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1/4 wave Vehicle Antenna Tuning

28 January 2016

The ¼ wave antenna is the basic standard vehicle antenna for several reasons:

  • It is simple; A straight conductive rod.
  • Practical; Easy to make, fit and of a manageable length for land mobile frequencies.
  • It provides a good impedance match to the radio meaning RF power generated in the radio is radiated via the aerial with minimal losses (assuming the coax and connectors a fitted correctly).
  • It radiates well in the horizontal plane giving good coverage in the directions other radios are likely to be located.


The ¼ wave antenna consists of two parts;

  1. The Ground-plane. This is the horizontal surface on which the aerial is mounted, almost always the body of the vehicle on which it is mounted. The Ground-plane is as important as the antenna whip because it forms part of the final antenna system. The ground-plane should be well connected to coaxial ground and it should extend at least one ¼ wave radius horizontally from the base of the whip. This is usually easy to achieve on a vehicle roof which is much larger.
  2. The antenna rod (or whip as it is sometimes called) must be cut to a length suited to the centre frequency that the radio transmits on. This is often called 'Tuning' because you are trying to achieve resonance of the aerial much like you might tune a musical instrument. A correctly tuned antenna gives the best power transfer from the radio to free-space.


Cutting or tuning the aerial;
Tuning can be done 'on paper' or using a special meter (VSWR Meter). On paper is more than accurate enough for most applications (but a VSWR meter will always give a check that the antenna and cable system are fitted and working correctly). Tuning on paper is a two step process:

  1. Find the centre frequency the radio transmits on. Add the highest and lowest transmit frequency that the radio is programmed to transmit on and divide by 2. EG 154.5000 + 161.1250 Mhz = 315.625/2 = 157.8125Mhz. This is the frequency the antenna should be tuned to.
  2. Calculate and cut to length using the following formula which gives a metric cutting length in metres: 300/Frequency/4 . This is based on the speed of light divided by the frequency giving the full wavelength. One quarter of this is the ¼ wavelength.

    This length in m (Metres) can also be expressed as 75/Frequency.
    EG 300/157.8125/4 = 0.475m. (475mm)

    This length in mm (Millimetres) can also be expressed as 75,000/Frequency.
    EG 300/157.8125/4 = 475mm

 

Notes
The antenna length is the total conductor length above the ground-plane and this should be the total length including any joints on the antenna base.

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