|Feeding an End-Fed Antenna|
|This is Page 1: The Transformer
Go to Page 2: Grounding
|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 an "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 greately 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. Without getting into "Lightening Safety" theory and practice, sufice it to say that not having a positively charged static generator sitting around waiting for a very large negatively charged cloud to pass over, is a good thing. There is still a lot of conjecture about this, but most 'lightning protection system' engineers agree: Having an Earth-neutral antenna over (near) your home is, in effect, like placing that area closer to ground. Well, maybe. Some "experts" disagree. In any case, as any physicist will tell you, there is NO absolute protection against a direct lightening strike, (even underground, in a cave!) but you can certainly take measures to NOT attract a strike ... and this is one of them!
|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).
|Photos property of N1KPR.|
|An Important Note on this Design|
|I selected the seperate (spaced, opposite or complementary) winding format over bifiler or twisted-pair winding in order to minimize the coupling effects of stray capacitance between the primary and secondary windings. This is important to the quality of common mode noise rejection, which is one of the balun's attributes.|
|Construction of the 9:1 balun/unun: I used an old plastic box and a type 75 torroid core. My windings, in this case, are for transmitting, using No. 10 and No. 14 solid copper wire. For SWL-ing a smaller core can be used along with No. 18 or smaller wire. The turns ratio is 3:1 for a transformation of 9:1.
The impedance is the square of the turns ratio (Z=T^2). The antenna terminals are brass speaker terminals and the low impedance side is a standard SO-239. The grounds of the two windings have been isolated but, can be strapped together by the screw contacts (shown in the top photo). The entire assembly was potted with marine fiberglass. Note that I also added a hanger strap for convenience.
|Testing the 9:1 balun/unun:
Properly designed and constructed torroid transformers are not lossey!!!
On the test bench with a 52 Ohm source and a 450 Ohm load the transformer shows a loss of no more than 0.6 dB with about 0.45 dB being average, and the low reading of 0.2 dB.
|Installation of the 9:1 balun/unun: The End-Fed is connected to one terminal of the "high" winding and the other terminal is connected to a good "EARTH" ground. One or more copper rods or pipes driven, at least, 4 feet into the earth near the dwelling entry point (2 or 3, is better, being spaced about 4 feet apart). The coax is connected to the SO-239 and fed to your radio. The best place for the balun for receive-only antennas, is mounting it directly to the primary ground rod and keeping the ground conductors as short as possible. Use a heavy copper wire (not steel or aluminum) or flat copper braid or copper sheet metal strap. This, in effect, gives you an inverted "L" antenna. For transmitting, it's best to place the transformer at the actual feed point, up, and away, from possible contact by humans and pets. Of course, extra care must be taken to properly ground the balun by running the ground conductor down the tree, house, etc. If your antenna is attached to your tower, as mine is, then you are home free ... just be sure your tower is properly grounded ... you'd be crazy if it wasn't suitably grounded the day you put it up!!!|
|A word about grounds. We are talking about RF (signal) ground on this page. These are the grounds that provide the antenna counterpoise (true ground plane), and afford static bleed to NOT tempt lightning. These grounds ARE NOT about electrical service safety. Some people tie their radio grounds to a common point "service ground" ("BONDING"), I do and this is okay if ALL the other ground measures discussed here have been met. Simply put: electrical service ground is NOT a substitute for a good RF Earth ground, which is required for a good antenna system. In fact, your service ground will probably be a very good source of noise, if not outright dangerous to you and your equipment. Remember, the "neutral" wire in your circuit breaker box travels outside of your home and all around the neighborhood ... it's a noise antenna and (going from pole-to-pole) a lightning rod!!! Best to consult with a licensed electrician about "bonding" the service, cable, telephone, and RF grounds at your homestead.|
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