Dual Loop Antenna Designs
I have experimented with a dual loop antenna patented by Maurice Hately, GM3HAT for roughly one year now. First, I built an original loop (picture 1) which was described in the patent drawings. It was easy to get tuned and I made several contacts with amateurs at a distance of 700 km on 7 MHz band. I tested several loop diameters from 5 cm to 2 meters with the corresponding frequencies of 145 MHz to 1.5 MHz. Then I tested dual loops with a magnetic drive to the second loop. With these loops, the operation of the DLA was easier to understand and I made several different loops. The last one was not actually a loop, instead it was a short-circuited piece of straight coaxial cable and It was quite a good performer as a transmitting antenna. It was during New Year’s eve 2000 when I contacted amateurs in northern Sweden, about 750 km north from my station with 100 watts using the antenna in my shack inside the house.
Figuring it Out
From the original picture of a DLA (Figure 1) it was difficult to understand the operation of the antenna. The loops plus capacitors were connected parallelly to the input connector (a step down transformer). The phasing capacitors were at the same end of the dual wire conductor but the phasing of the loops were different when compared to the quad phaser. The quad phaser circuit resembles the DLA and maybe it can be made to radiate, although I have not tried it. This original design is among the best operating designs that I have been using. The antenna is quiet, no local disturbancies and the antenna matches to 50 ohms input with ease even without a step down transformer (Figure 2). The loop diameters have been around .8 meters to 1.3 meters and the capacitors have been 20 – 300 pF variables with 1200V breakdown voltages. The best DX on 40 meters has been UA9CM on the eastern side of Ural mountains in Siberia, some 2000 km east from Finland.

Joel Hungerford, KB1EGI mentioned a balun tuner (Figure 3) when I was testing different tuners for the CFA and indeed this circuit developed 90 degrees phase difference between the two tuning capacitors at the end of the balun. This circuit also operates as a dual loop antenna. The circuit is simple and easy to understand. The phase difference at the input is 180 degrees as in the original DLA circuit. All currents are going to right directions and the circuit comprises a transformer with a tuned secondary, so there must be 90 degrees phase difference at the capacitor end of the loops. I thought I had designed a new DLA but M. C. Hately had included this in his patent, so nothing new this time.



When changing the capacitors to the input, the end of the loops are grounded so there is zero degrees phase and zero-volt voltage. The shorted end could be left floating (Figure 6). I tested it by simulation and it was okay with +/- 45 degrees at the capacitors and zero degrees at the shorted end.

Now it was tested on the 40-meter band.
On receive the antenna was noisy, the noise level was high, but most stations were over the noise and I talked to Jallivaara in northern Sweden, about 750 km north from my station on the 40m band. I heard Ahti, SM2JKI 59 +15 dB and my report from Jallivaara was 58 to 59. The 4-meter shorted coaxial cable was stretched straight inside my house and the lights were flickering as I talked to Ahti in Jallivaara. It seems that the shorted end of the antenna should be kept grounded and the loop shape should also be conserved. I tought I had again designed a new type of an antenna, but thinking more about the operation of the antenna, it was in fact a dual loop antenna with an autotransformer circuit at the coupled loops. Again GM3HAT has this design included in his patent.
I have made some drawings to understand better the operation of the DLA. I misunderstood the operation, I was wondering why currents and voltages within the antenna seemed to be low. The capacitor voltages are lowered by the capacitive reactance between the loops. This reactance is parallel with the tuning capacitors.
The operation of a magnetically excited DLA can be understood so that the capacitor C1 at the input matches the loss resistance of the loop and the whole circuit to 50 ohms input impedance. The capacitor C2 pulls up the second loop end impedance so that 377 ohms free space impedance and +/- 45 degrees phase difference for the Poynting Vector Synthesis are achieved. Of course the capacitors interact with each other (Figure 7).


Figure 8 is a simulator schematic of a DLA with floating shorted end. There are resistors in the schematic for getting the voltages and phases visible. There is also the cable capacitance inserted parallel with the cable.

Figure 9 shows the actual simulation results with varying frequency. V5 and V4 are the phases at the capacitor ends of the cable and V3 is the floating cable end phase. The marker line is adjusted to 7.236 MHz. The phases of V5 and V4 are not exactly +/- 45 degrees but +/-47 degrees and V3 phase is 3.2 degrees. If the 90 degrees phase difference was shown, the actual phases would have been about +43 and -47 degrees and the floating end phase would have been something like +4 …5 degrees. With this Micro-Cap V program we cannot simulate the radiation of the antenna, only the voltages and phases on the conductors.
It seems that I have not found anything new but have tested the circuits which GM3HAT must have tested long ago. This has been fun and I have really enjoyed the time I have been testing the different circuits and maybe I have learned something in the process.
Update
After writing the original text for the different dual loop antennas, I continued with my DLA tests and found that the dual loop with floating shorted end could be tuned with only one capacitor and the length of the cable should be closer to lambda/10 to get a good match to 50 ohms. The inner wire of the coax is grounded. This antenna matches to 50 ohms quite well the operating range on 80m is 250 – 300 kHz with below 2:1 swr ratio.
Then I received the info about EH antennas. My new DLA and the EH antenna circuits resemble each other quite a lot. My opinion is that the main radiating parts in these antennas are different. In a DLA the +/-45 degrees difference occurs at the same end of the dual inductance and it is the radiating element.
In the EH-antenna the +/-45 degrees phases are on the different ends of the one inductance, so it doesn’t radiate. The capacitor’s electric and magnetic fields are in phase as described by the writer of the text. The capacitor is the main radiating element in an EH antenna.
Originally posted on the AntennaX Online Magazine by Heikki , OH2BGC
Last Updated : 22nd May 2024