antenna prob.
04-27-18, 08:17 AM
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antenna prob.
1990 Lexus LS400 radio antenna will go down when tuning fm radio 96 and above on fm dial. Antenna stays up below 96 and all of am tuning. Befuddled.
04-27-18, 10:10 AM
#4
not sure what you mean exactly by "down." If it goes down a few inches, that is normal. If it goes all the way down, that is not.
The reason for it to go down a couple of inches is to optimize the length of the antenna to match a quarter wavelength of the transmit frequency. Typical Lexus overkill that may lead to early failure of the antenna. Mine only lasted about 24 years, I think.
Here's a brief post from a few years ago on the details of the antenna operation, which should help with debugging:
https://www.clublexus.com/forums/ls-...ml#post8320232
That's for my '91, at least.
It switches at 96 MHz (96.1 and above is the shorter setting; 95.9 and below is the longer setting), and it is correct for the 1/4 wavelength calculation.
For AM, it goes to maximum length, again to optimize reception based on frequency (1/4 wavelength of an AM signal would be yuge, so they just make it as high as it goes).
OK, here's the quick calculations so you can see it is pretty basic:
c, speed of light is about 3 x 10^10 cm/sec
f, transmission frequency it switches at is 96 MHz = 96,000,000 / sec
the 1/4 wavelength is since the antenna transmit/receive power is optimized at that length, assuming a reflecting ground plane
so the length of the antenna, optimized at 96MHz (where it switches from shorter to longer) = c / f / 4
So that's (3 x 10^10) / (96 x 10^6) / 4 = 78 cm
When the frequency is below 96 MHz, the wavelength is a little longer, so it extends to the slightly longer position. When tuned above 96 MHz, shorter wavelength, so it retracts a little.
I KNOW this does not actually matter, and I bet Lexus does too, but they did it, and I was impressed to see that they did it accurately.
The reason for it to go down a couple of inches is to optimize the length of the antenna to match a quarter wavelength of the transmit frequency. Typical Lexus overkill that may lead to early failure of the antenna. Mine only lasted about 24 years, I think.
Here's a brief post from a few years ago on the details of the antenna operation, which should help with debugging:
https://www.clublexus.com/forums/ls-...ml#post8320232
That's for my '91, at least.
It switches at 96 MHz (96.1 and above is the shorter setting; 95.9 and below is the longer setting), and it is correct for the 1/4 wavelength calculation.
For AM, it goes to maximum length, again to optimize reception based on frequency (1/4 wavelength of an AM signal would be yuge, so they just make it as high as it goes).
OK, here's the quick calculations so you can see it is pretty basic:
c, speed of light is about 3 x 10^10 cm/sec
f, transmission frequency it switches at is 96 MHz = 96,000,000 / sec
the 1/4 wavelength is since the antenna transmit/receive power is optimized at that length, assuming a reflecting ground plane
so the length of the antenna, optimized at 96MHz (where it switches from shorter to longer) = c / f / 4
So that's (3 x 10^10) / (96 x 10^6) / 4 = 78 cm
When the frequency is below 96 MHz, the wavelength is a little longer, so it extends to the slightly longer position. When tuned above 96 MHz, shorter wavelength, so it retracts a little.
I KNOW this does not actually matter, and I bet Lexus does too, but they did it, and I was impressed to see that they did it accurately.
Last edited by oldskewel; 04-27-18 at 10:48 AM.
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