Update Nov. 2018
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. They create a pattern shown in figure 1.
Figure 1: horizontal pattern of a single loop
To improve the front to back ratio and the opening angle, Double Loop systems are recommended. Waller flag, the Apex Loops and the LZ1AQ systems show improved patterns when simulated by EZNEC. The simulations use two different sources at a phase shift of 180 degrees being totally separated. When building the antenna, Waller just reverses the feeders of one of the loops, no further separation. The rest of designs use combiners. By SPICE simulations it can be proved that these combiners do not create the phase shift required. These systems then have the same pattern as single loops, giving just a little increased output. I made the same mistake with my first designs.
So I was looking for a double loop system which can be fully simulated by EZNEC, just one source and one load. I will introduce it as DK6ED Double Loop V2. The schematic is shown in figure 2, the patterns are in figure 3 and 4. The dimensions can be scaled, the value of the loads stays unchanged. The loops are connected by a reversed line cord giving about 100 Ohms impedance.
Figure 2: electrical dimensions of the DK6ED Double Loop (m)
Figure 3: horizontal pattern of the DK6ED Double Loop
Figure 4: vertical pattern of the DK6ED Double Loop
The mechanical design is shown in figures 5 and 6.
Figure 5: mechanical dimensions of DK6ED Double Loop
Figure 6: centre of the DK6ED Double Loop
This system can still be turned by a small rotor for TV antennas. With this antenna, simulations and practical experiences match, resulting in small openings and an excellent front to back ratio.
The attenuation of the system is rated – 46 dB on 160m, so an amplifier is needed.
How much gain is needed? Most authors say that this negative gain must be compensated. I disagree. With my transmit vertical noise gets above the S9 level. So I set this as the isotropic radiator. S 5 signals would give arm chair copy. That makes a difference of 24 dB which must not be compensated. So just an amplifier with a gain of only 22 db is needed. I experienced that even less will do.
Loops have output impedances of around 600 Ohms, so generally step down transformers are used for matching the 50 Ohm feed line to the receiver. In this feed line low impedance preamplifiers are used. We have to keep in mind that any load at the output will attenuate the antennas output by reverse transformation.
By the ideas of Dallas Lankford and Chris Trusk I designed a new preamp, with a step up transformer, so the medium impedance output of the antenna is matched to the high impedance of the common Drain FET preamp. This design is much more helpful to avoid IMD than any other preamp using a common source circuit at the same gain. The preamp shown in figure 7 gives a power gain of 13 dB. With this single stage amplifier the output level is high enough to feed the receiver. For additional gain a Norton preamplifier is recommended.
Figure 7: schematic of the new ultra low noise preamplifier using noise compensation
When using this new antenna system I was surprised by the low noise from the receiver. It is of course because of the antennas attenuation. But the most important reason is the fact that the horizontal and elevational openings are reduced significantly. Also noise form the back of the antenna is cut off. So I was able to hear stations covered by the noise caught up by the omni directional transmit antenna. I added some comparative examples of reception between the TX antenna (Sendeantenne) and the RX antenna (Empfangsantenne) to prove it. Also one example is added on the front to back ratio when turning the antenna by 90 and 180 degrees. That makes the DK6ED Double Loop V2 the most simple design but best dx system I ever had.
Comparison rx- to tx- antenna 80 m
Comparison rx- to tx- antenna 80 m
Comparison rx- to tx- antenna 160 m
Front to back 160 m
Figure 8: view of the DK6ED Double Loop
So good luck with the low band dxing!