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.

 

Reduces engine noise on AM.

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.

 

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.

 

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

 

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.

 

Maybe to block DC in case something comes in contact with the antenna.

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

 

0.85uF? It would have an impedance of about 1/3 ohm at 540kHz--
essentially a short circuit.


Best regards,
Spehro Pefhany

 

Probably that 0.85uF number is wrong.. perhaps 0.85nF (850pF), 1000:1
less, which would fit that function.



Best regards,
Spehro Pefhany

 

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.

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...

 

..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 shit like this, WHY did they do this and WHY did they do
that.

JURB

 

Why not stick that cap inside the radio?

 

Probably because there are different sized antennas on different
models. It might also help to match it to the coax itself.

JURB

 

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.