Prius antenna mod

Would it be possible to strip out the spiralling wire from the stock antenna "nub" and fit it inside the shark fin?

 

Well, I took engineering at Purdue in West Lafayette and aced it. Really.

A couple of comments here.

First one: Taking a 1/4 wavelength antenna and making it a 1/2 wavelength antenna is Not A Good Idea. That is, if you're looking for better reception, anyway.

Reason: a 1/4 wavelength antenna has is designed to have a low impedance point near the ground plane, or, in the shark fin case, next to the coax connector at the bottom. This impedance is around 30-70 ohms (depending upon design) and has relatively high current and low voltage.

This impedance gives a relatively good match to the input of the antenna amplifier and maximum power transfer from the air to the amp.

At the other end of the antenna you get (relatively) high voltage and low current. At the very tip you get Zero current because, well, there's nowhere for the current to go. Do the Maxwell's equations math and it's that high-Z at the tip that forces the other end to low-Z. (And, if you're wondering, that's one of the major reasons that AM transmitting antennas are 1/4 wavelength long. And, yeah, the high end of the AM band is more crowded for a couple of good reasons, but one of the major ones is it's cheaper to build an antenna up there. They're shorter.)

Now, if you just double the length of the antenna to 1/2 wavelength, well, the end that's not connected to anything is still sitting there at high Z. Maxwell's equations kick in and, 1/4 wavelength away, you get a low-Z point - right in the middle. And, continuing down the length of the antenna to where the connector is, the antenna wants another high-Z spot - but the coax is low Z! Oops. What the actual Z is on an antenna like this takes some serious math, but what you're going to get is a lousy antenna.

If one is going to extend an antenna like this then making its total length a multiple of odd 1/4 wavelengths works. So, you can have antennas at 1/4 wavelength, 3/4 wavelength, 5/4 wavelength, etc., and they'll all have high Z at the tip and low Z at the coax connector.

However, this then runs into a second effect: bandwidth. Generally, there are two things that effect bandwidth on a simple "vertical" antenna like this:

• How thick the effective antenna is.
• How electrically long the effective antenna is.

If you want a really narrow-band antenna that can (he says, almost jokingly) only pick up a single FM station, then make it 101 1/4 wavelengths long with a wire that looks like nylon monofilament. You want it wide enough to pick up the entire FM band? Then make the wire really thick and 1/4 wavelength long. So, there's a good reason that the standard antenna is thick and has the wire going around in circles: That circling motion is what makes that little stub wide band.

Of course, it also makes that antenna look inductive, too. So there's some impedance matching games that very likely go on inside the shark fin/standard Prius mount that try to keep things vaguely matched.

Oh, yeah. I saw that nifty picture of that guy who doubled the wire up inside the shark fin. Besides the fact that the wire was 1/2 wavelength long (Wrong!), there was another fun thing going on. When you lay two wires more-or-less parallel to each other like that they couple to each other. Fields cancel. Reception goes bad. Oops.

There's another issue with all this, too. Antennas on your standard car aren't just single band - they're at least two, and, with satellite radio, three band antennas. That means that the overworked RF engineer who had to make this thing worked had to consider ridiculously low frequencies down in the AM band, relatively easy to work with stuff at FM, and satellite radio at 2.3 GHz. I do not know, since I haven't done serious antenna modeling since my undergraduate days, but I have this image of some poor guy with 3-d RF modeling software, SPICE, and a lot of digging.

What this all means is that that antenna amp and such there in the back is very likely tuned to the physical layout of said antenna. Sticking funny wires on it.. Well, I wouldn't hold my breath waiting for better results. Of course, given that one can see components, wires, plastic, and you know that the coax to the radio is 50 Ohms or so, one could probably reverse engineer the thing and go playing to see if better results could be found.. But I wouldn't hold my breath for that, either. If that was true, the market would be flooded with people who could tack better antenna/amplifiers back there since, relatively speaking, the parts are cheap and there'd be a huge market of audiophiles with a wish for better radio reception. Since I don't think that's happening (but maybe I don't read the right magazines), I don't think there's much room for improvement in there.

One last thing: In general, the radiation pattern (which is also the reception pattern) for a vertical antenna looks vaguely like a donut with the wire running through the hole. This means that overhead extraterrestrials broadcasting in the FM band (note: Did not say XM!) aren't going to get picked up all that well, but antennas all around the horizon (hey! that's us!) do tend to do OK. Hence, the popularity of antennas on cars that point straight up, or nearly so.

A horizontal (typical: 1/2 wavelength, driven from the middle, low-Z spot) antenna in free space also shows that donut shape. When one places said antenna near the ground (yeah, the stuff we stand on!) things change. Think of as starting with the donut standing on its side; then smash the bottom of the donut and, while you're about it, flatten the top, too. Fun and all that: But, the main point is that the reception/radiation off the ends of the antenna is, by default, pretty blame rotten.

