Something about antennas and gain
When you read the UV-5R Yahoo group, one of the most asked questions is: which antenna should I buy? Which antenna delivers the most gain? The answer is not that easy, but here are a few laws and rules of thumb.
What? How come almost every manufacturer claim they do? I’ll explain. First of all, manufacturers measure the gain of their antennas in dBi, which means that they compare it to an isotropic radiator. This is a more of a theoretical antenna than anything else. Such an antenna radiates (wastes) its energy into every direction: left, right, up, down and every direction in between. For some frequencies even a paper clip would deliver more gain than an isotropic radiator. If you see something like 2.15 dBi, you’ll know that you’re being conned.
Ham radio operators measure gain in dBd. This measurement compares an antenna to a 75Ω half-wave dipole, or a 50Ω 1/4 wave ground plane. These antennas deliver a gain of exactly 0 (zero) dB. The approximate length of a 1/4 wave antenna, measured in centimeters, is easy to calculate: divide the wavelength by 4. So, an antenna for 2 meters would be about 50cm in length.
If you look at the stock antennas which come with dual-band HT’s, you will notice that none of them are 50 centimeters long. They’re much shorter! Yet they are a 1/4 wave or 1/2 wave in length, depending on the frequency. This is done by coiling up the necessary length of wire around a non-conducting core. This has two advantages: the antenna can be shorter and sturdier. Unfortunately there’s no such thing as a free lunch. By coiling up the antenna wire, the efficiency of the radiating element decreases considerably.
The real gain of a good dual-band helical antenna
So we learned that gain should be measured in dBd, not dBi. If we do that, the average dual-band helical antenna delivers a gain of minus 5 – 6 dBd on 2 meters, and something in the range of minus 3 dBd on 70 centimeters. 6dB or 3dB doesn’t sound like much, but don’t be fooled. -3dB will set you back by a factor of 2, and -6dB by a factor or 4. A Baofeng UV-5R is capable of delivering 4 Watts on 2 meters. but the antenna will eat up most of that power, radiating only 1 Watt into the air. Bummer, huh?
Single-band antennas always outperform dual- or triple band antennas.
More bands = more compromises. Sometimes it’s easy to make an antenna perfectly suitable for 2 bands, such as 4 meters and 2 meters. In this example the 1/4 wave for the 4 meter band will be a near-perfect 1/2 wave for 2 meters. It becomes trickier when you combine 2 meters and 70cm. In most cases antenna designers have to revert to ‘traps‘ in order to keep the SWR on both bands within reasonable limits. This system works well, but will decrease the efficiency of the antenna even further.