The following tool gives the length of the gap between the two elements of a dipole antenna at a specified operating frequency.
Example Calculation
For a 144 MHz (2 meter band) dipole antenna, the max gap size is 15 mm. The gap can be smaller but not zero.
Background
The size of the gap in a dipole antenna, especially at its feeding point, is relatively small compared to the overall length of the antenna and does not need to be large for effective operation.
Ideally it is zero. However this would result in a short.
The exact size of the gap can vary based on the specific design, the frequency of operation, and the method used to feed the antenna.
However, there are general principles that can guide the determination of an appropriate gap size:
- For Direct Coaxial Cable Feed: If you are directly connecting a coaxial cable to a dipole, the gap is essentially the space needed to accommodate the feedpoint hardware, such as soldering lugs, insulator, or a balun. This gap might be only a few mm wide, just enough to ensure mechanical stability and electrical connectivity.
In the case of the dipole antenna for 144 MHz to 430 MHz applications shown below, the gap is 15 mm or less than 1.5% of the total length.
- For Use with a Balun or Center Insulator: When using a balun (balanced-to-unbalanced transformer) or a specific center insulator designed to facilitate the connection of the feedline to the antenna, the gap will be dictated by the physical dimensions of these components. Even in these cases, the gap remains relatively small, often within the range of 1 to 2 centimeters (10 to 20 millimeters), primarily determined by the size of the balun or insulator housing.
- Minimal Impact on Performance: The electrical performance of a dipole antenna is largely unaffected by the small physical gap at the feedpoint, as the antenna’s operation depends more on its total length and the distribution of currents along its length. The critical factor is ensuring that the total electrical length of the antenna is appropriate for the desired frequency of operation. Use this calculator to find the length of the dipole.
- Practical Considerations: The gap should be large enough to ensure that there is no electrical short between the two halves of the dipole and to accommodate any hardware used for feeding and supporting the antenna. It should also provide sufficient insulation to handle the maximum voltage present at the feedpoint, which can be significant, especially at higher power levels.
In practical terms, the gap in a dipole antenna is a small but important feature. It relates more to mechanical and electrical connectivity and less to the theoretical aspects that determine the antenna’s radiation pattern and impedance.
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