HF Marconi Antennas
As we go higher in frequency (above 160M), different tradeoffs and options become important.
80M Marconi
On this band, we could go for a full sized 1/4 wave radiator, but this is still a very tall antenna for most of us to manage (66 feet). Most of the design considerations for the 160M version will apply, with some tradeoffs.
40M Marconi
On this band there is no reason not to use a full sized 1/4 wave radiator (33 feet):
30M Marconi
This band shares some of the characteristics of the lower bands and some of the higher bands. A 1/4 wave radiator is ~23 feet tall.
20M Marconi (and above)
For these frequencies, we definitely want to elevate the antenna. It will not be very effective at ground level because signals will be blocked and absorbed by trees, bushes, and structures in the near field of the antenna.
Summing it up
No matter what band you enjoy operating, a Marconi antenna is a worthwhile addition to your station. On 160M, it's the key to DX. Depending on available trees, towers, and other antenna supports, the same is true on 80 and 40M because for effective long distance propagation, a horizontal antenna needs to be elevated at least 1/2 wavelength. On 20-10M, it's relatively easy to put up an effective horizontal antenna, but a Marconi is a neat looking antenna and doesn't require multiple high supports. Of course, you can also consider an inverted vee, at a slight penalty in performance compared to a dipole, and this might be a better choice in some situations.
Hats off to Guglielmo Marconi, inventor of today's modern vertical antenna!
80M Marconi
On this band, we could go for a full sized 1/4 wave radiator, but this is still a very tall antenna for most of us to manage (66 feet). Most of the design considerations for the 160M version will apply, with some tradeoffs.
- We lose some ground conductivity compared to 160M, but not drastically.
- On the other hand, the suggested radial system of 60 radials each 60 feet long becomes highly effective on 80M, as these radials are now about 1/4 wave long; especially when combined with a ground screen (exact radial length is unimportant as they are detuned by the ground).
- The benefit of using additional ground rods and auxiliary grounds like cold water pipes and fences starts to diminish because the wires used to connect these will become a more significant fraction of a wavelength. But they won't hurt anything if they are used.
- The possibility of using multiple monopole radiators is still attractive even with a full sized 1/4 wave radiator if we want to use this antenna across the band. But a remote antenna tuner at the base can also work. If we just want to operate CW or FT8, it's probably unnecessary.
- If using a shorter 32' radiator, the use of capacitive top loading, perhaps in combination with center or upper end inductive loading is still the best way to go. The amount needed will be much less than on 160M.
- An interesting alternative to the use of capacitive and inductive loading is the addition of a horizontal wire connected to the top of the Marconi antenna, turning it into an inverted L. There will still be a lot of vertically polarized radiation, but now we will also have a horizontal component, making the antenna useful for shorter distances. We will want to use a lightweight wire to avoid bending the antenna; perhaps #18 or smaller. With this arrangement we won't need a fancy matching network at the base because the impedance will be closer to 50 ohms, although if we put a remote tuner there, we can use the antenna anywhere on the band.
40M Marconi
On this band there is no reason not to use a full sized 1/4 wave radiator (33 feet):
- At this frequency we will have lost a significant amount of ground conductivity compared to 160M. We need a good ground system!
- Auxiliary ground connections to additional ground rods, cold water pipes, and steel fences are less effective due to the length of the connecting wires. An exception would be the case where the antenna is mounted on top of or adjacent to a large metal object such as a steel fence, such that the connection can be kept short.
- Fortunately, a good ground system is fairly easy to install. We can put in 60 1/4 wave (33') radials in almost any backyard, and a ground screen would be a further enhancement.
- The antenna bandwidth won't be much of a problem even with an efficient ground system. If we use a tuner at the base, we can cover the entire band with an excellent match.
- The wavelength is still long enough that the antenna will be able to "see" over most nearby structures, such as a house or garage, when ground mounted. We could put this antenna on the roof, but it would be rather tall. The required elevated ground system would not effectively shield the antenna from ground losses because we could probably not put in enough radial wires, and the antenna would be too close to ground in terms of wavelength. At a typical roof height of 13 feet we would need almost as many radial wires as at ground level to make the ground screen effective. If we could boost the antenna to twice that height, we could get away with 10 or less quarter wave long radials (about 33 feet).
30M Marconi
This band shares some of the characteristics of the lower bands and some of the higher bands. A 1/4 wave radiator is ~23 feet tall.
- If we mount this antenna on the ground, it will work satisfactorily, although it will not necessarily be a spectacular performer due to absorption by nearby objects.
- For the ground system, the same considerations apply as on 40M. The radials will only need to be about 23 feet long but we should put in a large number of them for good efficiency (60 recommended). A ground screen is also still a good idea.
- This is a narrow band, so we won't need a tuner. We will see a good match to 50 Ohms at the base.
- If we mount this antenna on a pole or on the roof, an elevated ground system will be a compromise. At 13' the antenna is less than 0.2 wavelengths above ground. A minimum of 10 quarter wave wires would be recommended. This will not look good on most people's roofs. We really need about twice that number to create an effective ground screen. We only need four to get the antenna to resonate, but its efficiency will be reduced.
20M Marconi (and above)
For these frequencies, we definitely want to elevate the antenna. It will not be very effective at ground level because signals will be blocked and absorbed by trees, bushes, and structures in the near field of the antenna.
- The newer ground independent antennas appear to be the best choice for operation on 20-10M.
- We don't want a rat's nest of wires on the roof!
- The size is manageable. For example, a Hy-Gain AV-620 vertical is about 23' high and weighs only 12 pounds. No guy wires are normally required.
- At 13' (the height of a typical suburban roof), it is still a bit low from the standpoint of ground loss.
- The short counterpoise wires at the base of the antenna are not going to fully shield it from the ground underneath it, no matter what they say in the advertising literature!
- But the situation won't be too bad because it will be 0.2 wavelength above ground on 20 meters, which is the point where an elevated ground screen starts to become more effective.
- At roof level, the antenna will be able to "see" over most of the close-in ground clutter, which will significantly improve performance.
- If we raise it further, we'll pick up more high angle radiation, which can be useful on shorter paths but can also increase QRM from domestic stations.
- In general, on these frequencies, "height is might" and the higher you go, the better the antenna will work.
Summing it up
No matter what band you enjoy operating, a Marconi antenna is a worthwhile addition to your station. On 160M, it's the key to DX. Depending on available trees, towers, and other antenna supports, the same is true on 80 and 40M because for effective long distance propagation, a horizontal antenna needs to be elevated at least 1/2 wavelength. On 20-10M, it's relatively easy to put up an effective horizontal antenna, but a Marconi is a neat looking antenna and doesn't require multiple high supports. Of course, you can also consider an inverted vee, at a slight penalty in performance compared to a dipole, and this might be a better choice in some situations.
Hats off to Guglielmo Marconi, inventor of today's modern vertical antenna!