Short Vertical Antennas for Low Frequency HF Bands
For DX operation on low-frequency amateur HF-ranges of 80 and 160 meters it is desirable to use antennas with vertical polarization for good DX results. The ground absorbs the horizontal polarized signals to a large degree. This is due to propagation by the ground wave, that contacts the surface of the earth parallel to the surface of the earth, which is excellent for absorption. With respect to ionospheric propagation, the greatest losses in reflected radio waves from the ionosphere occurs when the radio waves are horizontally polarized. Many experienced radio amateurs are convinced that the short vertical antenna on low HF bands often works much more effectively for DX than a full-scale dipole at a low altitude above ground. In this article, we shall consider how it is possible and practical independently to make and to tune a short vertical antenna.
As is known, the radiation resistance of the short vertical is rather low. In Figure 1, the graph of the radiation resistance of the short vertical in the 80 and 160 meter ranges is shown. The resistance of radiation of the short vertical by an altitude up to 0.1 percent of a wavelength, which is 8 meters for the 80 m band and 16 meters for a 160 m band, as is visible from this graph, shows less than 5-Ohm radiation resistance for each.

The effective use of such electrically short antennas without methods for tuning that antenna to resonance is impossible. In practice tuning to resonance is realized with the help of a lengthening spool (loading coil) in an antenna base (Figure 2), and with the help of a lengthening spool (loading coil) in the top of an antenna (Figure 3). For the short vertical to be tuned to resonance, it is necessary that the circuit derived by the inductance of a lengthening spool L and capacity of an antenna. It is therefore important to know the total capacity of the short vertical conductor of an antenna. The total capacity of a capacitive load (antenna) and its counterpoises can then be tuned to resonance on these frequencies by proper selection of both base and top loading coils.


To increase the capacity of an antenna at “ground” in short vertical antenna, a capacitive load must be used. Usually such a load consists of from three up to six conductors of a length from 10% to 50% of the full length of the vertical antenna. These are usually connected to the top, in parallel or directly under the top to ground via insulators at the opposite ends (Figure 4). To ensure stability in the capacitance of the top hat and the counterpoises, it is necessary that the number of counterpoises in an antenna is not less than the number of wires of a capacitive load. Their length is 1.5 times the length of conductors of a capacitive load. Also, only non-resonant counterpoises are necessary for creation of a capacitive antenna. The capacitive component will exist between a capacity hat and ground. For an effective antenna and further lowering of losses in its ground system, it is desirable to use resonant counterpoises, even if only a few elements for each band.


With a vertical height of 10-15 meters and a capacitive load made as describe by the above guidelines, the capacity of an antenna concerning its ground system can fall within limits of 10-60 pF. This capacity depends on the number of conductors in a capacitive load, ground quality for a real antenna and quality of materials used for the antenna conductor. The theoretical calculation of this capacity in radio amateur conditions is complicated, but it can be measured in a practical manner with a battery-powered RLC-meter, connected between the antenna and “ground” (Figure 5). Before making such a measurement, it is necessary to remove any static charge accumulated by the vertical part of an antenna, by grounding it to a solid ground for a few seconds. The measurements should be made in dry, calm weather.

In bridging to the antenna and ground a capacitor of 1000 pF must be introduced. The capacity of the antenna can be determined by the amount of the increase in capacity above the 1,000 pF capacitor. The capacity of an antenna can be measured accurately using short connective conductors from an antenna and its “ground” leading to the RLC-meter. Having defined the capacity of an antenna and “ground”, with the help of the graphs from Figure 6, you can define the correct magnitude of inductance of a lengthening spool (loading coil). The inductance defined with the help of these graphs will require a reduction of its value by 20-40% since the length of an antenna and the capacitive load that goes into calculating this inductance was not taken into account when these graphs were first drawn.


A quarter-wave antenna, which is the theoretical model of a truncated antenna (Figures 2 and 3), has maximum current in the antenna base. In operation, the antenna during transmission has maximum current in the material that makes up the counterpoise and lower section of the antenna. The distribution of current in a radiator with a top-lengthening spool (loading coil) is exhibited in Figure 7. The distribution of the current with a spool (loading coil) in an antenna base is exhibited in Figure 8. As demonstrated by these figures, the maximum area of the current will be only in an antenna with a loading coil on its top end.

