The Omni Flyswatter was recently discussed on the UK microwave group email reflector.

John G3SEQ has written this up but seems to have made fundamental errors in understanding the gain of the omni flyswatter antenna. While this sort of antenna may have some practical use as a serach or beacon antenna, the omni flyswatter cannot possibly have the same gain as a similar dish correctly alinged. The difference is likley to be about 20dB, which in many cases is not enough to prevent contacts being made over reasonable distances, but will mean no contacts with distant stations under marginal conditions.

As John has updated his site following the open discussion, but still claims the same gain for the omni flyswatter, by referring to the rotation of the beam through 360 degrees of azimuth in one cycle, which demonstrates a poor understanding of the fundamentals of antennas. I have provided this information to help ensure that the errors that the rest of the microwave community perceive are highlighted. You may draw you own conclusions. I have to say, as John has been an educator and is an amateur radio instructor, I find this rather perplexing and disturbing.

In essence the antenna is based on a standard dish antenna with circular polarisation pointing straight up at a conical reflector surface that is large enough to be capture and reflect most of the energy in the beam of the dish. With a cone angle of 90 degrees, the conical omni flyswatter projects a beam 360 deg in azimuth with a vertical beamwidth equating to the bearmwidth of the illuminating source. The polarisation will have a horizontal component on all bearings because of the circular polarised launching antenna.

The omni flyswatter antenna cannot have the same gain (less the 3db losses associated with circular to linear polarised systems) as the illuminating source as claimed by John G3SEQ

The energy in a standard dish's main lobe is concentrated in a small notional area (projected on a distant sphere), while the omni flyswatter spreads this same energy into a disc with a significantly larger area on the same sphere. The power density is then much lower, and the gain associated with the omni flyswatter is considerably below that of the illuminating dish .. about 20dB less if the illuminating dish has a beamwidth of about 3.6 deg. The gain of such a system may compare favorable with that of a slot antenna, and bearing in min the difficulty of machining small slots at 24GHz and above, and the high losses in any feeders the omni flyswatter may prove to be a practical beacon antenna.

As a quick experiment, (similar to one actually suggested by John G3SEQ), shine a laser pointer at a highly polished conical object (I used a lathe dead centre). This is an optical omni flyswatter. The resultant arc of light is about the same width as the original beam but a fraction of the intensity at the same distance from the source when compared to the direct beam from the laser. This applies even if the laser beam is deliberatly poorly columnated (very like the beam from a high gain dish)

QED

As a breif note to follow up this, there are possible reasons for the observations that John has made leading him to the conclusion that the gain of the omni flyswatter is the same as his dish alone.

If his normal dish is incorrectly aligned in the vertical plane, even by a few degrees, he will not benefit from the full gain, and in fact may be peaking up signals on sidelobes. This is horribly easy to do and it is hard to tell that you are not on the main lobe in some cases. (A clue is getting 2 or even 3 peaks on the signal, or sometimes one wide peak). In most cases the signal is tens of dB lower than boresight , but given the nature of microwave propagation, often there is enough in hand that this is good enough to be usable.(Sidelobes are often in the range -15 to -20 dB on the main lobe)

Back to Technical Info Page

 

Free Web Hosting