Marconi Antennas for 160 Meters
I was recently reminded that vertical antennas are also known as Marconi antennas. It's cool to think of the important role that these antennas played in the development of wireless communication. They remain very popular for Amateur Radio use, especially for the 40-160 meter bands. For these bands, most amateurs would have difficulty putting up a horizontal antenna high enough for effective long distance propagation. For example, a 160M dipole would need to be about 260' (one half wavelength) above ground, whereas a vertical antenna can be ground mounted.
I have enjoyed re-reading "All About Vertical Antennas" by Bill Orr, W6SAI and Stu Cowan, W2LX. Another good book is "The Short Vertical Antenna and Ground Radial" by Jerry Sevick, W2FMI. From these, you can learn most of what you need to know to make an effective vertical antenna system. The Low Band DX'ing book by ON4UN is another valuable resource, particularly for Top Band (160M).
I have a low inverted L antenna for 160M. It does not do a very good job beyond a couple hundred miles. I can receive DX stations in FT8, but they don't hear me. For this band, the best way to work DX is with a Marconi. But none of us can put up a 130' high tower, so we are forced to use shorter versions. Short antennas have a very low radiation resistance, which exacerbates the problem of ground losses. But there are things you can do to mitigate them. One is to use a ground screen in addition to a radial system. Short verticals have a high concentration of return currents near the base, so if you can reduce the losses in that region it will improve antenna efficiency somewhat. For an effective ground system, you need fairly long radials for 160 meters. Ideally they should be at least 1/4 wavelength or 40 meters long, which is difficult for most of us. If you can't make them that long, you can compensate to some degree by using a lot of them (for example, 60 or more) and making them as long as possible.
An interesting thing about 160M is that we get a bit of a break on ground conductivity compared to the higher bands. Ground rods are fairly useless for RF at higher frequencies, but they do apparently help somewhat on 160. Also interesting; multiple ground rods can help, but they need to be spaced apart. I have usually not bothered with multiple ground rods, believing them to be useless for improving ground system efficiency, and this is true at higher frequencies. The inductance in the connecting wires makes them less effective. But those wires are a shorter fraction of a wavelength on 160. Some references suggest using 3 or 4 ground rods spaced at least 6' apart.
Another thing that can be done to improve your ground system for 160M is to tie into your cold water pipe and any large metal objects in your yard, such as a steel fence. Steel is a poor conductor, but it is a thousand times better than earth. That's why a chicken wire ground screen makes a big improvement even though it's a relatively poor conductor compared to copper. One thing I did not know that I learned from the Orr & Cowan book is that most water systems using iron pipe have poor conductivity because the cement used between pipe connections is an insulator. If you have copper pipe, you are in business. If you have PVC pipe, it is useless. The water in the pipe does not significantly improve conductivity for any of them. If you could jumper around some of the interconnects that might help, but I think that would be a monumental task.
As far as the antenna is concerned, top loading is by far the most efficient type of loading. For most of us it will be difficult to get enough loading capacitance to achieve resonance on 160M for the length of vertical conductor and size of top hat that we are able to use, so it will be necessary to augment that with inductive loading. The most convenient inductive load is a coil at the base of the antenna, but it is also by far the most inefficient. According to Jerry Sevick, placing the loading coil in the center or upper part of the antenna works reasonably well, especially when combined with capacitive top loading (i.e., a top hat).
Another technique I read about is the use of multiple monopoles. There are a number of ways to do this. The simplest is to mount two identical vertical antennas fairly close together and feed them in parallel. This has the effect of increasing the size of the radiator, which broadens the bandwidth. I have not tried it, but it sounds intriguing. Bandwidth is a problem on 160, especially if you manage to install a reasonably efficient antenna and ground system. The 2:1 VSWR bandwidth of such a system will typically be < 50 kHz.
Here is what I have come up with as a reasonable compromise antenna system for 160 (and it could also be adaptable to the 80 meter band as well):
For the ground system (which is very important for good efficiency), use two or three ground rods spaced at least 6' apart and connected with heavy wire (#6 or larger). Tie the ground system into your cold water system and also to any large metal structures you may have available, such as a steel fence. Use at least 60 radials that are as long as you can make them, which will probably be no more than 60 feet. This is really too short, but probably the best most of us can do. Add a ground screen at the base of the antenna. Make it as large as practical; 5' x 5' perhaps or 10' x 10' if you can. The ground screen helps collect return currents efficiently, which are highest near the base of an electrically short antenna (less than 1/4 wavelength).
A 1/16 wavelength antenna is probably about the best most of us can do (32 feet high). Add the largest top hat you can manage. Typically these consist of multiple spokes with the outer ends connected via a conducting ring. The capacitance is lower if you don't use the ring, but that's easier to make. Use a loading coil in the middle or upper part of the antenna to achieve resonance. The best loading coils are air-wound coils of large diameter tubing. Obviously you have to insert an insulator in the antenna tubing that is mechanically strong enough to support the upper part of the antenna. It's better if you don't wind the coil on top of it. Use a self supporting air-wound coil instead (like the Butternut vertical antennas use).
For the feed system, a remote tuner installed at the base of the antenna would facilitate frequency changes. The VSWR goes up quickly as you move away from resonance. If you don't use a tuner, you will probably at least need an L match at the antenna base (shunt coil) because the resistive component will be very low if your ground system is even moderately efficient. An alternative is a Sevick-design Unun to step down from 50 ohms to whatever the antenna impedance is (typically 10 ohms or less for an electrically short antenna). An antenna analyzer is a very useful tool for measuring and adjusting these antennas.
