How efficient is your antenna practically?

 Efficiency of the antenna is the ratio of power radiated by the antenna to the power supplied to the antenna.

Antennas can be very simple dipole antennas or very complex designs like stacked Yagi-Uda antennas or Cross Yagis for LEO Satellite operations. Either way, their basic function is to convert electrical signals to electromagnetic waves and vice versa, depending on whether it is used for transmission, reception or both.

Important parameters of an antenna are the gain, directivity, radiation pattern including the angle of radiation and polarization, beamwidth and effective aperture. All these will determine the efficiency of the antenna for a given application. Gain of an antenna is usually mentioned in comparison with the gain of a theoretical isotropic antenna (dBi) with spherical radiation pattern all around or sometimes in comparison with a dipole antenna (dBd). Gain of the antenna will be equal to the product of efficiency and directivity of the antenna.

Directivity of the antenna is usually mentioned in terms of its front to back ratio. In otherways, how efficient the antenna is in directing the signals in one direction, while cutting of radiation in other directions. High directivity will usually be associated with high gain and low beamwidth. Both can be useful in receiving signals and transmitting signals in one particular direction. But will be a disadvantage when you want to work stations from multiple directions. Then an antenna rotator will be needed to point the antenna in the direction of the station. For a point to point contact that will be highly efficient. But if you are conducting a net this can be a great disadvantage. Same is the case of repeaters. In both instances, we will go for an omnidirectional antenna. Omnidirectional antenna gain can be increased by having more vertically stacked elements with appropriate phasing coils in between.

Dipole antennas have dumb bell shaped pattern of radiation to both sides. Radiation is much less along the ends of dipole antennas. Hence dipole also has some directivity along its broad side, though it is bidirectional rather than unidirectional. The bidirectional radiation pattern gives the dipole antenna a gain over the theoretical isotropic antenna of 2.15 dBi. That is why, when gain is mentioned in comparison with a dipole as dBd, it will be 2.15 lower than dBi. Highly directional antennas like Yagi-Uda antennas have much more gain along the direction from reflector to director elements.

Polarization of an antenna represents the orientation of its electric field. Polarization of the antenna is vertical for vertical antennas and horizontal for horizontal antennas. These can be called as linear polarization.

For working LEO satellites, antennas with circular polarization is better than those with horizontal or vertical polarization. Cross Yagi is a typical antenna with circular polarization. Another antenna with circular polarization is the Turnstile antenna. During construction of these antennas, if the design parameters are not followed exactly, you may end up with an antenna having elliptical polarization instead of circular polarization, due to inequality of the pattern in both axes!

Gain of an antenna determines how efficiently the antenna radiates electrical power delivered to it. On the receiving side, it determines how efficiently it converts the electromagnetic energy received by it to electrical signals for the receiver input. Gain of the antenna is determined not only by its physical structure, but also by the surrounding environment. Antenna size, elements and the dielectric constant of the surrounding medium influence the antenna gain. Antenna height is also a very important parameter while considering the efficiency. A minimum height of more than a quarter wavelength of the operating frequency is highly desirable, though often not practical for lower bands. Lower height in relation to wavelengths tends to direct electromagnetic waves towards the sky and is often considered a wasted energy, except in case of NVIS or Near Vertical Incidence Sky Wave propagation meant for medium distance contacts. Sometimes such antennas are called 'cloudburners' in a lighter mood.

Simple way of finding out how efficient your antenna is to try it out on the air or like 'proof of the pudding is in the eating'. But that may not always give the intended results as it depends on band conditions and the number of operators available at that particular time to reply to your calls. You have to have a basic knowledge about which band is good at the particular time of the day and year as well as the solar cycle. In general, higher bands perform better during day time and lower bands better at night. Higher bands perform better when the solar activity is more and lower bands perform better when the solar activity is lower. That is due to ionospheric reflection properties depending on the level of ionization.

There are other ways to find out how your antenna works now. The Reverse Beacon Network or RBN can pick up your CQ calls if sent in a machine like precision. From the RBN stations which are picking up your signals, you can get an idea how well your signal is propagating at the given time. Of course, it will depend on the band conditions at the moment. Another option is to listen to multiple webSDRs while calling CQ on the band.

In radios with recording facility like FT-710, you could also transmit a recorded CQ call while listening simultaneously on different webSDRs. WebSDRs are software defined radios connected to the internet, which can be tuned to different frequencies by different users simultaneously. Different types of webSDRs with varying coverage of amateur radio bands are available online.

Antenna gain can be calculated using antenna modelling software like EZNEC if the construction matches the model exactly. But it is not as easy as parameters which have to be included in the model will including all nearby conducting structures and other antennas in the vicinity of the test antenna. If these are not included properly in the model, the gain, directivity and efficiency calculated using the modelling software will not match what you actually get in the real world. For real measurement, you need large amount of space for HF antennas and anechoic regions which will prevent echoing of the signals from nearby structures. That is not practical at the level of most amateur radio operators.