In real amateur conditions such construction of an antenna is used infrequently. Due to the requirements for the 80 and 160 meter amateur bands, coils have large overall dimensions and weight which hampers waterproofing and mounting on top of an antenna. During the setup of an antenna, any need to change the value of inductance will be inconvenient when the coil is located at the top of an antenna. Usually radio amateurs place a loading coil at the top of the antennas (Figure 9), having inductance by a magnitude approximating 70 percent from a calculated value of loading inductance. With the addition of a coil of variable inductance of small value in an antenna base loading setup of an antenna, tuning becomes easier. So, it is reasonable to consider this method of constructing a vertical antenna for the low-frequency bands as being optimum in practical terms.
After tuning the vertical element to resonance, it is necessary to measure its input impedance. The input impedance of an antenna at resonance is a pure resistance which can be defined under the graphs in Figure 1. Loss resistance includes a resistive loss from the type of material used in the antenna construction and from the losses in the grounding system. The lower the input impedance of a short antenna, the lower the total distributed loss will be between an antenna and its grounding system. And, the closer the antenna input impedance is to the radiation resistance, the easier the antenna will be to match. As the display shows, the input impedance of short antennas in the low-frequency ranges with a lengthening spool can fall within the limits of 5-20 Ohms. This low input impedance of an antenna makes it necessary to match it to a coaxial cable in the 50-75 Ohm range using known methods of matching. The matching circuit in this case is at the antenna base.
It is easier to feed a shortened vertical antenna through the quarter-wave transformer as shown in Figure 10. The quarter-wave transformer is made from two or three lengths in parallel of the same coaxial cable as used for the antenna feedline, and taking into account the velocity factor for that particular cable to figure the correct lengths. In Table 1, the value of resistance on the end of the quarter-wave transformer is exhibited. This depends on the velocity factor of the cable and the number of cable sections used for determining the value of the quarter-wave transformer. Use of more than three cable sections for creation of the quarter-wave transformer is ineffective.


Applying a combination of a feed cable of 75 Ohms and a transformer made from 50-Ohm sections, it is possible to reduce even further the resistance on the output of a feeder system. In Table 2, the value of this resistance is reduced by using the above-stated matching circuit constructed in this manner.

If the shortened antenna has shifted frequency out of the limits of an amateur band, the antenna system must be re-tuned. The shifting of frequency is possible due to deterioration of the capacity between the antenna and “ground” due to corrosion, which usually means installation of a new antenna. Deterioration of the counterpoise and, probably other factors, including general deterioration of the system, can cause problems of this nature. In these kinds of cases, it is usually not expedient to tune a spool (coil) installed at the top of an antenna. First, it is rather difficult, and secondly, the other repairs will eliminate the factors causing instability in the antenna frequency range and the antenna parameters will return to the former range.
If there was a reduction of capacity between the components that make up the antenna and its ground, the antenna will shift above the desired frequency range. The antenna can be re-tuned to the required range with the help of a small inductance, added to the base of the antenna as shown in Figure 9. If the total capacity between the components of an antenna and its ground has increased, the antenna will shift below the required frequency range; its tuning can be corrected by adding a variable capacitor to the base of the antenna (Figure 11).

Tune Slowly
Because the normal resonant frequency of an antenna usually changes in small increments, the tuning of an antenna should be adjusted slowly as well. Therefore, the capacity of a variable capacitor for amateur ranges 80 and 160 meters should be large—about 1000 pF. For this purpose, it is possible to use a three-section variable capacitor from the old tube©type receiver, having connected all of its section in parallel. Since many of these three-section capacitors have a built-in vernier with a reduction of 1:3, the tuning of an antenna with the help of a variable capacitor becomes much easier. The capacitor works on small voltage, but on a large high frequency current transiting through it. With power inputs to the antenna of up to 200 watts, it is possible to use regular three-section capacitors from old broadcasting receivers.
On the upper end of the loading coil there will be a high HF-voltage during transmission. This means that there must be adequate weatherproofing and good insulation used in the construction and mounting of this coil.
I hope you will find this helpful should you consider using one of these particular antennas. Until next month:
Originally posted on the AntennaX Online Magazine by Igor Grigorov, RK3ZK
Last Updated : 27th May 2024