From: rdewan@casbah.acns.nwu.edu (Rajiv Dewan)
Newsgroups: rec.radio.amateur.misc
Subject: Re: Discone Antenna
Date: 10 Nov 92 15:41:32 GMT
Article-I.D.: news.1992Nov10.154132.20941
Organization: Northwestern University, Evanston Illinois.

In article <1689A9713.D1SAR@VM1.CC.UAKRON.EDU> D1SAR@VM1.CC.UAKRON.EDU (Steve Rimar) writes:
>A while back there were some articals on building the Radio Shack antenna.
>I have a few questions on the construction of the antenna.  What was the
>the size of the screen needed an the cone size when completed? How is the
>2ft disk attached to the screen?(I think that was the size of the disk)
>Where is the best place to attach the coax to the antenna?
> 

This is a copy of an article sent to my local club's newsletter.
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                  The Discone Antenna

                  Rajiv Dewan  AA9CH



                        A
                    __________
                        |
                     B/|F|\C
                     / | | \
                    /   |   \
                   /  so239  \
                  /   feed    \
                 /             \ 
                /               \
              D/                 \E
             
 

                   Figure 1. 



How would you like to have a single antenna that covers all the amateur bands
between 140MHz and 1300 MHz and that is cheap (less than $5) and easy to
build? Well, the discone meets all of these requirements and more. Its wide
bandwidth makes it the antenna of choice for scanners.

The discone antenna consists of a cone with a disc above it. One terminal of
the feed line is connected to the disc and the other to the cone. The
arrangement is shown in Figure 1.  Many designs, including the one in this
article and the one in the ARRL Antenna book, are based on work done by J. J.
Nail for the Department of Navy.  [J. J. Nail, ``Designing Discone Antennas'',
Electronics, Aug. 1953.]  Nail's paper has a plot of SWR of a discone with a
400MHz design frequency with different cone flare angles. As is evident from
these, the 60 degree discone antenna has an acceptable SWR from about 1.05 of
the design frequency to about 10 times the design frequency. The discone
antenna behaves like a high pass filter with the design frequency as the cut
off frequency.

Let us first derive the basic geometry of the discone antenna for 2m ham band
and up. Consider Figure 1 above. The truncated cone is shown by BCDE.  
With a 60 degree flare angle, ADE forms an equilateral triangle. 
The connector for the coax is placed at BC and hence it is a little
larger than the OD of the connector: about 0.6 in for a SO239. The triangle
ABC is also equilateral and hence AC is 0.6 in long. The length AF can be
derived easily as Sqrt(0.6^2-0.3^2) = 0.52 in. The ARRL Antenna Book
recommends 0.6 in. The side CE has a length of a quarter wavelength in air of
the design frequency. For 135 MHz, this is about 22 in. Hence AE and DE are
22.6 inches each. Nail's experimentation showed that a disc with diameter of
0.7 of cone base diameter results in good performance. This works out to 15.4
inches. The ARRL Antenna book recommends 15 inches.

Some notes:

1) For feed point details consult the ARRL antenna book.  Essentially,
the coax is best placed in the center of the cone.  The flange connects
to the cone whereas the center conductor connects to the disc.

2) While the antenna impedance (and the SWR) depends generally on the 
cone flare angle, small distortions of the geometry near the feedpoint
cause large swings in impedance.  So keeping the gap between disc and cone
is critical.

3) For indoor applications, the truncated cone may be easily made using
galvanized hardware cloth.  Plans for this may be found in the ARRL
antenna book.  It is important to note that the RF currents in the cone 
traverse up and down along the length of the cone.  Hence if the cone is
made from hardware cloth, the cloth must be of the kind that is galvanized
*after* weaving.  The direction of the currents also explain why the ICOM,
Diamond and Radio Shack designs build the cone out of rods.

4) The RF currents in the disc are radial and so radial rods may be 
substituted.  

5) The iso-potential lines on the cone are circular and parallel to the
base.  So the antenna will not be affected much if it is placed on a mettalic
base.

6) This geometry can be scaled for other bands, including HF.   The ARRL
Antenna book has plans for an HF discone.

-------------------------------------------------------------------

It is a great antenna and fun to use.  I have built mine using galvanized
hardware cloth and place it in the attic.  Snow on the roof does not seem
to affect it much.

73

Rajiv                                 
aa9ch    
Address: r-dewan@nwu.edu
Phone:   None. Only CW.
And occasionally AMTOR.