Antenna, how to make a DIY internal or aerodynamic external antenna

[saand]

Thanks for the replies,
I have seen the defogger antenna idea on here but im personally not a fan since i use my defogger still and i would like to think a properly thought out internal antenna which would be very little work would work better ( i may be wrong though)
I have also seen the internal antenna idea, it is a very good idea but a simple piece of wire should be just as effective and a little easier to route in the car.

I might have to try out some tests with different orientations, lengths and arrangements maybe i can get some sort of signal strength meter so i can work out how to do it the best way. Then i can post it on here for everyone else.

Im still interested to hear from any RF engineers around if they come across this post so i can get a bit of guidance or if i have missed anything.

[Zerohour]

Where is your current antenna located? Is it coming out through the trunk? Front Fender? Roof?

Your profile says Mazda 626 mildly similar to the Ford Contour. I'm assuming the trunk is the location. If this is true, I want you to do something very simple.

Remove the antenna from the base. Open the trunk, remove the antenna from the interior mounting. Reassemble it inside your trunk and drive around. If there isn't a noticeable loss in reception, then remount it inside your trunk. You won't have to buy another antenna. You'll still be using the OEM equipment, and you'll have all the parts if you decide to put it back (reversible decision).

You can quickly plug the hole with a washer/bolt/RVT silicone combo. Or you can find another antenna base to plug the hole (Junkyard salvage might not even charge for just an antenna base lol!).

Just use any precaution that you don't mount the antenna where the trunk lid is going to cut the wires or bend the antenna when you shut it close.

-Z

[ConnClark]

As some one who designs radio equipment, first I would like to say your not going to approach the receive sensitivity of your stock antenna located outside the car. The simple fact is the roof and support pillars are going to act like a shield and attenuate the signal.

Now that we have that established that There are several things that will have an effect on your radio reception. The key thing is matching your antenna impedance to your radios impedance. If you are going to use a loop antenna around the rear window it will work better or worse depending on which direction your car is facing. If your going to use a whip antenna it will work best normal to a ground plane. Also you want to make sure you have nothing close to the antenna as it will interfere with it (think iPhone4 boondoggle).

[Big Dave]

Did you get any MPG improvement by taking off the antenna?

I took mine off completely and plugged up the hole.

No improvement whatsoever.

[saand]

ZeroHour, The antenna was already removed, Since my last post i have mounted it inside my cabin instead of my length of random wire. I now can pick up more radio stations however not all of them. This is expected though since the antenna is inside the cabin which shields some of the signal. I am still interested to have a DIY antenna since the stock car antenna in the cabin just looks very wrong.

ConnClark thanks for your reply, good to hear from someone with detailed knowledge about antennas.
When matching the impedance of the antenna to the radio is that only the impedance of the wire between the antenna and the radio or is the antenna specifically constructed to have an impedance. Naturally at 100 MHz there will be reflections in the signal wire if the impedances are not matched. But I am not sure how an antenna (as in the piece of wire of a particular length and geometry) will be of a specific impedance.
Also im an electronic engineer so feel free to get a bit technical about the subject

BigDave,
I did a test a while ago with the antenna up compared to down and the improvement level was in the noise so i didn't post the results. I would have to believe it would provide some improvement because the antenna is no longer in the wind but other than that i dont have any test results for you. I could estimate less than a 0.5 % improvement, anyone doing a simulation of the drag would be able to give you a better estimate of the improvement but personally i am happy with any improvement.

[ConnClark]

saand,

yes you are correct, for a correct impedance match you need to have the receiver, the antenna, and transmission cable all the same impedance. your best bet is to use the cable that was used to connect the existing antenna. I'm not sure off hand what the standard antenna impedance is used for car antennas (my gut tells me its probably 75 ohms due to cost). I know TVs are 300 or 75 Ohms. For radio communications equipment its most often 50 ohms.

In general an antennas impedance is determined by its design. A dipole is about 73 ohms. A monopole is roughly about 30 ohms. A loop antenna is usually around 300. In a lot of cases they aren't a perfect match and have some tuning circuit to match them.

some good reference material

Monopole antenna - Wikipedia, the free encyclopedia
Why Fifty Ohms - Microwave Encyclopedia - Microwaves101.com
Dipole antenna - Wikipedia, the free encyclopedia

[NiHaoMike]

Put ferrite cores on the wires going to the defroster, then connect one end to the radio through a 0.1uF capacitor.

[Tango Charlie]

Originally Posted by Clev

I thought I remembered reading here about somebody who took a regular car antenna and stuck it behind the plastic trim piece on the A-pillar. If I actually cared about radio reception, I'd probably try that.

Johhny Mullet's thread from back in the day.