An Uncommonly Good Vertical
After months of anticipation and waiting, it has finally happened—I have just gotten some of the Ham shack put together at my new QTH. Perhaps it was good to wait, it gave me more time to consider which antennas to build and use. Over the next several months I will tell you more about my new antenna farm, but this month I am excited about a basic concept every Ham should be aware of, but few have put into practice.
It is common to hear remarks like “a vertical antenna radiates equally poorly in all directions.” That is a true statement for most verticals, simply because a very poor ground radial system is installed and rather than radiate well, the antenna system spends most of its energy heating the earth worms. It isn’t that most Hams don’t want to install a gooood set of radials, they just simply can’t because of limited real estate or whatever other impediment sets the limit. They probably chose the vertical because of the small amount of real estate it requires above ground, but that is only half the antenna.
My Antenna Requirement
There is nothing wrong with a good vertical – note that the word good is underlined. In my search for a good antenna at the new QTH, I first had to answer the question – what do I want to use the antenna for? Since I moved from Florida to Georgia, and prior to that I was in Texas, most of my desired QSOs will be in the range of 400 to 1200 miles. However, as we all do, I want a good antenna to chat with foreign stations, even though I don’t go out of my way to chase DX. Also, I don’t want the hassle of a big beam and the associated tower, etc. And, I want a design that can be replicated for various bands. The bottom line is—I need an antenna that is vertically polarized (for long haul) and non-directional for rag chewing, but must be efficient, since I prefer to run 100 watts and let the big rig rest unless it is really needed.
How to Build a High-Efficiency Vertical
Now, that is settled. I will use a vertical. So, what do I do to get the efficiency to be high? Obviously, it is necessary to eliminate the ground loss. That means the use of elevated radials or no radials at all. I chose the latter. How do you build a vertical without radials? Use a half wavelength vertical. A “conventional” vertical is a half wavelength antenna with half of it buried. Actually, the radials create the effect of an image antenna, making it appear that half the antenna is buried and half is above ground. With poor radials you get a poor image.
The first band I want an antenna for is 40 Meters, so that means the antenna must be = 468/7.2 = 65 feet (19.8m) for operation at 7.2 MHz. Since Georgia is noted for it’s pine trees, and there is a row of them that pass within 15 feet (4.6m) of the Ham shack and they are approximately 85 feet (25.9m) tall, the support structure is already there (lucky me).
Antenna Efficiency
After giving birth to a new antenna, it must be properly fed. A half-wave vertical could be fed in the center, the same as a horizontal 1/2 wave dipole (it is nothing more than a dipole mounted vertically). That would not be convenient, so I chose to feed it at the bottom end. That says it must be fed against ground and now we must have a good ground—wrong! A half-wave antenna has a low impedance at the high current point in the center of the antenna and high impedance at the low current points at the ends of the antenna. Based on an analysis using EZNEC, this antenna will have an input impedance of about 3300 ohms. A single ground rod will have an impedance of less that 800 ohms. Therefore, if I used a single ground rod, the antenna efficiency would be (output/input) = 3300/(800 + 3300) = 85%. Just a few short buried wires will reduce ground loss to less than 100 ohms with a resulting efficiency of 3300/(100+3300) = 97%. Just for fun – the efficiency of a “conventional” vertical is based on an antenna impedance of about 32 ohms and a ground loss of about 100 ohms, or 32/(100+32) = about 24%. That says my 100 watt transmitter will radiate 97 watts on one antenna and only 24 watts on the other, or a power ratio of 4 to 1.
Antenna Gain and Loss
There is another significant contributing factor to radiation, and that is the antenna pattern. If you look at the graph in Figure 1, you can see the antenna patterns for four antennas as follows:
1/2 Wave Horizontal Dipole 1/4 Wave Above the Ground
A great antenna for local QSOs out to about 20 degrees which is equivalent to about 800 miles, then the gain falls off rapidly. Thus, it is not a good DX antenna – and you are looking at the broadside which is the best pattern. If you looked at the end of the dipole it would be down much more. This antenna does not meet my design criteria.
1/2 Wave Vertical
I had thought this would be my desired antenna – until I decided to slope it to get from the house to the tree.
Sloping 1/2 Wave Vertical
A great antenna at low angles, virtually non-directional in the azimuth plane (within 3 dB), and is a good performer at short range for local QSOs. This was a pleasant surprise. Note that at low angles, it is far superior to the horizontal Dipole and almost as good as the 1/2 wave vertical. To lose a little low angle performance due to the slope is a good tradeoff to gain high angle performance. Now I can change my initial requirements and include good close in performance. In fact, I can now say I have a very good antenna for use at all ranges.
Typical 1/4 Wave Vertical
lives up to it’s expectations, i.e., is equally poor in all directions compared to the other antennas.
Delta Loop vs. 1/2 Wave Vertical
I did not include it here, but I ran an analysis on a delta loop that some friends were using in DX contests and bragging about the results. The 1/2 wave vertical is much better.
When reviewing the chart in Figure 1, it should be noted that 6 dB is the equivalent of 1 S-Unit on your receiver. Now look at the chart again and note that there isn’t all that much difference between antennas.
The J Pole
The performance of a J Pole antenna is legendary. I have used one on the mobile for 2 Meters for years with excellent results. It’s advantage is the placement of the high current point in the antenna high above ground, which is exactly what the 1/2 wave vertical does. Remember, it is the current flowing in the antenna that creates the magnetic field that results in radiation. High currents high above ground lead to less ground loss. The J Pole is really a 1/4 wave section of transmission line used to end feed a 1/2 wave vertical. The transmission line can be replaced with a parallel resonant circuit. See Figure 2.
A simple alternative to feeding this antenna is the use of a antenna tuner that includes the capability to feed a high impedance unbalanced antenna. Just bring in a good ground and the end of the antenna wire to the tuner and you are in business. Keep the ground wire short to minimize RF in the shack. Also, the feed through for the wall in the Ham Shack should be considered. For an antenna with a feed point impedance of 3300 ohms and 100 watts, the voltage will be the square root of the product of power and impedance = (100 x 3300) 1/2 = about 574 volts RMS = 574*2.8 = 1617 volts peak-to-peak. The center of a piece of coax with the braid removed but the insulation left on does just fine for a feed through insulator.
If you don’t have a convenient pine tree, note that a 65 foot (19.8m) length of wire sloped at 45 degrees requires a tie point only 46 feet (14m) above ground. If you prefer using this design on other bands, note that above 40 meters, a signal radiated at high angles will penetrate the ionosphere rather than being reflected back. Therefore, make the vertical antenna as near vertical as possible. The support mast should not be metal unless it is aluminum due to the significant loss the antenna will have, which simply means you will heat the mast rather than the earth worms—and still have a poor antenna. -30- and hope I can hear you on the bands. Ted – W5QJR.
Originally posted on the AntennaX Online Magazine by Ted Hart, W5QJR
Last Updated : 17th March 2024