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1994-04-02
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DISCONE v1.0
Mike Davis, AB6SL
03-29-94
DISCONE v1.0 is a quick and dirty but very accurate utility
program that calculates every dimension necessary to construct
a discone antenna. Discones are famous for their tremendous
bandwidth (SWR of 1.2:1 from 14 to 144 Mhz, for example.) They
are also infamous for being large, unfortunately. They have
the low angle of radiation offered by a vertical from the minimum
frequency of operation through about 4 times that frequency, after
which the angle of radiation increases out to the maximum usable
frequency (10 times the minimum.) They have the gain of a dipole
without the nulls of a dipole. The current distribution is
greatest at the top of the antenna, unlike a vertical, so they
can be operated much closer to the ground, with almost no ground
losses! They mate to 50-ohm coax with no feedpoint matching.
If your 80-Meter dipole is suffering ground losses because it's
not 3/4 wavelength high (263-feet !) build a 71-foot tall discone
and enjoy a 360 degree azimuth plot with a low angle of radiation.
Work every freqency from 3.5MHz to 35MHz with 1.2:1 SWR and little
ground loss!
71 feet too tall? OK, how about 35 feet for 40-Meters through 6?
Using wire radials to make the skeleton, these antennas are cheap!
Given the desired minimum frequency of operation and optionally,
a user-specified critical dimension, Cmin, this program will
calculate all the dimensions necessary to construct a discone
antenna. The "ARRL Antenna Book" 16th Ed., Chapter 7, pp. 17-20
is required reading! All formulae were derived from this source.
Construction methods and graphic figures provided there,
especially figure 33 on p. 7-18 make this program's output of
value instead of just confusing.
Antennae constructed using this program will exhibit the optimum
60 degree discone parameters: S=0.3Cmin, D=0.7Cmax, and Ls/Cmin>22.
Bandwidth with a 1.2:1 SWR will be 10:1 !
Try running the program with the same design criteria used for
the 7 to 70 MHz discone documented in the article. With an Fmin
of 7.0 MHz, a Cmin of 13.5 inches, 8 disc radials and 24 cone
radials, this program spits out the same dimensions derived the
hard way in the article. It's accurate and you can trust it.
This program assumes a skeleton discone will be constructed but,
works just as well for solid surface discones... just ignore all
input and output regarding "radials." All relevant dimensions
will be correct. It is inadvisable to reduce the number of
disc radials to less than 4 or cone radials to less than 8.
The ARRL article indicates a 0.02 wavelength at Fcutoff as a guide
for spacing the radials at the base of a cone. No matter what
frequency you design for, this works out to 40 cone radials!
The article overlooks a contradicting example they give of an HF
discone with only 24 radials in the cone (8 in the disc)!
Extensive testing was done by WA4RHT, D. Wilson Cooke, with
skeleton discones, as published in the "ARRL Antenna Compendium,
Volume 3" pp. 140 to 143. He concluded that more spokes are better
but, 8 in the cone and 4 in the disc is as few as you can have
without having to increase the Ls (Slant Height of the cone) to
1/3 wavelength at Fcutoff instead of 1/4 wavelength.
Most commercially manufactured VHF/UHF discones have no more than
8 cone radials and 4 disc radials, but more cone radials will yield
a flatter SWR curve accross the 10:1 bandwidth of the discone.
To use DISCONE.EXE, just execute it at a DOS prompt. It accepts no
arguments. You'll be asked to supply the Fmin (Minimum Frequency
at which you desire to operate with an SWR 1.2:1.)
The SWR rises steeply below this frequency. For the first run, I
would suggest you provide no value for Cmin, the diameter of the
cone at it's narrowest point beneath the disc. This is where the
cone radials are attached. READ THE ARTICLES MENTIONED ABOVE!
By taking the default Cmin the first time you run DISCONE for a
given Fmin, DISCONE will calculate the largest allowable diameter
for Cmin that fits within an important performance optimization:
Ls / Cmin > 22. READ THE ARTICLES! After determining the largest
allowable value of Cmin, you can run subsequent calculations with
manually entered lesser values for Cmin that better suit your
construction needs. Remember, smaller Cmin means better bandwidth!
Changing the number of radials from the defaults of 4 disc and 8
cone radials will only change some of the reported values that are
provided for ease of construction. Calculated values of Fcutoff,
the 4:1 and 10:1 multipliers of Fmin, etc. are not affected by
changing the number of radials from the defaults. Remember
that you are venturing into inaccurate territory if you specify
fewer than the default. The formulae used here assume the defaults
to be the minimum number of radials. No effort is made to calculate
an increased Ls (Slant Height) to compensate for fewer than 8 cone
radials for example.
If a printer is attached, do screen prints to capture results.
Crude, huh?
This program is freeware! Use it, copy it, distribute it with a
clear conscience. It's accurate and the price is right.
Enjoy,
Mike Davis, AB6SL