9:1 Balun For End Fed Antenna

A 9:1 Balun For Your End Fed Antenna

End-Fed antennas do have their place in radio and other communications work. Like everything else, antennas come in many forms and flavors. It’s up to the designer to select the best design for the job at hand and to utilize that design in the most efficient way.
* * * A Bad Rap? * * *
– End-Fed antennas are NOT balanced systems; but neither are verticals, ground planes, discones, windoms, zepps, Marconis, half-slopers, et al. Additionally, the low-impedance antenna port of your transmitter/receiver is not balanced.
– End-Fed antennas are noise magnets. Really? That’s because most hams and SWL-ers don’t bother to interface them properly.
– End-Fed antennas have wild impedance swings. So do all antennas, but not at the design frequency – there, at the design frequency, the terminal impedance is quite predictable.
To make the best use of an End-Fed antenna, it should be fed with a transformer. Here are some photos of one of my 9:1 baluns. However, when using it with an End-Fed antenna it is wired as a (so-called) “unun” transformer (unbalanced to unbalanced). The raw End-Fed antenna will go through impedance swings as high as 5K Ohms, or more, at even multiples of its 1/4 wavelength design frequency. At every odd multiple it will be at a more civil impedance of between 36 and 90 Ohms. Using the transformer, the magnitude of the impedance swings is greatly reduced. This is due, in part, to the ratio balancing of the transformer’s turns (windings), and to a few complex reactance and other physics attributes that I won’t try to cover here. Additionally, the unun will eliminate (virtually) all “common mode” currents on the feedline. This is important for eliminating the pickup of local electrical noise from homes and power distribution lines. The coax, being connected to ground through a DC path will eliminate all but the differential currents … pretty cool, huh? Finally, since the antenna is connected directly to Earth ground through the secondary of the transformer, static buildup cannot occur – the antenna is a dead (DC) short to ground. This is important to sensitive, solid state radios.
The 9:1 transformer provides: 1. a much flatter broadband impedance response, 2. a static elctricity-free antenna system (no buildup), 3. common mode noise immunity (if space wound as shown).

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