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EE audio help needed
Those specs are not all that good. I don't know how they compare to other car radios, but I do know how they compare to my good amateur radio receivers and they are not even in the same neighborhood.
I had an Audiovox stereo in a car a few years ago that had terrible sensitivity. I bought a signal booster for it and it completely cured the problem. It just plugs into the antenna line and connects to 12VDC. It cost about $30. They are probably available at auto parts places and on ebay.
I don't know if they still do this, but it used to be quite common to have an unlabeled adjustment screw on the radio to "peak" up the antenna for AM reception. The antennas used for the AM band are quite a bit shorter than optimal, but you can't make them too big on a moving vehicle. So, this peaking adjustment tunes up the system to do the best it can. If this adjustment is not optimal, then it can make the receiver seem as dead as a fencepost.
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dBu means Decibels above a microvolt. I have no idea what dBF means. Lower numbers are better if you are not comparing apples and oranges. I will do a little more digging. But that is for my own education.
34 dBu means that it takes about 25 microvolts of radio signal to generate an acceptable audio signal. This is normally accompanied by a resulting signal to noise ratio which defines how much signal you have compared to the noise or static in the background.
A very good radio receiver will be able to detect a signal on just a few microvolts of signal. In some modes I can get by with less than one microvolt, so 30 is not a particularly good number. AM and FM will generally take a few microvolts for a good audio signal.
I still think that the preamplifier/signal booster is the least complicated way to solve the problem. I have heard of some systems that will disable large sections of the car electrically if the radio is removed from the integrated electrical system. On my Hummer, the radio is connected to the On-Star, GPS, and who knows what else. The other interesting point about the Hummer is that the radio antenna is only about eight inches tall, but the radio is very sensitive.
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OK, I found the answer, but it is not an easy conversion. dBf means 1 dB relative to a femtowatt or 10e-15 watts.
To further complicate the issue you need twice as many dB difference for voltage measurements to be on equal par with dB related to power (watts).
On a low impedance transmission line I normally use dBm which is dB relative to a milliwatt. A milliwatt is a trillion femtowatts. A good receiver will be capable of sensitivity in the -110 dBm range. That is 10e-11 milliwatts or 10 femtowatts or 10 dBf. This is compatible with the sensitivities that I found on JVC, Pioneer, and other high quality receivers.
So 10dBf is equivalent to 0 dBu or 1.00 microvolt. At 34 dBu on AM, your receiver in the Toyota will take 25 uV to give you a good signal. That means that the Toyota receiver will have a dBf sensitivity of around 24 dBf which is quite low compared to the aftermarket radios. On FM the story is a bit better, but the radio will still be around 17 dBf in sensitivity. So, the comparison specifications back up your observations.
Most preamplifier/RF boosters will give you at least 10dB and probably closer to 15dB. That number subtracts directly from the 24 dBf sensitivity to put the radio right into the 10dBf range which is good compared to the aftermarket radios.
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With respect to WLS...
Propagation of the signal is better at night as Mark said. Also these stations use large arrays of antennas designed to shape their transmission pattern. I don't know specifically, but WLS probably shapes their pattern to the east or maybe E/W.
When I was growing up in Missouri, we could hear the Little Rock station better than WLS. Later I moved to Kankakee and WLS was very strong there.
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