bill Freeman wrote: Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. Car antennas are really, really short at AM wavelengths; for that reason they are sensitive almost entirely to the "E" component of the EM field only. Interestingly, if that kind of antenna is hooked to a truly *infinite* load, the voltage induced in it is entirely independent of its length (it depends only on the total voltage swing from the E field of the signal that it encounters). With a very-high-impedance load, it doesn't take very much shunt capacitance to really attenuate the signal. The lead-in from the antenna isn't properly a "coax" at all; it's just a shielded cable, and it's constructed in a way that minimizes the shunt capacitance as much as possible -- very large shield diameter, very small inner conductor, nearly all air dielectric; concentricity is of no concern since its impedance doesn't matter. If cost was not an object, an insulated gate FET directly at the base of the antenna would produce the best results. Isaac |
#6
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Car radio antenna has an inline .85mfd capacitor, why?
In article ,
Bill Freeman wrote: The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? At the frequencies used for AM reception lead capacitance is critical. Older radios used to have a trimmer to get an exact match- newer ones do this automatically. So all aerials should have approx the same capacitance. If you have a longer than usual lead - for a rear mount etc - the cable capacitance goes up, so you add a series one to bring it back to that standard. Same as an extension lead will have. -- *Where there's a will, I want to be in it. Dave Plowman London SW To e-mail, change noise into sound. |
#7
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Car radio antenna has an inline .85mfd capacitor, why?
On Fri, 28 Sep 2007 20:13:10 -0400, the renowned Bill Freeman
wrote: I have an 87 Nissan van with an antenna wire that has a 0.85 mfd capacitor soldered between the center wire and the center pin of the antenna connector, on the end that plugs into the radio. This vehicle actually has two antenna wires that connect side by side at the radio. One antenna is printed onto the centerline of the windshield, and the other is a conventional whip antenna mounted at the drivers side of the roof. The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? Maybe to block DC in case something comes in contact with the antenna. Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. Probably an impedance matching issue. The coax characteristic impedance is proportional to the the log of the ratio of the outer and inner diameters, so if you made the inner conductor 5x the size the OD would have to increase by 5:1 to get the same Z. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
#8
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Car radio antenna has an inline .85mfd capacitor, why?
On Sat, 29 Sep 2007 10:01:50 +0100, the renowned "Dave Plowman (News)"
wrote: In article , Bill Freeman wrote: The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? At the frequencies used for AM reception lead capacitance is critical. Older radios used to have a trimmer to get an exact match- newer ones do this automatically. So all aerials should have approx the same capacitance. If you have a longer than usual lead - for a rear mount etc - the cable capacitance goes up, so you add a series one to bring it back to that standard. Same as an extension lead will have. 0.85uF? It would have an impedance of about 1/3 ohm at 540kHz-- essentially a short circuit. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
#9
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Car radio antenna has an inline .85mfd capacitor, why?
On Sat, 29 Sep 2007 11:16:41 -0500, Spehro Pefhany
wrote: On Sat, 29 Sep 2007 10:01:50 +0100, the renowned "Dave Plowman (News)" wrote: In article , Bill Freeman wrote: The roof antenna lead is the one with the capacitor. Anyone know what the purpose of an inline capacitor would be on a car radio? At the frequencies used for AM reception lead capacitance is critical. Older radios used to have a trimmer to get an exact match- newer ones do this automatically. So all aerials should have approx the same capacitance. If you have a longer than usual lead - for a rear mount etc - the cable capacitance goes up, so you add a series one to bring it back to that standard. Same as an extension lead will have. 0.85uF? It would have an impedance of about 1/3 ohm at 540kHz-- essentially a short circuit. Probably that 0.85uF number is wrong.. perhaps 0.85nF (850pF), 1000:1 less, which would fit that function. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
#10
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Car radio antenna has an inline .85mfd capacitor, why?
On Sat, 29 Sep 2007 13:23:13 -0400, Bill Freeman
wrote: In article , Spehro Pefhany wrote: 0.85uF? It would have an impedance of about 1/3 ohm at 540kHz-- essentially a short circuit. Probably that 0.85uF number is wrong.. perhaps 0.85nF (850pF), 1000:1 less, which would fit that function. Spehro, You are correct, it is 0.85nF, not 0.85mF. The symbol on my DMM screen is so small I misread it without my reading glasses. Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. Probably an impedance matching issue. The coax characteristic impedance is proportional to the the log of the ratio of the outer and inner diameters, so if you made the inner conductor 5x the size the OD would have to increase by 5:1 to get the same Z. I was estimating by eyeball when I said the original core wire was .010. It actually measures .0065. I replaced the .0065 core wire with an insulated wire that is .0185 (.036 O.D. of the insulation). Sounds like maybe I should put the original size wire back inside for optimum performance. Thank you and everyone else who answered my questions so promptly. Regards, The diameter (gauge) of the wire is important... |
#11
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Car radio antenna has an inline .85mfd capacitor, why?
