DK6ED Double Loop Receive Antenna Systems

design of the different antenna systems

with a new ultra low noise preamplifier

comparing single loopreceive antennas like Flag, EWE, K9AY and double loops like Waller Flags against this double loop.

Update Dec.  2022

A new receive antenna system for the low bands

For serious dx-ing on the low bands a receive antenna is a must. K9AY, Flag and EWE systems are common. Compared to other loops they are larger size, broad band and terminated by a resistor.

Resistive terminated loop antennas

These systems give a good front/back ratio and a small opening horizontally and vertically compared to other small size loops. That reduces local QRM and gives better reception in a pile up because other stations are getting attenuated. The radiation pattern depends on the terminating resistor. But it is important to know, that also the output must be terminated by the same resistance. Never use a high impedance preamp without matching at this point. Nearly all directivity will be lost.

Dimensions and radiation pattern of a single loop having a base length of 5 m are shown in figure 1.

Figure 1: single loop terminated by a resistor

The pattern can be improved by installing two loops behind each other and combining their output so that most of the sideways signals, reaching both loops at the same time, are extinguished. Signals from the front will reach each loop at a difference in phase, so these signals will not be fully cancelled and they remain at the output of the antenna.

The double loop system DK6ED V2 shown in figure 2 is a simple solution with one single feedline.

Figure 2: double loop with one feeder

The front/back ratio of the double loop system can still be improved. The radiation pattern of the DK6ED double loop V 1 with two separate feeders is shown in figure 3. 

Figure 3: double loop two feeders

For combining any two rf lines a combiner is a must, but this causes an attenuation of the signal by at least 3 dB. To avoid this problem, also two separate medium impedance preamplifiers could be used and the two output signals could be joint behind. This gives more sensitivity and better separation of the two loops. It is important to realize, that, despite the EZNEC simulations results, a combiner is needed otherwise you get the pattern of a single loop! This is why other double loops, for example the Waller Flag, do not work as expected. They are pure make believe!

Building one of the DK6ED double loops

This is not a major challenge mechanically. The bases for the antenna shown in figures 2 and 3 are the same. Details can be taken from figures 4 to 6. They are made from tapered aluminium tubes, all same length.

Figure 4: mechanical dimensions

Figure 5: centre of a system

The holders keep each sector isolated from the other.

Figure 6: the whole double loop

Preamplifier for resistive terminated loop antennas

With the front/back ratio increasing, the output of the antenna is decreasing. This is why designing a convenient preamplifier is challenging. Because of the low output the amplifier must be ultra-low noise. Its input must be in the Medium Ohms range.  The preamplifier should have a good IMD performance as the antenna is a broad band system. It is also important to avoid strew in of RF from the feed line, as this worsens the front / back ratio of the antenna significantly.

Recent developments of preamplifiers for WIFI and mobile applications show a simple way. The main idea behind is to use a common source stage at the input then feeding the signal to a common drain stage. For a medium input impedance and to improve IMD performance of the first stage a current feedback resistor is used. That of course is also feeding back noise of the first transistor to the input of the preamplifier. This noisy signal is feed to a second common source stage where the input signal is amplified and also turned in phase. At the output of the preamplifier the signals from both paths are added up but the noise from the input stage is getting subtracted as it had undergone a phase shift in the second common source stage. The major advantage of this circuit is that we do not need any transformers as the input is matched by the feedback resistor and the output is low impedance by coupling a common drain and a common source stage. The circuit is show in fig. 7. It has a gain of 15,5 dB, a noise figure of 0,4 dB and the OIP3 is 20 dBm, all at 2 MHz. Just the variable resistor must be aligned for maximum output.

The proposed circuit must be installed at the base of the antenna in order to avoid strew in of unwanted signals and losses on the line, increasing the noise figure of the whole system. Also the power is applied through the feed line. This is why a transformer and a choke are needed for the line, also shown in the diagram.

This amplifier can be used for a DK6ED double loop V2 with a single feedline shown in figure 2. As the double loop with one feeder is a symmetrical system also the preamplifier should be strictly symmetrical otherwise the radiation pattern of the antenna will have sideways openings. So a broadband balun is needed. For optimum performance a design with a second winding on the same core is useful. As DK6ED double loop V1 with two feeders, shown in figure 3, is an unsymmetrical system we can also use this preamplifier, but a splitter is a must in front of the amplifier! Otherwise the two loops influence each other by feedback through the feeders, making them work like a single loop as I learned from the Waller Flags. NEC simulations show the wrong way.

For all different versions a single board was designed, it is shown in fig. 8.

Figure 7: diagram of the preamplifier

Figure 8: Preamp built

Power supply

The preamp is feed through the coaxline, so a bias T is necessary. Details can be taken from fig 9.

The feeder is not grounded, as the choke built in my TS 590 is doing a good job suppressing the RF picked up on the feeder. The power supply does not use a regulator which might create noise at lower frequencies. By the switching transistor supply voltage will only turned on when receiving. It is switched by the T/R relay of the transceiver, when the contact is closed. Turning off the supply voltage will be sufficient to protect the preamp against RF when transmitting. The BF 981 transistors are already have built in diodes at the gates. 

Aligning the system

First the shunting resistor of the antenna needs to be checked. It must be aligned for maximum front/back. With the antenna from figure 3 you do it just with one single loop and use the same resistor for the other loop too. Also make sure that the combiner for the two loops is subtracting the signals, which can be checked by reversing the feeder from one of the loops.

That’s all, my results with the DK6ED Double Loop with two feedlines could be checked with the audio files attached.

Comparison rx- to tx- antenna on 160 m

Front to back 160 m