I have enjoyed re-reading "All About Vertical Antennas" by Bill Orr, W6SAI and Stu Cowan, W2LX. Another good book is "The Short Vertical Antenna and Ground Radial" by Jerry Sevick, W2FMI. From these, you can learn most of what you need to know to make an effective vertical antenna system. The Low Band DX'ing book by ON4UN is another valuable resource, particularly for Top Band (160M).
I have a low inverted L antenna for 160M. It does not do a very good job beyond a couple hundred miles. I can receive DX stations in FT8, but they don't hear me. For this band, the best way to work DX is with a Marconi. But none of us can put up a 130' high tower, so we are forced to use shorter versions. Short antennas have a very low radiation resistance, which exacerbates the problem of ground losses. But there are things you can do to mitigate them. One is to use a ground screen in addition to a radial system. Short verticals have a high concentration of return currents near the base, so if you can reduce the losses in that region it will improve antenna efficiency somewhat. For an effective ground system, you need fairly long radials for 160 meters. Ideally they should be at least 1/4 wavelength or 40 meters long, which is difficult for most of us. If you can't make them that long, you can compensate to some degree by using a lot of them (for example, 60 or more) and making them as long as possible.
An interesting thing about 160M is that we get a bit of a break on ground conductivity compared to the higher bands. Ground rods are fairly useless for RF at higher frequencies, but they do apparently help somewhat on 160. Also interesting; multiple ground rods can help, but they need to be spaced apart. I have usually not bothered with multiple ground rods, believing them to be useless for improving ground system efficiency, and this is true at higher frequencies. The inductance in the connecting wires makes them less effective. But those wires are a shorter fraction of a wavelength on 160. Some references suggest using 3 or 4 ground rods spaced at least 6' apart.
Another thing that can be done to improve your ground system for 160M is to tie into your cold water pipe and any large metal objects in your yard, such as a steel fence. Steel is a poor conductor, but it is a thousand times better than earth. That's why a chicken wire ground screen makes a big improvement even though it's a relatively poor conductor compared to copper. One thing I did not know that I learned from the Orr & Cowan book is that most water systems using iron pipe have poor conductivity because the cement used between pipe connections is an insulator. If you have copper pipe, you are in business. If you have PVC pipe, it is useless. The water in the pipe does not significantly improve conductivity for any of them. If you could jumper around some of the interconnects that might help, but I think that would be a monumental task.
As far as the antenna is concerned, top loading is by far the most efficient type of loading. For most of us it will be difficult to get enough loading capacitance to achieve resonance on 160M for the length of vertical conductor and size of top hat that we are able to use, so it will be necessary to augment that with inductive loading. The most convenient inductive load is a coil at the base of the antenna, but it is also by far the most inefficient. According to Jerry Sevick, placing the loading coil in the center or upper part of the antenna works reasonably well, especially when combined with capacitive top loading (i.e., a top hat).
Another technique I read about is the use of multiple monopoles. There are a number of ways to do this. The simplest is to mount two identical vertical antennas fairly close together and feed them in parallel. This has the effect of increasing the size of the radiator, which broadens the bandwidth. I have not tried it, but it sounds intriguing. Bandwidth is a problem on 160, especially if you manage to install a reasonably efficient antenna and ground system. The 2:1 VSWR bandwidth of such a system will typically be < 50 kHz.
Here is what I have come up with as a reasonable compromise antenna system for 160 (and it could also be adaptable to the 80 meter band as well):
For the ground system (which is very important for good efficiency), use two or three ground rods spaced at least 6' apart and connected with heavy wire (#6 or larger). Tie the ground system into your cold water system and also to any large metal structures you may have available, such as a steel fence. Use at least 60 radials that are as long as you can make them, which will probably be no more than 60 feet. This is really too short, but probably the best most of us can do. Add a ground screen at the base of the antenna. Make it as large as practical; 5' x 5' perhaps or 10' x 10' if you can. The ground screen helps collect return currents efficiently, which are highest near the base of an electrically short antenna (less than 1/4 wavelength).
A 1/16 wavelength antenna is probably about the best most of us can do (32 feet high). Add the largest top hat you can manage. Typically these consist of multiple spokes with the outer ends connected via a conducting ring. The capacitance is lower if you don't use the ring, but that's easier to make. Use a loading coil in the middle or upper part of the antenna to achieve resonance. The best loading coils are air-wound coils of large diameter tubing. Obviously you have to insert an insulator in the antenna tubing that is mechanically strong enough to support the upper part of the antenna. It's better if you don't wind the coil on top of it. Use a self supporting air-wound coil instead (like the Butternut vertical antennas use).
For the feed system, a remote tuner installed at the base of the antenna would facilitate frequency changes. The VSWR goes up quickly as you move away from resonance. If you don't use a tuner, you will probably at least need an L match at the antenna base (shunt coil) because the resistive component will be very low if your ground system is even moderately efficient. An alternative is a Sevick-design Unun to step down from 50 ohms to whatever the antenna impedance is (typically 10 ohms or less for an electrically short antenna). An antenna analyzer is a very useful tool for measuring and adjusting these antennas.