Rear defroster trivia

OK, that brings up another question - Lots of glass, not much vertical a woosy little wiper that misses over half the window and the top third isn't heated. What prevents snow buildup as one drives along in the winter?

 

Nothing prevents snow buildup. And the wiper is indeed a bit too small. Toyota could at least have put it on the right side of the car for us left hand drivers.

 

Took another look at the lines on the back window glass again and I question weather the funny short bit at the top is a second FM antenna. I say this because if you look carefully at the anchoring conductors for the defroster lines, you will see a ~1/2 inch strip on both sides. Following the right hand strip up, above the last whole window line it narrows to ~ 1/4 inch, goes up a little more and then crosses the top of the glass until it attaches to the lines in question. If you then follow the lines in question that only cover the left 1/3 of the top of the glass, while they do loop around, they eventually get to the Left hand 1 inch conductor for the defroster.

It would appear that this is designed to help free the rear wiper blade from winter ice. The defroster's role as a second FM antenna I would know nothing about (or even if it is possible).

My Observation,

Jon

 

This can happen if you overpack the trunk and close it. You've scratched a defogger trace on the glass.

To find the broken trace, turn on the defogger. Use a voltmeter's 20- or 50-volt DC range, attach the black (-) lead to a metal ground and use the red (+) lead to test different spots on the traces. Be careful not to scratch off those delicate traces or you'll make it worse. One side of the window should read 12 volts, then 6 volts in the middle, and 0 volts (ground) at the other side. Find a trace that acts differently, and keep narrowing it down until you find a break in the trace. Get a brush-on patch kit from an auto supply store to fix the trace. If they can't help, get a tube of Circuit Writer from Radio Shack orhttp://www.amazon.com/dp/product/B0002BBVQO?tag=priuschatcom-20 (about $20). Or, you can go to the dealer and they'll try to get you to replace the entire back window (figure $300-$400). Your choice.

Now, as far as the antenna goes, without trying to sound like a know-it-all, but explaining one of those curious things that no one else will ever tell you about, and you'd drive yourself nuts trying to Google . . .

The AM/FM antenna is the short whip that sticks up from the roof. (The rest of this message is just engineering details. It can be safely ignored.)

The metal body of your car acts as a 'ground reference' to give the antenna something to relate to. That's why turning your car may affect a weak station, since there's more sheet metal ahead/behind than left/right. It's why radio transmitter antennas often have 'ground radials' and commercial broadcasters prefer antenna installations in areas with a high water table -- they get a better ground reference and a stronger signal. Without a ground to compare itself to, an antenna's value is very limited.

AM antennas in cars are not made of a black ferrite rod like portable radios -- cars use a completely different method. (You'll find a tiny fine-tuning spinner [trimmer capacitor] where the antenna lead comes into a car radio, or underneath the main tuning knob -- set it to perfectly match the electrical length of your car antenna to your favorite distant AM station.)

If they used the same AM ferrite rod antenna as a portable radio or your clock-radio, you'd have to point your car directly at (or directly away from) a weak radio station to hear it. A ferrite rod antenna is cheap, short, and effective, but directional. That zillion turns of extra-fine wire wrapped around it represents a compromise/fractional of how long the perfect antenna would be (around 1,000 feet long -- 300 meters or so -- the actual length of a 1000 kHz / 1 mHz middle-of-the-AM-broadcast-band radio wave). Your radio is marked 10 or 100 at this spot.

A car's AM whip is short because additional circuitry matches it to the radio's needs. Ditto for the FM band, which would like to see a 1-meter long (3+ feet long, or some fraction thereof) antenna at 100 mHz.

If you look carefully, you'll notice that the Prius whip is actually a solid rod with a longer thin wire wrapped around it; one for AM, one for FM, both on one connection (the radio 'loads' the antenna it matches to best).

The odds of the second wire being a diversity element are nil -- you'd need an additional connector to use it that way, and they'd have to be separated further from each other (that's what makes a diversity system work -- it eliminates anything that's different between two antennas' signals, like the slightly off-sync FM signal reflected from a water tower or some aluminum siding).

You'll hear electrical noise when you remove the whip because you've crippled the radio's ability to receive a solid radio signal, so noise is more able to get in (poor Signal-to-Noise ratio, or S/N). The radio automatically becomes much more sensitive.

The Nav GPS system uses a special microwave "antenna" mounted in the dashboard. It must be able to see the sky to work well. Glass or plastic in the way is OK, metal is not OK. Water (fog, rain, clouds) and even the water in trees block GPS signal too, since microwaves in this range are nearly the same size/wavelength as a water molecule (this same feature lets a microwave oven do its work by resonating/shaking water molecules to cause friction. No water = no microwave heating).

So, AM signal wavelength (long wave) is about 1000 feet. FM signal (very short wave VHF) is around 3.3 feet. A GPS signal's wave (they don't really use antennas, but waveguides at this point) is about the size of a water molecule.

I'll shut up now. Thanks for listening. Hope that cleared things up. ;-)