MOBILE ANTENNAS                                              

We all want the best mobile antenna, preferbly this has to be the smallest one aswell.
As usual this is never the case. Luckly for us there is a great variatie in mobile antennas in length, looks and again price. 

You could say there are two kinds of mobile antennas. The one who depends on coaxlenght for "grounding" , these can be used b for example on polyester boats. By changing the length of the coaxcable the SWR will change. 
The second type uses the metal to wich the antenna is connected as its counter system (grounding).

We know that almost all antennas are based from the dipool, this is the case aswell with the mobile antenna.
You can see the vertical mobile antenna as one half of the dipool, the otherhalf is the car itself or the outherbraid of the coaxcable.
Since the lenght often is shorter then one half of a dipool (¼ wavelenght), the lenght missing has to be made up with a coil. (the coil, makes the antenna electricly "longer")
 
But with a coil comes losses although they are not large, it is still a factor to deal with.
These can be kept to a minimum by choosing the mobile antenna wich can handle the highest power. This will mean that this is the larger coil (diameter) and therefor has the lowest loss.
Another important factor is where the coil is put. The best placing beeing right at the top of the mobile on a second place we have the mobile antenna with the coil in the middle at last the one with the coil at the bottum. There is only one problem...no one wants to have a coil on top and the mechanical construction  will be quite weak aswell.

GAIN.
Sadly all mobile antennas have no Gain, only loss, logical since the lenght of  a mobile antenna is to small to create any.
Also the often seen additional remarks, like this is a 5/8 wave lenght mobile antenna!!,  well...not true ofcourse, If a mobile antenna is 1 meter long it is 1/11 wavelenght simple as that.

THE BEST?
So the best mobile antenna is indeed the longest antenna. When you desided what your maximum lenght and still have a few models to choose from, then you should go for the antenna wich can handle the highest power. And preferbly with a coil on top.
The position of the antenna is preferbly the highest central point. When used on a car such an antenna has some directivity when it is placed on the back of the car. The drawings below will explain: (a quaterwave lenght vertical highly depends on ground)
                                                                                          

At last, dont think you are lost with only a mobile antenna. In December 2005 I worked stations from 43 division who were using only 20 watts and a mobile. (almost at the bottum of the solarcycle)
So just think what is possible with perhaps a bit luck, and patience when the sunspots are high...
There are a few guys who have over a hundred DXCCs  on 11 meter with a mobile.
And there are guys who have a real good DX-spot for example on a hill near water and use the mobile set-up there for DXing, with better results than at home.

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