Car radio antenna questions

[kurnia.2]

Hi all,

I own a small Honda Jazz (Asian market). The one thing I don't really like is its antenna is too long.

So, I had decided to have a try & have somebody make 2 straight, 7 cm long, 10mm diameter antennas made out of brass and stainless steel, threaded on one end for installation.

I tested both antennas (or maybe I should just call it brass & stainless steel shafts) but they did not work good. Only about 40-50% of the original antenna. They looked good though.

I wonder how to build a good, functional one. What's the theory behind it? What kind of material should I use? Any details that I should know?

I appreciate your help.

Thanks

 

[David Bridgen]

its antenna is too long. ..... I had decided to have somebody make 2 straight, 7 cm long, 10mm diameter antennas. ..... I tested both antennas ... but they did not work good. They looked good though. Any details that I should know?

I'm not surprised that they didn't work "good." In fact I am surprised they did work as well as you say.
The original antenna was of a particular length for a good reason. And even that length was a compromise selected after many trials carried out by the vehicle manufacturer. All vehicle antennas for use on the broadcast bands, whether for medium frequencies or v.h.f., are a compromise because they are very short in comparison to the wavelengths they are intended to receive (or sometimes transmit) and they all have some method of compensation for this. Replacing one with another of radically different length, especially a much shorter one, is a major step in the wrong direction.
If you want good radio reception leave the original antenna as it is/was and put up with its looks.
If you want something pretty you will, in all probability, have to put up with poor reception.

It is often said that any radio system is no better than its antenna/s.

 

[kurnia.2]

Thanks for the reply David.

I've tried to find information on how this vehicle radio antenna works but I couldn't find any.

I've browsed google and had no luck. I only found how people build big antennas (not for automobile). So I thought why don't just try it and at least I'll know if it works or not.

I've heard about the vehicle antenna's compromised length.

The thing that still is in my mind is that I know of some late cars produced, they use about 15 cm long of look like rubber material. And yet they receive very good. Lots of FM stations.

I'll buy one, break it, and see how they are constructed. I'd like to see if I can make one that has the same receiving capability as the OEM :lol:

Well, thanks again for your time.

 

[bryan1]

Eh Kurnia,
My father-in-law is an avid Honda fan, that said he owns a heap of classic Honda's but his nu girlfriend suggested he buy one of those Jazz's as they are a honda in which he did. He said the same thing and had a similar problem so he got a mate to replace the radio with a previous Honda radio that worked better. Now he's happier than Larry would ever be.

Just a thought Bryan

 

[mstechca]

use this equation:

300/<frequency wanted in Mhz>

Now you know how long of an antenna you need in meters for 100% reception. Most people use 1/2 wavelength antennas instead of full wavelength ones, so use this equation:

150/<frequency wanted in Mhz>

Since you are dealing with a car radio, and assuming you are using FM, you will need an antenna about 2.5 meters long (or 1.7 meters long for 1/2 wavelength).

 

[Nigel Goodwin]

use this equation:

300/<frequency wanted in Mhz>

Now you know how long of an antenna you need in meters for 100% reception. Most people use 1/2 wavelength antennas instead of full wavelength ones, so use this equation:

150/<frequency wanted in Mhz>

Since you are dealing with a car radio, and assuming you are using FM, you will need an antenna about 2.5 meters long (or 1.7 meters long for 1/2 wavelength).

300 divided by the frequency (in Mhz) gives the wavelength, so (assuming 100MHz - mid FM band), this comes to 3m wavelength. A whip aerial is commonly quarter wavelength (which gives 50 ohms impedance), so this gives a whip length of 750cm.

If you make the aerial longer, it won't tune properly, and you have to add tuning coils etc. to bring it back in tune, a popular length for the 2m amateur band is 5/8th, which uses a bottom tuning coil to make it work.

If you're talking a dipole aerial, then it's half a wavelength long overall, each side is quarter wavelength - a quarterwave whip is actually the 'same' as a dipole, the other half is reflected in the ground plane of the car body.

As you're wanting to cover a fairly wide bandwidth (20MHz), then the aerial will be a comprimise anyway - the shorter aerial are generally helical ones, with the aerial rod spiralled round.

 

[Dean Huster]

Usually, car antennas aren't that terribly caring about antenna length as long as it grabs enough signal. They use short ones, collapsable ones, dipoles that are part of the windshield (windscreen), etc. Usually, at least in the old days, the FM section of the radio took what it got and did well with it. The AM section had a padder capacitor on the front of the radio under the tuning knob that was peaked for whatever antenna was supplied. These days, AM is thrown in on a car radio with some el cheapo circuitry just so it's available and they spend most of the design on the FM portion.

If the antenna length was that critical, then they'd mount a power antenna on the car and it would automatically adjust for antenna length according to the frequency selected on the radio.

Dean

 

[kurnia.2]

Thanks all for the rich information.

However, I wonder how Bosch (usually on Mercedes sedans) could make an antenna that sticks on a windshield, extending along the width of the windshield. That's it. It's just as high as the windshield.

There must be something else here. Might that be that antenna length is not really the only ultimate factor to consider when designing an antenna? Are there something else? Is spiralled wire (Nigel had mentioned) one important thing?

Bryan,
Was it about the antenna not receiving properly or the radio unit wasn't really receiving signals? If replacing the radio unit made it receive better I presume that your father-in-law had a bad unit?

 

[Sebi]

The secret of small (short) aerial an in-built FET preamplifier, but nothing compare with a good full-wawe aerial...