Antenna Placement in a Mobile Installation

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Doppler Systems Blog

Frequently we are asked, “What antenna spacing should I use for my mobile antennas?”  This is a good question because many people think that antenna spacing would be critical for good performance and people are surprised when our answer is, “Somewhere between an 1/8 and 1/4 wavelength.”   In this post I’m going to present to you the results from the detailed model of our antennas to show the effect on error of antenna spacing and also antenna gain and phase errors.  I’m also going to make some recommendations not only on spacing but on placement of the antennas on the vehicle.

Figure 1 below shows the effect on bearing error due to antenna spacing.  Results for both the 4 element and 8 element antenna are shown.  Notice that the curve is fairly flat from 0.125 wavelength to 0.35 wavelength and then it increases rapidly.  This is really good news if you are going to be operating over a relatively large portion of the band since you don’t want to have to jump out and change the spacing when you change frequency.

Antenna spacing of 0.125 to 0.3 wavelengths works well for 4 element mobile array.

Another source of error in the DF is mismatch in the gain of the antennas. Figure 2 illustrates this effect. You may ask why do we care about this since all the antennas are identical?  The answer is that the antenna’s ground plane has an effect on the gain of the antenna.  Therefore it is important to have adequate ground plane around all of the antennas.  Typically we suggest at least a 1/4 wavelength of ground plane around each antenna.

Antenna Gain Variation Cause Bearing Errors

In conclusion use a spacing somewhere between 1/8 and 1/3 of a wavelength but try to make sure that each antenna has at least 1/4 wavelength of ground plane all around it.  The table below is a good reference.

Frequency Range (MHz)
LowHighSpacing (in)Spacing (cm)
10012023.760.2
12014419.850.2
15018015.840.2
18022013.133.2
22025011.128.1
2503009.524.1
3003607.920.1
3604306.616.8
4305005.614.2

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