Parent Category: 2017 HFE
By Hans Van Bruggen
Monopole antennas, verticals excited against a groundplane, are among the most used antennas. Generally, they are fed at their bases. In the case of the full-size quarterwave monopole, the antenna as such is already resonant and its base feedpoint impedance matches to 50 Ohms quite well. Electrically short monopoles however, i.e. verticals with lengths substantially shorter than a quarter of a wavelength, are not resonant and exhibit complex impedances that deviate strongly from 50 Ohms. Using these more compact antennas efficiently in 50 Ohms systems necessitates both a resonating and impedance transformation action. One example of how an electrically short monopole can be made more efficient is by adding a capacitive top load to the vertical radiator (Fig. 1). This increases radiator current to a maximum at series resonance. The low series-resonant feedpoint impedance at the base must then be transformed to 50 Ohms, by means of an external matching network. This introduces additional losses and compromises overall antenna efficiency.
Fig. 1 • Classical top-loaded short monopole on a groundplane.
Meet the CMMA
The present design idea, to the best of author’s knowledge new and original, provides a way to build an internally matched electrically short monopole antenna, dubbed the Compact Matched Monopole Antenna (CMMA). Instead of one, the CMMA has two capacitor plates stacked on top of the radiator (Fig. 2). While the connector is still at the base, excitation is at the top instead of the base. The signal from the connector is coaxially fed upwards, through the interior of the radiator, to excite the antenna between the centers of the two plates. The center conductor is soldered to the upper plate, the shield to the lower plate.
Fig. 2 • Compact Matched Monopole Antenna (CMMA).
The radiator base is not insulated but grounded, allowing for sturdy mounting to the ground plane. The two capacitor plates bring the antenna into resonance while also providing a capacitive tap to step-down the high-value parallel-resonant antenna impedance to match 50 Ohms. By choosing appropriate values for plate dimensions and distance between the plates, radiators of virtually any length between zero and a quarter of a wavelength can be made to match 50 Ohms perfectly. Please note that the plate diameters need not, like Figure 2 might suggest, be the same. Either plate can be bigger than the other. Also, they need not necessarily be of circular shape.
The idea can be applied at every frequency where the antenna structure is not too bulky or too tiny. A radiator length of approx. 1/12 of a wavelength offers a favorable volume filling factor. For higher frequencies such as 2.4 GHz, the radiator can simply be the outside of the same semi-rigid cable that is used to run the signal to the top of the antenna. There is no need for separate tubing for the radiator in this case. The upper plate can be held in place with small insulating standoffs to the lower plate, the latter rigidly mounted on top of the radiator. For lower frequencies the solid capacitor plates can also be replaced by radials or chicken-wire screen constructions.
The radiation diagram is identical to the classical top-loaded monopole, i.e. omnidirectional with a null at zenith and a maximum roughly along the groundplane. With a sturdy design and no requirement for an external matching network, power efficiency can be good. Efficiency values exceeding 90 percent have been measured. Typical relative bandwidth is 5 to 10 percent, for VSWR better than 2:1.
A model for 2.45 GHz fits inside a small plastic (acrylic) jar of 30 mm diameter and 14 mm height, with a female SMA bulkhead connector protruding through the bottom (Fig. 3). Implemented with 0.4 mm (.015”) thick brass plates, the total weight is 10 grams. This model, for instance, would be perfectly suited as a ceiling radiator for WiFi, mounted upside-down on partially conducting ceiling tiles.
Fig. 3 • Pigtailed 2.45 GHz prototype CMMA (top) in comparison with a regular WiFi dipole (below).
Figures 4 and 5 respectively show S11 and VSWR of the 2.45 GHz CMMA model as measured on a 80 X 80 mm rectangular groundplane. Results shown here have been measured without plastic enclosure, but there is little difference.
Fig. 4 • Measured S11 of the 2.45 GHz CMMA model.
Fig. 5 • Measured VSWR of the 2.45 GHz CMMA model.
Figure 6 shows measured return loss (RL) with the antenna (including its plastic enclosure) inside a conducting cavity (analog to the Wheeler Cap method) for measuring efficiency. The antenna resonance frequency shifted marginally. The result suggests an efficiency of roughly 92%.
Fig. 6 • Efficiency measurement (linear RL) of the 2.45 GHz CMMA model including plastics inside a conducting cavity.
There is an excellent correspondence between the measurements to a physical model, and simulations in NEC-Win Pro of the corresponding wire frame model (Figs. 7 and 8). The model includes the outlines of the SMA bulkhead connector at its base (the part of the connector that is above the groundplane). Please note that total antenna height is only 10 mm.
Fig. 7 • Wire frame model of 2.45 GHz CMMA in NEC-Win Pro.
Fig. 8 • Simulated S11 of 2.45 GHz CMMA wire frame model in NEC-Win Pro.
Another interesting application example for the CMMA is DVB-T transmission or reception (digital terrestrial television on UHF). A model for 480 MHz (or 482 MHz to be precise, UHF channel 22 in Europe) fits within a cylindrical boundary of 100 mm diameter and 50 mm height. A construction drawing is given in Figure 9.
Fig. 9 • Construction details for a 480 MHz CMMA (side view).
Figure 10 shows a rough equivalent circuit for this 480 MHz CMMA model, with typical component values. In reality, the various capacitances, inductances and resistances are distributed rather than lumped components. The input of this two-port circuit is the actual point of excitation, not the antenna connector. The output port is the radiation resistance.
Fig. 10 • Equivalent circuit diagram for the 480 MHz CMMA model.
In this diagram, the ground symbols do not stand for groundplane-ground, but rather the center of the lower capacitor plate, being the reference point for excitation. Actual groundplane-ground is denoted by the capital ‘G’ in the diagram. R1 to R4 are loss resistances with (fairly uncritical) estimated values. C1 is the capacitance between the two plates. C2 and C3 are the capacitances to groundplane-ground, from lower and upper plate respectively. L1 is the radiator inductance.
Figure 11 shows the measured S11 of the 480 MHz CMMA model, and for comparison Figure 12 shows the simulated S11 of the equivalent circuit, with added delay for the short length of transmission line between connector and actual point of excitation, not included in the equivalent circuit.
Fig. 11 • Measured S11 of the 480 MHz CMMA model.
Fig. 12 • Simulated S11 of the 480 MHz CMMA equivalent circuit.
The highest practical frequency for building a CMMA based on a female SMA bulkhead connector was about reached when approaching 6 GHz. Figure 13 shows a 5.8 GHz model without plastics and Figure 14 shows its RL characteristic.
Fig. 13 • A 5.8 GHz version of the CMMA with plastics removed.
Fig. 14 • Return loss curve of the 5.8 GHz CMMA model.
This new way of constructing CMMA’s, compact monopole antennas (verticals) with built-in matching is believed to have advantages over present state-of-the-art monopole designs. It is the author’s choice to disclose and share this idea with the antenna building community rather than protecting it. It may be a valuable addition to the antenna design techniques arsenal available today, of interest for both professionals and amateurs, and useful from HF up to microwave frequencies.
About the Author
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