A Series Resonant Dual Loop Antenna
and the Poynting Vector Synthesis
Heikki Antman, OH2BGC

INTRODUCTION
have tested GM3HAT's crossed field Dual Loop Antenna and its many derivatives. My latest antenna,developed from the original DLA, is a series resonated loop antenna. This antenna is formed with a series resonating dual loop radiator loaded with a separate resistor. The radiating element is a piece of dual wire or coaxial cable resonating at the operating frequency with its own capacitance and inductance.

Compact Antenna

The loss and radiating resistance of the antenna element are added to the resistance of the external resistor and the transmitter sees the total resistance. If we use a 50-Ohms load resistor, the total resistance is around 50 to 60 Ohms and the antenna can be connected directly to the feeder cable. The length of the series resonating piece of cable is around wavelength, depending on the cable type used. When this length is turned into a circle, the diameter is less than 0.03 WL, so the antenna can be called a compact antenna or a huge dummy load.

The efficiency of this design is low, of course, maybe somewhere near 5 to 10%. Most of the transmitted RF-power is converted into heat in the load resistor. The power handling capacity of this antenna depends mainly on the power rating of the dummy load resistor. I have had contacts with this antenna on 80m band using 100 Watts SSB up to a distance of 300-400 km, and the antenna was inside my house, below a sheet metal roof. Similar distances were reached when the antenna was outside in the open. See Fig. 1.

Multi-Band Antenna and Higher Efficiency

This basic idea even allows a design of a multi-band antenna. This is done by paralleling separate loops for every band wanted. I tried this design on the 80- and 40-meter bands. The parallel loop did not affect the other antenna loop connected, because of being a series resonant circuit tuned to a different band. See Fig. 2.

I simulated the circuit with Micro-cap V program (Fig. 2 & Fig. 3), which shows how the reactive voltages are about zero over the loop elements on both frequency bands. This program cannot show the RF radiation or radiation resistance of the antenna.

I tried different load resistor values to reach higher efficiency and found that lowering the resistor value will make the antenna resonance sharper. Of course, the efficiency was a bit higher when the radiating resistance was higher compared to the total resistance of the system: 50 Ohms - 1.1 MHz broad antenna, 25 Ohms - 500 kHz, 12.5 Ohms - 250 kHz and 0 Ohms - 20 kHz between 2:1 standing wave ratio values. Using lower load resistors led to the use of matching components at the input of the antenna. A suitable capacitor across the input and part of the antenna loop as an inductance formed the matching circuit. Using small load resistor and L/C matching components means a single band antenna. A multi-band antenna could be fed with a step down transformer.

The radiating resistance appeared to be around 5 to 10 Ohms. This value seems to be high. Perhaps the loss resistance of the loop wire insulation, made of PVC, is higher than I have thought.

The NEC4WIN95 program can simulate a 5-m wire antenna on 80 meters, and it gives a radiation resistance of near 6.5 Ohms for a 5-m vertical wire tuned with a simple wire as a capacitive hat and a counterpoise. The radiating resistance value of 10 Ohms found in practice includes both loss and radiating resistances of a 5-m piece of PVC isolated dual wire, and the loss resistance seems to be about 3.5 Ohms.

Multi-Element Single Band Loop Antenna

After reading about RASER-antennas, utilizing L/C series resonating parts in a dipole antenna, I made a prototype antenna with several series resonated, series connected, loop elements. I could omit the load resistor. I made the antenna for 80 meters by connecting 5 antenna elements in series. I fed it directly from my transmitter with a nearly perfect match at the resonant frequency. I used fiberglass fishing rods to raise the antenna to about 6 m above ground. I had to shorten the wire element lengths from 5.3 m to roughly 5.0 m before I got the resonant frequency to 3.7 MHz. The total length of the radiating antenna was something like 25 meters and the bandwidth was about kHz between 2:1 standing wave ratio points. I had several contacts with this antenna to distances from 50 km to 400 km using 100 Watts of power. My signal was also heard in Oulu, a city located some 600 km north of my transmitting site. See Fig. 4.

Poynting Vector Synthesis

In a series resonant circuit, the reactive voltages are suppressed as shown by the simulation picture and only the RF-current is flowing from the transmitter to the load resistor trough the antenna element. See Fig. 5. The current is decreasing from full value to zero in the wire connected to the input and increasing from zero to the full value in the wire connected to the load resistor. Inductor and capacitor are "built in" in the same piece of dual wire series tuned circuit. The summed current is roughly constant throughout the loop element.

The RF current and the loss resistance of the wire create a tiny voltage drop over the loop element. This voltage is in phase with the flowing current and the loop conductor directs the voltage field. The current makes a magnetic field around the antenna conductor. These two fields are in phase, have a similar form, and are perpendicular to each other. This set of conditions may trigger the Poynting Vector Synthesis and lead to the appearance of the 7 Ohms radiating resistance and the radiated RF power. This might be only part of the PVS story, because in a normal dipole antenna wire, the reactive fields are not suppressed. In a dipole, the RF current is not spread evenly over the length of the dipole as in the antenna made of a series resonant element. Using several of these resonant radiating elements in series, we may approach the dipole radiation or even go above it.

Conclusions

This design needs much more testing, thinking and studying than I have been able to do up to now, but I think that this is a good start for me to gain a better understanding of the operation of existing and new antennas. The original GM3HAT dual loop antenna seems to be better, but it has two series resonant, parallel connected loops, which are also phased correctly. My idea of a single series resonant loop antenna should be tested with a cable isolated better than the one I have used. Maybe I am simplifying things too much and the truth is hiding somewhere near but beyond my thoughts at the moment. -30-

BRIEF BIOGRAPHY OF THE AUTHOR
Heikki Antman, OH2BGC of Finland got his first novice licence in 1965 at the age of 17. Professionally, Heikki has been an electrical engineer since 1975 and involved in many activities from selling components to the Finnish electronic industry to designing tourist submarines. He has designed DC/DC power supplies for the very large global company, Nokia. In a smaller company Heikki designed GSM detectors for planes and hospitals. This company was also involved in the design of antennas and filters, and it was here, where he really got interested in small size antennas. He is presently working for a company, which hires experienced designers out to other companies needing such engineering skills in projects for a limited time.

~ antenneX ~ June 2003 Online Issue #74 ~

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