..85 nF makes sense.
I've quite a few car radios that had two complete tuners in them, side by side with LEDs on the board. There were two antenna inputs and you could plainly see it switch if you changed to the other input. Having worked on said radio, I think they did a pretty good job at making it switch without a transient. The theory is obviously that the multipath is not occurring in two places at the same time. However, it did not seem as though the AM section was duplicated in that manner. Looking at these boards, as this was years ago, they seemed to be strictly FM. Following the reason, for now, along these lines, there would obviously have to be a tapoff for the AM section, and this would operate pretty much like a speaker crossover, just a cap and coil. Both inputs, although seperate for the FM front ends, would be summed for the AM section. Not hard to do, pretty much like a summing subwoofer crossover. Only the frequencies have been changed to protect the innocent (c'mon it's Sunday). So, if my summary of the architechture of the radios on which I worked is correct we can assume the strange capacitor does something to the phase of the signal, but not to FM frequencies, only to AM frequencies. This makes sense because the two antennas are of different size. So the capacitor makes the two antennae into a basic, unsophisticated, yet probably effective phased array. Perhaps someone from Russia will ring in with some ideas on the phase they actually chose. The value of that cap will determine the directional sensitivity no doubt. I wonder what pattern they chose, hmmm, for one it is unreasonable to think that someone would drive right past the transmitting tower every day, I guess a cardioid pattern would be good. Actually that holds water because if you are driving right past the transmitter, what if you want to listen to a different station ? So cartioid it is right ? I dunno it might be spelled cardiod or something. I don't feel like checking right now, but it is most commonly used to describe the pickup pattern for a microphone. But you had me all screwed up at first anyway, a 0.85uF cap would serve no purpose except to let a static charge build up and blow the front end. I don't think that was their goal. Oh, the thing about Russia, as far as antennas and things, they are more advanced than just about anybody. We might have better ICBMs, but they have better antennas. So that's what I think it is, the cap brings the two signals into phase. Then as far as matching the radio to it, they are treated as one unit. That's my theory and I'm sticking to it. Pick it apart and I'll reconsider. JURB PS, I love **** like this, WHY did they do this and WHY did they do that. JURB |
#12
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Car radio antenna has an inline .85mfd capacitor, why?
On Fri, 28 Sep 2007 20:35:39 -0700, "Jerry G." wrote:
On the windscreen, in the winter dry snow will be brushing against the windscreen, and in the summer during a rain storm the windscreen can act in a way they could build up some DC static type voltages. The cap will block this effect. The cap basicaly acts as a protection against any DC from going in to the radio's front end. Jerry G. Why not stick that cap inside the radio? -- Boris |
#13
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Car radio antenna has an inline .85mfd capacitor, why?
Probably because there are different sized antennas on different
models. It might also help to match it to the coax itself. JURB |
#14
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Car radio antenna has an inline .85mfd capacitor, why?
Thanks again to everyone who answered my question about why the .85pf
capacitor was in the antenna lead. I put back the original size .0065" diameter center coax wire. Afterward, I could not tell any difference in reception between it and the .0185 wire I had also tried. Listening to stations is rather subjective though. I didn't have any instrumentation to actually measure signal strength. At any rate, it's now back to the way Nissan built it. The whole antenna repair project was unnecessary. In addition to what I learned from the replies I got here, I learned that if you look for continuity through a car antenna with an ohmmeter and there is none, doesn't necessarily mean the antenna is defective. If it ain't broke, don't fix it. In article , Spehro Pefhany wrote: 0.85uF? It would have an impedance of about 1/3 ohm at 540kHz-- essentially a short circuit. Probably that 0.85uF number is wrong.. perhaps 0.85nF (850pF), 1000:1 less, which would fit that function. Spehro, You are correct, it is 0.85nF, not 0.85mF. The symbol on my DMM screen is so small I misread it without my reading glasses. Another car antenna question, why do they use such a small diameter wire inside the coax? The wire inside this coax is about .010 inches in diameter. Is it this small to save on copper, or is there some technical advantage to a small diameter wire. Probably an impedance matching issue. The coax characteristic impedance is proportional to the the log of the ratio of the outer and inner diameters, so if you made the inner conductor 5x the size the OD would have to increase by 5:1 to get the same Z. I was estimating by eyeball when I said the original core wire was .010. It actually measures .0065. I replaced the .0065 core wire with an insulated wire that is .0185 (.036 O.D. of the insulation). Sounds like maybe I should put the original size wire back inside for optimum performance. Thank you and everyone else who answered my questions so promptly. Regards, -- Bill Freeman |
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