Perrin Shorty Antennas for 2008+ - Subaru WRX Forum
 
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#1 Old 02-02-2009, 11:40 AM
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Perrin Shorty Antennas for 2008+

Okay. I just came across these today. I wasn't too happy seeing these since I've taken electromagnetic and antenna classes, and I actually understand what exactly happens when you modify your antenna. The marking idea for them is that the long antenna that comes on the new WRXs are very unattractive, so they make a shorter version, and an ultra short version.

The problem is that an antenna is optimal when the length is one quarter of the wavelength. The wavelength is always going to be the same, so by swapping out with a shorter one, you are going to lose significant performance the further away your antenna length is from the optimal. Even making the antenna longer will affect your radio's performance.

Again, this is one of those appearance vs performance modifications. Most people that buy it probably don't even realize they're worsening it for them. I should also note that if you're in an area with strong signal reception, it shouldn't made a noticeable difference, so you should be okay if you know particular stations and stick close to their proximity.

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#2 Old 02-02-2009, 12:27 PM
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What do you think about this? Metra Electronics - Products
The antenna on my 02 was bent to crapola so I wanted something that looked a little more stylish.Seems to work OK but I'm in a bad area (in between stations) so it's hard to tell.

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#3 Old 02-02-2009, 12:44 PM
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Quote:
Originally Posted by Vew View Post
The problem is that an antenna is optimal when the length is one quarter of the wavelength. The wavelength is always going to be the same, so by swapping out with a shorter one, you are going to lose significant performance the further away your antenna length is from the optimal. Even making the antenna longer will affect your radio's performance.
I think you misunderstood some stuff from antenna class. Quarter wave antennas with a ground plane are nice and are the main design used in this type of application but they are FAR from what you would call "optimal". In the world of antennas, size does not always equate with performance. It is quite possible to make a BETTER antenna which is SMALLER than a quarter wave vertical.

I'm not saying that's what these products are, I'm just saying your not leaning on hard science in just tossing them out based on size. Verticals aren't the only possible car antenna either. A long wire would work very well if it were correctly done. You could wind a wire around the roof of the car and it would make an excellent FM antenna. Another superior option would be a J-pole. That would take more thinking to lay out correctly but it should be doable.

Seriously, nobody is going to drive around with a true 1/4 wave whip on their car anyway. 3 foot tall antennas went out of fashion a while ago. Besides, the car is FAR from a perfect ground plane at those sizes.
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What do you think about this? Metra Electronics - Products
The antenna on my 02 was bent to crapola so I wanted something that looked a little more stylish.Seems to work OK but I'm in a bad area (in between stations) so it's hard to tell.
They quote 3db gain which I don't believe for a second. A half wave dipole is only 2.15db. Remember that FM at longest is 87.5mhz (11.2 feet) and that makes a 1/4 wave about 2.8 feet. They're saying it has more gain than a 6 foot wide horizontal antenna. The 3db gives a clue into the design though. It's probably a wound coil which acts to lengthen the effective resonance of the antenna for a given length. A true 1/4 wave would be 3db so the fact they claim the same gain as a 2.8 foot tall piece of metal means there's a lot of coiling going on. What do I think of it? It's cheap.

How do antennas like that work when they're so short? The effect is called Electrical Lenghtening. It can be done cheaply though which is common in this type of application where price is a main driving force. The result is a low gain antenna with narrow bandwidth.

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#4 Old 02-02-2009, 03:48 PM
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I switched over to the ultra short and have zero problems. Sound/Clarity/Reception are exactly the same. (Navigation Package)
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#5 Old 02-02-2009, 04:07 PM
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Quote:
Originally Posted by mosc View Post
I think you misunderstood some stuff from antenna class. Quarter wave antennas with a ground plane are nice and are the main design used in this type of application but they are FAR from what you would call "optimal". In the world of antennas, size does not always equate with performance. It is quite possible to make a BETTER antenna which is SMALLER than a quarter wave vertical.

I'm not saying that's what these products are, I'm just saying your not leaning on hard science in just tossing them out based on size. Verticals aren't the only possible car antenna either. A long wire would work very well if it were correctly done. You could wind a wire around the roof of the car and it would make an excellent FM antenna. Another superior option would be a J-pole. That would take more thinking to lay out correctly but it should be doable.

Seriously, nobody is going to drive around with a true 1/4 wave whip on their car anyway. 3 foot tall antennas went out of fashion a while ago. Besides, the car is FAR from a perfect ground plane at those sizes.
Okay, I agree that neither antennas are really "optimal" and a poor choice of wording on my part. The optimal length is easy enough to figure out; you need to know the total wavelength first which can be found:
Wavelength = Speed of Light / Frequency
where the speed of light is 299792458m/s, frequency is assumed to be 88Mhz or 88x10^6Hz, which equals 3.41m. A quarter of the wavelength could be 0.85m and converting to inches would make it about 33.2 give or take a couple of inches which is very unrealistic for a vehicle.

Stock antenna ~6" which i can only assume it's a helical wound antenna and that allows for a shorter physical length. Although small, the location where it's mounted does make the metal roof a decent ground plane.

