Some basic antenna info

 Antenna thoughts August 2025

If one knows the basics of antennas then it becomes easy to choose what antenna is best for you. There is no one “best” antenna. There are very good antennas of course but even a very good antenna is not going to be right for all uses. I will discuss some antenna basics here and give some “rule of thumb” guidelines on how to think about antennas. 

The halfwave dipole can be considered the basic building block of antennas. I think understanding the dipole is the key to understanding antennas. Also for antennas we need to think in terms of wavelength, not feet or meters. 

For example a dipole 1/2 wavelength long and 1/2 wavelength high above ground is an excellent antenna. It has a gain of about 8dBi off the broadsides. It has a slight reduction in signal strength off the two ends. It’s peak radiation ( off the sides) is centered at about 30 degrees. The actual radiation is rather broad from about 15 to 45 degrees and gradually drops off above and below those angles. That same antenna if only 1/4 wavelength above the ground has stronger radiation at high angles which is good for shorter distances at frequencies where the ionosphere is dense enough to return high angle signals to earth. That same halfwave dipole is installed 1 full wave above the ground will have very little radiation straight up, and much more radiation at lower angles favoring long distance communications.

For horizontal antennas it is the height above the ground that determines the peak maximum radiation angles and nulls.

The straight halfwave dipole is considered a full size antenna. Any straight wire less than 1/2 wavelength long will not radiate as strong a signal as the 1/2 wavelength dipole all other factors being equal. Radiation from a wire close to a half wavelength long will not be noticeably weaker but as the wire becomes shorter and shorter the radiation will definitely become noticeable. While the directive pattern of the half-wave dipole is broadside to the wire, if that wire were increased to 1 wavelength long, (either by making the wire twice as long or by increasing the frequency by a factor of two) the pattern will change and instead of being directly broadside with one major lobe off each side, it will have two major lobes off each side with a shallow null directly broadside. This is due to the phasing of the current being significantly different is the wire.

We say the dipole when installed at 1/2 wave above ground has a gain of about 8 dBi. That is really pretty good. You have to double the size of almost any antenna to get an additional 3 dB of gain. That means going from a single dipole element to an antenna consisting of two such elements would give noticeable gain. To get the same increase again you need to double the antenna by going to 4 such elements. It’s not as simple as simply doubling the number of elements but the spacing between the elements also needs to be doubled. For example while my two element Yagi needed only 5 foot spacing between dipole elements, my 4 element Yagi needed more than a 20 foot overall length! Again these are just ballpark figures as there is a small amount of leeway in design depending on if you want maximum gain or maximum front to back ratio. 

Considering the half-wave dipole, it does not matter if that half wavelength of wire is fed power at the center, off center, or at either end. The result is essentially the same. There is some slight change in pattern shape but it is of no practical consequence. What is usually important is matching the feedpoint impedance to the feedline impedance.

Another good rule of thumb is that a good vertical antenna has approximately zero dBi of gain over real ground, while the halfwave dipole starts with 6 to 8 dBi of gain at normal heights. With the vertical the quality of the ground nearby is very significant and there is nothing that can change that other than relocating the vertical to a better part of the world! A vertical dipole over seawater has essentially the same gain as the horizontal dipole, however both the horizontal and vertical radiation patterns are different. The vertical will have a high angle null in the vertical pattern and be omnidirectional in the horizontal (azimuth) pattern. Vertical elements can be formed into directive arrays much the same as horizontal antennas. The same rule of thumb applies, double the number of elements and spacing and you get 3 dB improvement in gain.