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70-210MHz inverted skeletal discone

Introduction
A wideband aerial like the discone allows continous coverage of a 3:1 frequency bandwidth. Although the discone is a groud-plane aerial and therefore has a higher angle of radiation (Brewster angle) than its balanced cousin, the biconical, it has the convenience of not needing a balun. For VHF and below, the discone is usually formed from wires/rods instead of solid metal in order to reduce weight and wind load; i.e. a skeletal discone.

While skeletal discones made by Radio Shack or Diamond (D-130J) can be easily purchased in advanced countries, such aerials are either not readily available in other parts of the world or their import is hampered by red-tape (when this aerial was constructed in 1999, eBay did not exist). Additionally the commercial offerings exist only in the upright format; i.e. disc above cone. But, the upright discone has a negative Brewster's angle in free space [1] - meaning that it will radiate mostly below the horizon - which detracts from its usefulness in accessing hill-top ham repeaters. Turning the discone upside down -  to create the inverted discone - can beneficially reverse the down-tilted beam (fig. 1) [2].

Fig. 1 Elevation pattern of an inverted discone at 500MHz [2]

To realize a vertically polarized aerial that can cover most of the VHF range, we fabricated and tested an inverted discone made of multiple wire elements. This article describes the construction details and VSWR measurement.

Material and method

3mm dia. brass rods are the base material. The cone and disc use 9 and 8 rods, respectively. We expect that this quantity is sufficient to approximate a solid discone because a professional discone model was realized with 6 elements only [3]. The cone's rods are 1m long since that are their original length.The disc's rods are cut to ~33 cm long. The lowest operating frequency (fmin) estimated from the cone surface's quarter-wave length is ~75MHz. A small cone and disc fabricated from brass sheet form the connection points for the rods and also a BNC RF connector. Brass terminal blocks, commonly used for electrical wiring, are soldered to the sheet cone and disc. The ends of the rods are then inserted into terminal blocks and the screws tightened. A similar metal (brass) construction is emphasized to minimize corrosion. The cone, disc and connector are screwed to a 10mm thick insulating material.

The mast which the aerial is mounted on is deliberately bent to increase the separation from the wall. A ~1.2m total separation is achieved when the mast is mounted on a TV aerial bracket - corresponding to ~1/4 wavelength at fmin.

 
discone mounted on mast outside flat's window
 

The VSWR is measured by connecting a directional coupler to the aerial via a short (1m) coax. The importance of a short coax length for VSWR measurement will be elaborated on in the result section.


Results
The VSWR fluctuates from 1.5 to 2.7 in the target 70-210MHz operating range. The worst VSWR is 2.7 at 90MHz; this is comparable with the aforementioned professional discone guaranteed <3:1 VSWR. Although BW as large as 10:1 are often claimed, the upper frequency limit is reached when the cone length reaches 5/8 wavelength because the radiation angle because too large to be useful for terrestial communication [1]. VSWR ripples are observed at 90MHz and 120MHz - they are likely caused by the rods being a poor approximation of a solid discone; doubling a biconical's elements from 8 to 16 has been shown to reduce max VSWR from 4 to 2.5 [4]. An alternative cause is interaction with the small room where testing took place. The VSWR then stays below 2 from 140MHz to 250MHz.

VSWR measured at the base of the aerial

When the VSWR measuring instrument is connected through a cable with finite loss, it can make the aerial VSWR appears deceptively better. To study how feeder losses influences SWR readings, the VSWR measured at the aerial base (i.e. 0dB loss) was modelled with additional losses of 1dB and 2dB  to represent cable losses. It is apparent from the modelled results that 1-2dB of cable loss can make the VSWR appear better than reality. However, it must be noted that the real coax cables exhibit increasing loss with frequency.

VSWR modelled with different cable losses

Postscript: this discone has been operating since 1999 without needing repair.

References

[1] L. B. Cebik, W4RNL, "Notes on HF Discone Antennas," [Online] Available: http://www.cebik.com/
[2] K. Nagasawa and I. Matsuzuka, "Radiation field consideration of biconical horn antenna with different flare angles," IEEE Tran. Antenna & Propagation, Sep. 1988, pp. 1306-1310.
[3] Harris product specification, "RF-072-AT001 low-band VHF discone antenna," Sep. 2003. [Online] Available: http\\www.harris.com
[4] M. Lu and C. Shi, "A high-quality ultra-wideband omni-directional antenna," Proc. Intl. Symp. on Electromagnetic Compatibility, May 1997.


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