Antenna mod

We have the standard antenna with the JBL stereo and find our reception to be pretty good, even in our hilly rural area. A shark fin antenna should always be worse than a properly designed whip due to the smaller element length. You could always switch to a longer whip, or a base loaded design, but I don't suspect you will get much better reception. If radio is important to you, you might want to consider a satellite system.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Jul 3 2007, 09:33 AM) [snapback]472245[/snapback]</div>

are you pretty sure that the shark fin antenna has a smaller element? for example say you have a 10" antenna, you can either have a 10" antenna, or you wind a 10" antenna around a pole, and it would be the same. that is how they make rubber ducky antennas.

might need someone to purchase a shark fin, and disassemble it, and measure how much electric wire there is in it, if any


http://en.wikipedia.org/wiki/Rubber_Ducky_antenna

 

Over on Darelldd's pages, where he shows all the mods to his Prius, he has a photo of the inside of the Shark Fin... it appears a VERY short pigtail wire connects the Prius' antenna stub, to a small bit of "paper" with flat copper wiring 'printed' on it. (His assumption is that copper bit is the actual receiving bit of metalwork, for the Shark Fin.)
Website below; couldn't find how to give a link right to the photo... so go to his Mods link at the top, then look for Shark Fin in the list. (Darell, hope you don't mind me linking here!)

http://evnut.com/prius.htm

 

<div class='quotetop'>QUOTE(rigormortis @ Jul 3 2007, 11:41 AM) [snapback]472283[/snapback]</div>

It's a tad bit more complex than just measuring the amount of wire. Winding the wire produces a coil or load, which changes the electrical characteristics of the antenna. Coils are one way of making a short antenna appear longer.

Tom

 

If you look at http://www.sigmaautomotive.com/FoliaTec/antenna.php under "Sky Year Technology" banner, their shark fin antenna installation required feeding power to it. Only explanation I can think of is the antenna has a built-in amplifier to make up for the shorter element length. They claimed it has better reception. I personally have no experience with this antenna. Seems like the installation would be more involved than the other shark fin antenna that does not required external power. Anybody care to be the guinea pig?

 

Any source for cheap, longer antennas that'll screw right on?

 

<div class='quotetop'>QUOTE(rigormortis @ Jul 3 2007, 08:41 AM) [snapback]472283[/snapback]</div>

Well, looking at a Shark Fin website, it says (and I quote),

Through testing, we found that the VG Shark Fin Antenna performed consistently better than any stubby antenna fitted on the same car.

By "stubby" do they the mini antennas? If so that implies a smaller element.

 

<div class='quotetop'>QUOTE(aaf709 @ Jul 3 2007, 12:30 PM) [snapback]472433[/snapback]</div>

Hey Guys, I hate to spoil the party. But when it comes to antennas, having more metal in the air (like a 1/4 wavelength whip at FM frequencies in the center of the roof) is far more efficient than a stubby antenna or a small "Shark Fin" with an rf amplifier. (Ignore the marketing BS.)

Keith

 

Hi All,

Yep, Tech Guy is right. Although small antennas can work effectively.

1/4 wavelength on the geometric mean of the FM band is about 30.3 inches long.

The question is, can one just put a 30 inch rod on the existing antenna element mount? Or is there matching circuitry which tunes the short rod for optimum reception inside the plastic base? This matching circuit that improves the performance of the short rod, might detune the effectiveness of a 30 inch rod.

Another issue is that FM broadcasts are typically done in 45 degree slanted circular polarization. Ever see the crazy slanted antennas on retail stores? That is to match the antenna to this polarization, as Muzak and other services are fed to the store by the auxiliary carriers on FM broadcast stations. The purpose of the slant is so that home horizontally polarized antennas will receive as well as a vertical car antenna, and better than if the signal was a pur circular polarization by a dB or so.

So slanting a car antenna may increase, or decrease reception depending on the slant of the particular radio station, and orientation of the car.

An aerodynamic shape made from copper, silver plated, and then coated with a white plastic might help the antenna effienciency. Black colors are loaded with soot (socalled carbon black) pigments. If the granules of the soot touch, there is conduction and resistive loss. Titanium dioixide pigments are low RF loss, and about the same dielectric constant as the non-touching dispersed soot particles. But touching TiO2 particles do not conduct, and cause resistive losses.

Long airfoils have a tendency to flutter. The airfoil shape would have more surface area than the thin wire wrapped around the the plastic rod which the stock antenna appears to be. This would reduce the RF resistance. Attaching a wire extension to the airfoil might help stabilize it in the wind. Such airfoil shapes are available at hobby shops.

Or one could wrap a large airfoil with 1/2 inch wide copper tape to increase surface area. Both sides of the tape will carry current, if not overlapped. This gives a large decrease in the RF resistance of the antenna.