Ultimately, my goal wasn't to debate antenna types and their possible performances (but it's kind of cool to find others intrigued and i don't mind at all either). I just wanted people to know that in the end, swapping out their stock antenna for a shorter one like this will degrade performance. Some people may not experience any worse reception, but there are those that will. I have read some comments were people claim it improved their reception, but I seriously have a hard time believing that.

Everything aside, if anyone is curious, the math behind antenna design is absolutly horrific. There is a lot of vector math and partial fractions.


(in reference to the antenna posted)
Sadly, I think your stock antenna is going to work better then that one. Not saying it won't work at all.

Jon
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#6 Old 02-02-2009, 04:24 PM
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Quote:
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Stock antenna ~6" which i can only assume it's a helical wound antenna and that allows for a shorter physical length. Although small, the location where it's mounted does make the metal roof a decent ground plane.
Believe it or not, the ground plane is actually the limiting factor here, not the 6" antenna. Cars used to be big sheets of metal and make very effective ground planes but it's not the case anymore. Between the sides is very little metal. Also, these things often get mounted on the EDGES of the car which is far from ideal. You're actually better off using your trunk for a ground plane than the roof. Course, I haven't seen a car company with the balls to stick up a vertical in the middle of their trunk yet

Vertical 1/4 wave designs really need full ground planes. That means they need to be at the center of a 6 foot by 6 foot flat conducting surface (for FM). When you reduce this, they stop behaving like mirrored dipoles and it can particularly be a problem if you have an electrically lengthened coil instead of a whip. It creates lots of resonances in strange places that make it a pretty crappy antenna.

I don't know why car companies don't sell it as an option but the best thing to do would be to run a long wire around the top of the car frame. Should be easy to get >12 feet of length out of it and maximize height. You should be getting closer to 3db with that setup and no antenna sticking up at all!

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#7 Old 02-02-2009, 04:32 PM
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Actually, I wasn't aware that the ground plane was the limiting factor. It does make sense though. I was actually going to post the ideal ground plane size, but it's just the full wavelength calculation in all directions from what I learned in class.

I can somewhat see why they wouldn't wrap all that wire or even offer it as an option. That is a lot of "unnecessary" additional labor and cost. If they don't do it to all their cars, I doubt they'd even do it at all since it would be difficult in a manufacturing environment on their scale. But wouldn't the metal panels of the car affect it anyway?

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#8 Old 02-02-2009, 05:40 PM
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Well, you don't really need a full wavelength for a ground plane. You also don't need a complete sheet but a few quarter wavelength long strips really help. I worked at a ham radio guy's place for a couple of contests and had the privilege of working on one of the world's few 160m 4-squares (basically a quarter wavelength vertical). 160m is a huge wavelength. You're talking more than a hundred foot tower and ground lines going off over an ACRE of land. God if that thing didn't get out though. Anyway, it was an excellent case study on simulating a ground plane. The result was that you really only needed 4 perpendicular wires. More didn't do TOO much. Anyway, they're 1/4 wave, not a full wavelength.

On the metal in your car, it's effectively an electrical ground. You could, and some people have, use the entire car's frame as an antenna. Generally you don't want to do this because it's also used as an electrical ground for the battery/accessories and you could get some substantial currents going into your radio in ways you don't want. Anyway, it's not going to interfere with the antenna unless you put the wire through the middle of it. Laying it on top would be fine. Will the wire pick up RF from your car? Absolutely! Huge amounts. Any car antenna will. Fortunately though, this is proportional to frequency and most machine noise doesn't get past 10mhz or so. FM is up in the VHF range so it's basically too high to be effected much. Certainly a long wire would not be worse for it than any other car antenna but yes, it's a problem in general having an electrical noise source that close to your antenna. I just don't see a simple way around that one. Tow your antenna?

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#9 Old 02-02-2009, 07:23 PM
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Hahaha, you never know, that's how submarines do it! Actually, this sounds like it'd be a fun experiment to try if you ever have to remove your headliner or something. Supposedly if you have a very good antenna and pick up enough power, it can exceed the quality of high definition radio. At least that's what I read.

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#10 Old 02-02-2009, 07:59 PM
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Originally Posted by mosc View Post

They quote 3db gain which I don't believe for a second. A half wave dipole is only 2.15db. Remember that FM at longest is 87.5mhz (11.2 feet) and that makes a 1/4 wave about 2.8 feet. They're saying it has more gain than a 6 foot wide horizontal antenna. The 3db gives a clue into the design though. It's probably a wound coil which acts to lengthen the effective resonance of the antenna for a given length. A true 1/4 wave would be 3db so the fact they claim the same gain as a 2.8 foot tall piece of metal means there's a lot of coiling going on. What do I think of it? It's cheap.

How do antennas like that work when they're so short? The effect is called Electrical Lenghtening. It can be done cheaply though which is common in this type of application where price is a main driving force. The result is a low gain antenna with narrow bandwidth.
The unit itself is based on the Fuba antennas that are factory on the VWs and Audis.It does have a coil wound mast with a 12v amplifier in the base.

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#11 Old 02-03-2009, 10:11 AM
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Do you notice it performs better at one end of the band than the other dumdum?

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