The EFDA antenna is somewhat similar to the BDA. It is also constructed with a number of equal, parallel and equally spaced elements, but the feed point is not located in the antenna center but at the center of the rearmost element. The antenna radiation pattern is something between the bidirectional and unidirectional patterns. The EFDAs perform better than the BDAs.
This is the simplest EFDA but despite that its performance is surprisingly good. The antenna is rather bidirectional, but when the frequency is increased it slowly starts showing somewhat unidirectional pattern. And on the 10 m band, it sports F/B gain ratio of 4.5 dB.
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One more element and SWR goes down and gain improves a little.
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One element more and the maximum SWR drops to 2.4:1. Such antenna can be driven even by a transceiver without an antenna tuner. You can expect SWR well below 2:1 at the TRX end of the coax.
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This antenna is slightly smaller than the 4-element version and has a little better performance. Thanks to its low SWR, it can also be driven by a transceiver without an antenna tuner.
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If we add to the 2-element EFDA described above converging elements, we will create a very interesting antenna. It has a bidirectional pattern on 20 m band, changing gradually to a true unidirectional radiation pattern on 10 m band. Its gain equals a dipole on 20 m band and 2-element Yagi on 10 m band!
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If the length of the basic 2-element EFDA is too large for you, you can modify the basic design by bending inwards the ends of the parallel elements. In this way, the antenna length can be shortened to 8 meters. The performance will suffer a little bit, but not too much.
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EFDA can be made even shorter. 7 meters is possible but EFDA performance will suffer again. You will have to accept SWR up to 4.2:1 at the feed point (which, in typical cases, results in SWR <3:1 at the transmitter end of the coax). Such a decrease in length will also affect slightly antenna gain, but still the 7 m long EFDA will be just slightly below HW dipole+coax performance (-1.1 ... +0.1 dB).
The picture below presents not only the wiring but also a construction idea. Solid green lines represent fiber glass poles, and dashed green lines - non-conducting strings. The construction like this will result in a very lightweight antenna suitable for telescoping masts.
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