HF Discone Antenna
10 MHz and Up
Reading the ARRL Antenna Book (17th Edition) I came across plans for a discone antenna that was usable for the ham bands from 40 meters through 10 meters. Seeing this, I decided it was time that I do a little experimenting and build one for myself.
Unfortunately, being an apartment dweller, I have no room for such a monster antenna (I'm lucky to have the antennas that I do have). Not to worry. My grandmother has a farm not to far from where I live and it will be no problem to erect a large structure made of wires out in one of the fields...once the corn is out!
So far, I have started the initial planning of the antenna. Using the formulas in the ARRL Antenna Book I found that I would need approximately 36 feet of mast to hold the antenna up. I was planning to use the mast from a military surplus antenna that I bought, but it is only 30 feet tall, so I recalculated and discovered that I would be able to build an antenna which 30 meters is it's lowest band.
Now for more fun stuff, I had to dig back into the deep dark depths of my brain and my HP48 calculator and work out some basic triangles to calculate the dimensions of the antenna on paper. Having done that, I was able to put the data into EZNEC and do some initial modeling and a little bit of experimenation before actually building the antenna. This also allows me to make comparisons between computerized antenna modeling and the real world.
I've never worked with any kind of antenna modeling program before, so up to this point it's been a steep learning curve (just how I like it). I plugged the wires into EZNEC - eight downward sloping wires and eight horizontal wires (like spokes on a wheel) and one wire connecting the two major pieces together which becomes the feedpoint.
The ends of all of the horizontal "spreaders" and the downward sloping wires are conneced together by skirt wires to form two complete loops. This further approximates the "disc" and "cone" that the discone is supposed to be made of.
Figure 1
Figure 1 shows what the antenna looks like when viewed in EZNEC. It is modeled so that the bottom is 2 feet off the ground.
This particular antenna description, while nifty to look at, is not very practical as each wire consists of only one segment and there are 33 wires. The EZNEC demo is limited to 20 segments. I was forced to then switch to NEC-Win Plus+ which handles up to 100 segments.
It took an hour or so to get used to NEC-Win Plus+, but now that I got the hang of it I think I like it better than EZNEC. I retyped all of the coordinates for the wires and used the Auto Segment feature to segment the wires properly. NEC-Win Plus+ produces some nifty Graphs and charts that are bound to please anyone interested in antenna design.
One of those wonderful charts shows VSWR over a range of frequencies. This is of course extremely useful in determining the broadbandedness (is that a word?) of the antenna. The results of this one are quite interesting, but are best described by taking a look at Figure 2.
Figure 2
In Figure 2 we see that the VSWR starts out quite high at the lower frequencies, but then rapidly falls off to a desirable level. The spike at about 13 MHz is also interesting and I may investigate that later as time permits. The nice part about this is that the antenna appears to be close to a good (but not perfect) match from at least 20 through 10 meters, especially if the transmitter power is turned down to about 75% or less.
Figure 3
I was actually surprised by the radiation pattern (Figure 3). I read some years ago that a discone is a great antenna for listening to sattelites at VHF and UHF because of its broadbandedness (there's that word again) and its good spherical pattern. While this may be true at VHF and above, at HF (and two feet off the ground) it's far from that. Nice lobes off to the sides should be excellent for DX. Not much going straight up into the heavens.
So far this has been simply an exercise in modeling techniques, using information that has already been discovered - there's no reason to research what has already been researched. I found it disturbing that there is little information on the web about HF discone antennas. A search for HF Discone on Google Groups returns only a handful of results.
When I find more information as well as get to the construction phase of the project, I will update this publication as necessary.
