Monday, May 2, 2016

Antenna Gains for Portable HT Radios

Antenna Gain is basically the antenna's ability to radiate energy in a particular direction. Portable HT radios have a small vertical antenna mounted to the case of the radio. Usually the manufacture's antenna has a mediocre gain or negative resulting in poor performance. This is not usually noticeable due to the fact that most people use these radios to hit line of sight repeaters for contact with others. If the operator wants to use this in a simplex mode then antenna gain becomes evident. (continued below...)

Keep in mind a vertical omnidirectional (radiating in all directions perpendicular to the antenna) is never as good as a directional antenna such as a Yagi (offering high gain in one direction). Most people want a small portable antenna that radiates in all directions with an HT radio. Therefore the operator usually accepts the compromise. However, I have to ask is a -5 dB gain worth that compromise? If your HT antenna is rated at -5dB gain and you are transmitting at 5 watts then your actual output is only 1 watt. While most HT antennas are not rated that bad they are still in a negative gain design.

There are aftermarket antennas that can offer a greater gain than what comes with the radio. Most of these antennas are 1/2 wavelength for 2 meters making them considerably longer than those that came with the rig. Most radios come with a 1/4 wave or less antenna. A gain that is considered good for an HT vertical is 2 or more dB. Many will disagree with the fact that 2 dB is good considering that a Yagi can easily, depending on the configuration, achieve 9 dB's or greater. However, if one understands that the factory antenna are usually a negative dB, then seeing a 2 dB gain is actually good.

Without a reflector and director it is difficult to increase the gain of a vertical to any major degree. Also keep in mind there is not a good ground plane when it comes to HT's, therefore you are off to a bad start. The first objective is to get the antenna to the best electrical radiating design for the frequency that you will transmit. In theory this would be 1 full wavelength. In the case of the 2 meter call frequency the length would be over 6 feet in length. How would you like to carry that on the top of your HT?  Just a note, using a vertical antenna in a full wavelength design would better be served as a loop. I won't get into the electrical aspects of that in this post. 

At the 2 meter call frequency on an HT, it would be better to reduce the antenna by 1/2 wavelength. This would make the length of the antenna to a little over 3' 2". This would be more manageable and still better than the factory antenna. This should give you a little to think about when trying to improve your radio. There are plenty of antenna options on the market that would improve the efficiency. Let me encourage you to experiment and work with others who have tried different antennas of their rigs.

Many companies offer different high gain options for your radio. Below are a few after market options for dual-band antennas.

Diamond Antenna Dual-Band HT Antennas SRHF40
Rated at 6 dB gain

Comet Antennas SMA-24 
Rated at 2.3 dB gain on 2 meter and 3.5 dB gain on 70cm

If you are looking for a new antenna for your HT then make sure it can handle the output power you want to transmit. You will also need to make sure it has the matching connector that mates to your transceiver.

The subjects of antenna design including antenna types, length, loading, material, thickness, and engineering is well beyond this article. However if you are interested in designing and experimenting then the reference you need is the Antenna Book published by the ARRL. It is not for the faint at heart but is an invaluable resource.

Let me encourage you not to look for the smallest antenna you can find to put on your radio. It won't give you the performance you desire and certainly won't be good for your radio.

What antennas have you found that works best for your HT?

No comments:

Post a Comment