Unreasonable Antenna Claims

Recent claims of 10dBi gain with 18 dB front to back for a shortened coil loaded mini two element 20 meter beam were both unreasonable and unbelievable.

Many publishers are quick to divorce themselves from what they publish by claiming that the stories they print do not reflect their views yet they still print them. Okay, maybe doing so sells magazines, at least for one issue. Unfortunately it also turns off the thinking reader to the point of avoiding future issues.

A gain of 10 dBi? You can always bet on inflated gain claims when dBi is invoked. The dBi designation is a phony baloney scale based on how an antenna compares to an isotropic antenna. There is no such thing as an isotropic antenna in the real world. An isotropic radiator is a theoretical point source radiator. Even an ordinary dipole has about 2.5 dBi gain! What could possibly cause people to make comparisons to things that don’t exist? Could it be the money they seek by misrepresenting their product and defrauding potential buyers?

So far we have accounted for 2.5 dBi out of the 10 dBi claimed. Lets see if we can find the 7.5 dBi remaining.

A two element beam is most commonly composed of a driven element and a reflector spaced 1/4 wavelength behind the driven element. The reflector is energized by that half of the signal radiated by the driven element in the undesired direction. Once energized, the reflector re-radiates the signal it has intercepted. That 1/4 wave separation between driven element is important because it ensures radiation will travel 1/2 wavelength (from driven element to reflector and back to driven element) to ensure it is in phase with the signal being radiate by the driven element so it can be added to the radiation at the driven element.

Under the best of circumstances this mechanism can only double the effective radiated power for a gain of 3 dB. Now add the 2.5 dBi we get from a dipole apply some real world inefficiencies, and we arrive at a 5 dBi gain for a full sized two element beam. That is the historically accepted gain figure for this antenna.

Only 3 dB! We can do that by simply doubling the transmitted power. True but increasing transmitter power does nothing for the received signal. Three dB at the antenna doubles both transmitted and received signals.

The article in question now claims that by shortening and reducing element diameter, adding loading coils, and reducing the 1/4 wavelength separation between elements, we can double the gain to 10 dBi!!!

I believe you would have better luck sprinkling fairy dust on that isotropic radiator.