A quarter-wave vertical built on a grounded mast has been known for many, many years. You feed it with a coaxial cable via a gamma-type match, as shown in the figure below. The bottom part of the metallic mast is buried, what makes the mast grounded for direct current.

Yet, would it be possible to create not a quarter-wave but a half-wave vertical antenna using a grounded metallic mast? It turns out that yes. The picture below presents the initial concept of such an antenna  

A grounded half-wave tall mast is fed by a coaxial cable at half the height. The cable's braid is connected to the mast, and its center conductor connects to a radiator descending almost to the ground, parallel to the mast. This radiator should be a quarter-wavelength, but then its end would come into contact with the ground. To prevent this, a simple capacitive umbrella is used at its end in the form of two horizontal wires. The top half of the mast forms one arm of the dipole, while a radiator parallel to the mast forms the lower arm of the dipole. Although the lower half of the mast also has an HF current flowing through it, its value decreases as it approaches the ground point. This is a very important observation. Since a small value of current flows at the grounding point, this means that the grounding resistance of such an antenna is not critical and can have a large value, yet the performance of the antenna will not be adversely affected. In other words, a simple lightning protection grounding of the mast is sufficient, and it is not necessary to build an extensive system of radials. Simulation of the antenna with a simple vertical rod grounding and with a system of radials showed that the antenna performance remained almost unchanged. The simulation also showed that the input impedance of such an antenna had a not very "comfortable" value of about 20 ohms, the radiation pattern was not perfectly omnidirectional, and the SWR bandwidth was somewhat smaller than that of a center-fed dipole.

In the course of further work, the final version of the antenna was created (see the figure below). For it, an input impedance close to 50 ohms and an almost perfectly omnidirectional radiation pattern were obtained. The antenna gain even turned out to be slightly higher than that of the half-wave dipole. The SWR bandwidth remained smaller than that of the center-fed vertical dipole, but with good antenna trimming, it is sufficient for almost any HF amateur band, except the 80 m band. But a half-wave vertical antenna for this band is hardly practical anyway.  

The antenna has been enlarged from half to about 5/8 wavelength. Its feed point was raised to about 3/8 l. The radiator terminated with a capacitance hat was replaced by an elongated loop, what raised the feed point impedance. The vertical wires of the loop were placed exactly on opposite sides of the mast, resulting in an almost perfectly omnidirectional radiation pattern. The braid of the coaxial cable was additionally connected to the mast just above the ground. Perhaps this connection is not absolutely necessary, but by doing so, you almost completely equalize the potential of the mast and the potential of the coaxial cable braid.

An additional effect of such a revised design is that now every point of the antenna has a DC connection to the ground, including even the inner conductor of the coaxial cable. The antenna is short-circuited to earth for direct current and is therefore also grounded for lightning. This increases the safety of using the antenna system, and can also reduce the level of received atmospheric interference.

Table 1 shows the dimensions of the antennas calculated for different HF bands. In the calculations, it was assumed a use of a telescopic aluminum mast. The antenna should be tuned either by extending/retracting the mast, or by lengthening/shortening the loop height.

The antenna dimensions (in meters) for different HF bands.

In order to check whether the simulation of the antenna reflects its actual behavior, a prototype antenna was built for the 10 m band. The feed point and the connection point of the braided coaxial cable to the mast were secured using small plastic enclosures. The crossbars supporting the top and bottom of the loop were made using 5 mm thick polyamide boards. The plastic enclosures and the crossbars were attached to the mast using metal pipe clamps sized to fit the diameter of the mast at a given height. The clamps provided both mechanical attachment and electrical contact. Construction details of the prototype antenna are shown in the pictures below.

Overall antenna view.

Top crossbar (shown from both sides).

Crossbar fixing the bottom of the loop.

Coax-to-mast connection box mounted at the bottom of the antenna.

The standing-wave ratio measured immediately after the antenna had been built reached a minimum of 1.1:1 at 29.08 MHz and about 2:1 at the edges of the band (measured with 22 meters of RG58 coax cable). The antenna could have been trimmed down a little bit, but because I built it just to prove the concept rather than to use it permanently, I did not bother.

The radiation pattern of a half-wave vertical antenna is similar to that of a dipole mounted at the same height, but has a slightly higher gain.

The receiving and transmitting comparison tests of the antenna and a vertical half-wave dipole showed that their gains were practically indistinguishable from one another. A difference of about 1 dB is imperceptible on the HF bands.

The question arises, however, whether we would achieve a similar effect with a vertical quarter-wave antenna. Such an antenna would be more than twice as low. It is not difficult to perform appropriate simulations and compare the radiation characteristics of such antennas.

The figure shows the radiation characteristics in the vertical plane of a grounded half-wave antenna (red) and a quarter-wave monopole (blue). Antennas designed for the 20m band are compared, but for other bands the results will be virtually the same. The advantage of a half-wave antenna over a quarter-wave antenna depending on the elevation angle is: 3.8 dB for 15 degrees, 4.4 dB for 10 degrees and as much as 4.8 dB for 5 degrees. In order for a quarter-wave antenna to be received as strongly as a half-wave antenna for an elevation angle of 5 degrees, it would be necessary to increase the transmitting power by three times!

In conclusion, it is possible to build a vertical half-wave antenna on a grounded mast radiating at a low angle, and therefore good for DX operation. Such an antenna has a somewhat smaller SWR bandwidth than a classical vertical half-wave dipole fed centrally, but its mechanical structure is more robust because the metal mast does not have to be interrupted by insulators. Feeding it with a 50 ohm coaxial cable is very simple. You run the coax along the ground and then along the mast up to the feed point. The antenna does not require any matching circuit nor a common-mode choke. The antenna dimensions and the proportions of the loop are chosen so that the feed point impedance is close to 50 ohms. Although the antenna requires grounding, this grounding does not have to be of very low resistance. A simple earth stake should be enough. It may even be sufficient to use the buried part of the mast if it is long enough. But make sure your earthing is good enough for lightning protection!

Click here to download this antenna model for the 10 m band. Warning! You will need to use either NEC-5 or NEC-4 engine based simulator. The NEC-2 based simulators will produce erroneous results.