One sort-of-way around this is to consider a loop antenna which looks, pretty much, what it sounds like. These things broadcast well off the side: When you point the flat side at a target, the target gets nulled out. (Hence their use in WWII as direction finder antennas.) But, if you build a strange enough looking antenna, you might get some half-assed, semi-performing thing that's got the requisite bandwidth and does a vaguely reasonable job of picking up transmitters arrayed across the horizon.

This is called the "Welcome to the printed antenna on the rear windshield" approach to life. It's got lots of advantages to the manufacturer and buyer: There's no antenna to snap off, fewer piece parts and holes in the car, they had to put conductive wires on for the defogger anyway, etc. But if you were wondering why reception was good on a station when you're going one direction and not so good when you're going another, well, now you know.

Hope this helps.

KBeck.

 

@kbeck:

That was awesome... I felt like I was back in my electromagnetics courses, only more useful. Are you an RF engineer?

 

If you remove the radio antenna where do you fly your Pirate Flag from ?

 

[IMGLINK]http://priuschat.com/forums/attachments/gen-iii-2010-prius-accessories-modifications/21792d1266423301-sr-shark-fin-010.jpg[/IMGLINK]

kbeck,

That guy is me.

Curious, can you see anything in the picture that would explain my experience that the doubling of the wire had no effect (good or bad) on the reception of the shark fin antenna?

Maybe because the new wire is thinner and different shaped? Or the fact that I didn't solder the two wires together (I just lifted the tape covering the existing wire, laid the new wire on top of the existing wire and put the tape cover back down)?

 

Well, at one time I really, really wanted to be one. Walked into Harris RF Electronics with great hopes to be building the Next Big 800 MHz Amplifier or something. Then my boss asked me, "Do you know what a Karnaugh Map is?". Five months of doing ASIC logic design on paper with a little template and I decided to go back and learn DSP.

Now I beat on ASICs and transistors for a living, north of 10 Gb/s. Not too much radio, but I muck with spectrum analyzers and fast scopes enough to keep me happy.

Oh, yeah. Ex-Navy radar techie and ham radio licensee. Fun.

KBeck.

 

Well, a couple of things.

First, if you live in an area with relatively good signal strength, dropping the signal into your radio by 10 dB or so might not even be noticeable. All modern radios have Automatic Gain Control (AGC) on all bands; if the audio drops a bit the radio cranks up the gain or vice versa, the better for you not to notice how the radio signal's fluttering all over the place.

Second: Umm, dude, laying two wires on top of each other and using tape doesn't usually work too well. I'll bet my next paycheck that those wires have some serious anti-corrosion coating on them, bare copper having this disturbing tendency to corrode away and disappear. Just ask Lady Liberty. Said coatings tend to be non conductive, electrically, so I'm not sure you actually made contact. A little capacitive coupling where the wires are close to each other, but nothing at 100 MHz would consider a signal path.

Third: Even with no contact, you've got one antenna not connected to anything in space, and the other one connected to the wire inside the car, and they're right next to each other. That would not stop the two wires from coupling to each other; for reference, look at a random TV antenna up on a roof somewhere. A lot of those antennas only have a physical connection to one of the horizontal elements; the others just live out there, intercept the radio waves, then re-radiate the waves. You get all these funky interference patterns that, if done right, can give an antenna a serious amount of gain in a particular direction. You, on the other hand... Well, it's too close for gain, I think, so my gut reaction is to think it might make things a little worse. Or a little better. Or better at 98 MHz and terrible at 104. $DIETY knows.

Back in the days before PCs, and, in particular, before freebie antenna modeling software showed up at the ARRL, there used to be these VHF/UHF antenna contests over at the American Radio Relay League, the Ham Radio semi-official club. You see, there was this guy, Yagi, from Japan, a pretty brilliant dude, who, back in the early 50's (I think, may have been earlier) came up with the idea of the Yagi antenna. That's that antenna with many parallel elements, the ones in the rear being longer than the ones in the front. Really popular. Yagi had some serious Physics and math background, but, really, the true math behind an antenna like that is better for electromagnetic CAD than trying to work it out by hand. He did so, anyway, and came up with a cookbook approach that any old fool (and lots of fools kicked in!) could use to build a pretty decent antenna.

So, there's all these hams with just enough electronics background to be worse than dangerous, and they would show up at the VHF antenna contests with all sorts of antennas in the hopes of getting their name attached to a universally used antenna. The ARRL organizers would set up a decent measurement field (not trivial - you need a lot of space) and would then start doing measurements.

As you might imagine most of these antennas didn't do much. But the coup de grace was when a bunch of guys showed up with Queen-sized naked bedspriings, fed with their favorite UHF transmitter. Pretty sad when some fella has spent a year bending aluminum and some guys, fresh from the bedding department at Wards, have got a better antenna.

Fun.

KBeck.