Based on my own review and research in the technical literature, S.A Schelkunoff wrote the original paper on broadbanded antennas (Theory of Antennas of Arbitrary size and Shape." IRE Proc. 1941) In 1948 C.T. Tai wrote a similar article entitled: On The Theory of Biconical Antennas. One of the problems with their writings was that it was centered around a theortical model with a feedpoint Impedance (Z) of 3 ohms or so. For these fellows Z matching and maximum transfer of power (resonance) was of little concern. However, if you read further into their writings and others in this field, you will find various graphs and formulas that will lead you to the correct angle to produce the optimum Z for 50 ohm coax. The reader should note that this notion of 50 ohms as with the HF conical monopole does not mean exactly 50 ohms. However, the Z excursions are sufficiently close enough (with some exceptions) to keep the antenna from blowing up the radio with high VSWR (Voltage Standing Wave Ratio). That is, the amount of power delivered from the radio to the antenna, to how much of that power is reflected back down away from the antenna.
The biconical and the discone are from the same family of antennas. The theory is very similar. In my opinion the biconical provides true vertical polarization when compared to the discone. For an air to ground communications system, we always recomend the biconical, however the discone will work with no problems. It is the customers choice.
If you are interested in the VSWR statistics please send me an email. I will be happy to send you the VSWR characteristics of the Biconical on an EXCEL Spread Sheet. Please note, they really load from about (depending on how you design them) 120 to 300 MHz, with high VSWR spots dispersed in between. We have built a smaller version that starts at 300MHz, and goes up from there. I don't like the notion as seen in some advertisements that they will load from 30 MHz to 1000 MHz. The EXCEL Sprad Sheet provides a one MHz step interval VSWR analysis of the antenna, warts and all.
The reader should note, that by changing the angle of the spreaders the VSWR curve changes. For general purposes we have found that the 45 degree angle seems to statistically produce the closest approximation to 50 ohms. However the 45 degree version has it's high VSWR spots. If your frequency requirements are inside the highspots, we are able to change the angle and also the length of the radials, thus shifting the VSWR curve around a tad.
Two bottom biconicals were designed in INVENTOR software. I like them better than the photographs.
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