AM/FM Antenna

I'm in the same situation

 

One of the tricky bits is that there's an amplifier mounted directly under the antenna, at least in some cars. Not only does that antenna have to work with AM and FM, but, if you've got satellite radio, with that, too.

In general the more wire that's out there the better in general an antenna works. So maybe you could get a Gen II antenna.

The other tricky bit has to do with noise factor. Say you build a radio: What you want is a really good signal to noise ratio (SNR), where the signal is lots bigger than the noise. So, the noise has two components: That generated by the electronics, and that generated by environment.

At AM frequencies environmental noise dominates by several orders of magnitude. Hence, any old electronic amplifier stage will do since it's highly unlikely that the noise from said amplifier will be noticeable in the presence of the environmental (lightning, arcs, sparks, thermal noise, stuff from the ionosphere, etc.) noise.

As one goes up in frequency the environmental noise diminishes. By the time one gets to 100 MHz (FM band), unless you've got one of those liquid nitrogen cooled amplifiers, the electronic amplification noise will be the noise floor of your receiver. Make the noise less, one's SNR gets better, and one can hear signals from farther away.

It so happens that when one does the math, the noise from the first amplification stage dominates the overall noise generated in a radio receiver. So, the first amplifier is designed to have ridiculously low noise and, typically, not much gain (maybe a gain of 2x or 4x out of the total 1,000,000 that might be necessary to make the audio large enough to hear). The amps are designed with special low-noise transistors with good (low) shot noise, low impedances (high resistance leads to noise via Boltzman's constant), and careful design. Astronomers like to soak their radio receivers in liquid nitrogen or even liquid helium, since noise generated by resistances is proportional to K*T, where K=Boltzman's constant and T is the temperature in Kelvin.

And this is why that amplifier is situated directly under the antenna. The coax between the rear of the car and the radio receiver is a noise source; amplifying the signal at the antenna helps the overall noise factor of the system. Note that a 100 MHz resonant antenna, quarter wavelength, is around 3/4 meter long; that's a fair amount of wire picking up the power from radio waves. The Prius like a lot of modern cars has that shorty antenna back there which intercepts less of the radio wave; to make up for that, they put that low-noise amp right at the antenna.

Everything up to here is standard radio engineering. Now I get to start flapping my hands.

So, it's not just enough to stick a wire up: If one wants to receive a radio signal, it's a Real Good Idea to make an antenna that properly resonates at the frequencies one wishes to receive and has the appropriate output impedance. A proper, traditional quarter-wavelength antenna has a 35 Ohm output impedance over a ground plane, which is close enough to the standard 50 Ohm coax to provide a reasonable match. (Mismatches at the coax cause recirculating power in the coax due to reflections; these reflections aren't evil per se, but when one runs power back and forth like that, that means that what's recirculating tends to be absorbed/attenuated by the coaxial losses in the cable.). A half-wavelength antenna is typically tapped at the center and has a nominal impedance of 70 Ohms. (Which is why 75 Ohm coax is the next most popular coaxial impedance after 50 Ohm: One gets a better match.)

Now, a quarter-wavelength antenna at 100 MHz is 0.75 m long or thereabouts. (Which, if one thinks about it, is about the length of a standard car wire antenna). One can make it shorter, and still resonant, by adding an inductor to the bottom. Or curling the wire around and around, which makes it inductive, and then adding an inductor. After one has done all that, the impedance at the bottom of this much shorter antenna won't look very resistive, so one typically adds a capacitor to get the whole shebang back to something that looks vaguely like 50 Ohms. Note that making it shorter does mean one is intercepting less of the electromagnetic wave in the air.

However, and this is where life is tricky, the designers over at Toyota don't actually have a piece of coax down there at the base of the antenna: They've got an amplifier, instead. If one knows the impedance at the base of the antenna, and it's not resistive, it's trivial to add components to the amplifier to get good power transfer into the amplifier. For example, if the base-of-the-antenna impedance happens to be 10 + j50 (10 ohms resistive, plus 50 ohms inductive reactance), then one can make the amplifier have an input impedance of 10 - j50 (10 ohms resistive plus 50 ohms capacitive reactance), and thus get a match and maximum power transfer. Further, if the antenna impedance varies as a function of frequency (which it absolutely does), one can make the amplifier input impedance do the same thing in the opposite direction, in lockstep, at least over the frequency bands of interest.

So, into this environment, one takes out that 6" long antenna and puts in one that's 3/4 of a meter long. Urgh. The amplifier input impedance no longer matches the design for that 6" stub antenna. One might still get low-noise gain.. But, without data, I couldn't say whether things would be better. Or not. Of course, one could remove the amplifier and connect the 3/4 meter antenna to the coax directly. But then I would wonder what, if anything, would happen to the satellite receiver frequency band, not to mention the AM radio band, both of which are running at very different frequencies in the GHz and down at 1 Mhz, respectively.

Now, back in ye olden days, I've taken apart car radio receivers and discovered that, rather than getting the AM signal from the whip antenna, they get it from what looks like a ferrite coil with lots of turns, mounted somewhere in the dash. And I'm not sure that the Priuses with satellite radio have their antenna back there in the rear, or mounted somewhere near the top of the dash, underneath. And there may be aftermarket amplifiers or antennas that purport to give better radio reception, so YMMV. But at least with the above you've got something to start with.

KBeck.

 

Wow. Thanks for taking the time to post all of that but I wonder if there might not be too much information to digest.

Before messing with the antenna on a weak FM station, try hitting the HD button to turn OFF HD for that station.

And I gotta ask........with ALL of the other FM, AM and SAT stations out there, do you really HAVE to listen to that one station ???

 

Some radio stations stream online these days. You might check that, if they do, you could stream it through your iPhone and then Bluetooth or direct connect into the Prius to listen.

 

Rule of thumb, the closer to the transmitting tower the better chances of getting HD FM! The output power of most HD FM stations is so low, I wonder why they even bother!