 

<div class='quotetop'>QUOTE(ChristoB @ Jul 3 2007, 09:04 AM) [snapback]472298[/snapback]</div>

No problem at all. That's what my pages are there for. I definitely prefer a link, and not hot-linking of my imgages. I'll chose when to hot-link one of my images... thanks!


Is this the picture you were shooting for?

 

Doc Willie,

Where did you get the cool decals on the side of your car?

Blue Hoosier

 

I too would be interested in a basic mod to improve the Sharkfin. We replaced our stock antenna after the base lost a confrontation with the garage door. I believe stock replacement was $200.+ so we opted for the Sharkfin, but the reception is not satisfactory.

PA P

 

Hi Doc Willie,

Stick antennas have maximum horizonalling oriented gain at 5/8 's wavelength. The gain advantage of a 5/8'ths wavelength stick over a 1/4 wave stick antenna is about 3 dB. Above that length, the pattern of the stick antenna starts to shift to more on axis directions, versus horizontally omindirectional for a vertical stick. So, if you make the antenna too long, it will get best reception from some angle less than 90 degrees off the axis of the stick. A really long stick antenna, or what is commonly called a "infinately long wire" will have a biconical patter with a 17 degree axis between the axis of the wire/stick and the main lobes of the reception.

Additionally, as we can assume the original Prius antenna was professionally designed, there will be some sort of matching capacitors and inductors between the shorter than 1/4 wavelength stick the stock antenna is and the input impedance of the amplfier. We do not know what the amplifier is, but usually, these things are designed for a 50 ohm impedance, so various substitution games can be played during design and developement. RF test equipment is all 50 ohms.

A vertical stick antenna will also change its impedance with length, as well as its pattern. The 1/4 wave stick has approx. 35 ohm impedance, if its perfectly vertical, and mounted on a perfectly flat vertical ground plane and tuned to the 1/4 wavelength resonance. Mounted as it is on the Prius, the impedance will be a little more than 35 ohms. Because half the ground plane is missing (where the rear hatch window is). Off frequency the impedance changes, and reactive impedance componants will occur. Impedance is realy a complex value - as in a real part and an imaginary part - complex numbers remember those? If the imaginary part of the impedances is positive, the antenna is acting as if besides a disipative (energy leaking out into free space - real part of impedance) componant, it also has an inductive (like a little coil) part. If the imaginary part is negative, its like a little capacitor is connected to the resistive part.

So, now the geometric mean of the FM band is 97.5 MHz. 1/4 wavelength at 97.5 MHz is 30.3 inches, and 5/8 's wavelength is 75.7 inches.

The Prius antenna is only about 12 inches. So its impedance is much lower than 35 ohms, and besides that its capacitive. So, inside that little black plastic molding is a combination of electronics components that first make the impedance purely resistive, then converts it up to 50 ohms.

One interesting thing is these reactive componants repeat. At 1/2 wavelength (4/8 ths) antenna has a very high purely resistive impedance. With such a large resitive mismatch, very little energy from the antenna will make it into the stock circuits in the Prius molding.

A 5/8 ths wavelength antenna will become capacitive again, however. And the impedance comes down, to about 100 ohms.

An antenna that might have better performance, would be a 5/8 ths wavelength element, with a 4:1 matching transformer, into the stock circuit. Assuming the transformer can be made with lower losses than the improved gain of the antenna. An adjustment for reactance might also be included to peak up the performance.

So, then the issue is, will the little stock plastic molding hold a 78 inch long metal rod up without breaking ?

Another approach might be to match a 1/2 wave element into the stock element. This is done with a 1/4 wavelength transmission line shorted stub, or parallel tuned circuit off the end of the standard length Prius antenna element. The 1/2 wave element would need to be 60.6 inches long and attached to the standard Prius element by way of the resonant circuit mentioned above.

BTW, I enjoyed your car show story. Quite neet.

 

FYI, the sharkfin on my Prius out performs the stock mast types on our other two Subarus, at least, in the marginally fringe stations (KFWB am in LA) that we have compared, also some FM stations are better compared to the other cars "real" antennas. It depends entirely on the terrain, and strength of the stations, but the here, the Shark-fin is better. I got the fin to make putting on a car cover easier. Now I have garage space, but the sharkfin stays.

 

<div class='quotetop'>QUOTE(subarutoo @ Aug 23 2007, 04:42 PM) [snapback]501165[/snapback]</div>

This might be explained by the smaller directivity gain of the smaller shark-fin antenna. Whey you are down in a canyon, the stronger pattern higher up in elevation, would pick up high angle signals bleeding over the edge, or reflecting off the face of the canyon wall. A quad-filar helix antenna might be an even better antenna for this enviorment. Dual quadfilar, with polarization diversity might be the best.