Portable antenna gain
 |
| DP4801e with PMAD4119 |
Some background and theory.
Gain is achieved in an antenna when it is able to focus the signal is a specific direction. The isotropic - which only exists in theory - or dipole antenna are used as a baseline for measuring other antennas. The unit of measurement of gain is the Decibel (dB).
So in simple terms, a high gain antenna will be able to amplify a signal (think of a magnifying lens focusing the light from the sun onto a piece of paper to burn it).
The helical antenna is particularly practical for use with portable two way radios, as it reduces the overall length of the antenna. Some helical antennas are known as stubby antennas, these antennas are generally half the length of a regular helical antenna.
As the name suggests, the helical antenna consists of a spring steel wire wound into a spiral (a helix). The length of the wire is around one quarter wavelength.
What is wavelength?
If helical antennas were not used, the length of a VHF antenna would be around 50cm (19") - this would not be practical.
A helical antenna is also a compromise since it is not as big as a full length quarter wave (i.e. a 50cm antenna for VHF) hence the gain is not as good either. Stubby antennas will have lower gain when compared to regular helical antennas.
The portable radio itself is a compromise: the antennas used on them work well when they have a counterpoise - the counterpoise in this case exists in the body of the user. The radio is also usually worn on the belt or somewhere else on the body - this shields the radio off from signals and messes around with the antennas impedance.
UHF (403-520MHz) antennas are usually full-length quarter wave antennas - these are usually around 17cm (6") in length. There are also stubby antennas available in the UHF band.
What to expect.
Here are some examples of MOTOTRBO radio antennas and their gain:
| Antenna | Frequency Range | Length | Gain |
| PMAD4117A | 136-155MHz | 15cm | ≈ -3dBd |
| PMAD4119A | 136-148MHz | 9cm | ≈ -3dBd |
| PMAD4147A | 136-174MHz | 20cm | ≈ -4dBd |
| PMAE4079A | 403-527MHz | 15cm | ≈ 0dBd |
| PMAE4069A | 403-450MHz | 9cm | ≈ -3dBd |
| PMAD4138A | 300-360MHz | 16,4cm | ≈ -3dBd |
0dBd ≅ 2dBi.
Leave a Comment