Portable antenna gain

DP4801e with PMAD4119
Motorola portable two-way radios will either be supplied with a normal-mode helical (a.k.a. rubber ducky) or quarter wave antenna (a.k.a. whip antenna). These antennas generally do not have any/much gain.

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.

4 comments:

  1. My experience with 50 Ohm (SMA) UHF radios is that there are sometimes 2 or even 3 dB between both antenna types. However the MX thread connector radios usually have one dB or less between them. In practical use there is no difference when the radio is held in the hand. Just when it is clipped to the belt the helix antenna is more attenuated from the body. For UHF marginal, 160 MHz more, and 80 MHz much more :)

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  2. These values don't include the effects of body loss which are very significant. Casey Hill of Motorola wrote an in depth study on various portable antenna configurations including in vehicle effects. The Paper was published in IEEE Transactions.

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    Replies
    1. Thanks, I’ll look that paper up.
      Yes, the losses due to the users body can be significant.

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  3. I apply at least -8dbm on all bands when modeling portable coverage because they are rarely handheld. They are either kept on the belt at all times using a speaker mic for their traffic, or they are used within a moving vehicle. Often both. Many vehicles use vaporized copper to tint the glass effectively making the car or truck a rolling Faraday cage. I've demonstrated the isolation numerous times to techs and customers using their cell phone signal bars, and simply rolling the windows up and down.

    Bill George, Portland OR USA

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