**Next message:**Adam B. Kanis: "(no subject)"**Previous message:**Jim: "KnightLites & Pixie/49er test THANKS !!"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

There's been a lot of recent interest on qrp-l in the W3EDP

antenna. This is an antenna with a 84 foot wire on one side of

the output port and a 17 foot wire on the other side. The 17

foot side is sometimes called a "counterpoise." Here's my "take"

on the antenna's operation, along with a suggestion or two:

To begin, the "counterpoise" isn't a real counterpoise, or at

least it doesn't function like the usual counterpoise (except on

20 meters). The normal function of a counterpoise is to

establish a point of minimum rf voltage or "ground" potential. A

quarter-wave length of wire does this when the far end is open.

(That's assuming it isn't detuned by nearby objects or by the

ground itself.) Except on 20 meters, where it is approximately a

quarter-wave long, the "counterpoise" part of the W3EDP antenna

doesn't fit the bill here. The best approach is to forget about

the short wire as a counterpoise.

To understand the W3EDP, instead conceive of the short side as

one side of a feedline that's been separated or pulled apart from

the other side of the feedline.

Now in your mind move the short side so that it's parallel to the

first 17 feet of the long side and anywhere from several inches

to a foot or so away. What you have is a section of feedline.

This gives you a standard *end-fed zepp* (in other words, a true

zepp antenna, as in zepplin flying), with a fundamental frequency

of about 7 MHz. The flattop portion is 67 feet long, and the

feedline is 17 feet long. The impedance of the feedline, which

is not critical, is somewhere in the vicinity of 500-800 ohms,

depending on wire diameter and spacing.

An end-fed zepp will work on its fundamental frequency and on odd

and even harmonic frequencies (that is, where the flattop is an

odd or even multiple of a half wave). With our W3EDP-derived

end-fed zepp, the antenna will work satisfactorily on 40, 20, 15,

and 10 meters.

The principle of operation is this: At the feedline end of the

half wave flattop (or multiple half wave flattop), the impedance

is *very* high. The impedance at the antenna end of the *open*

side of the feedline is also *very* high. (Were it not for

capacitive coupling to space and various objects, the impedance

at the antenna end of the open side of the feedline would be

infinitely high, but in reality the current never quite falls to

zero.) If the length of the flattop is properly adjusted, then

the currents on the two sides of the feedline are *roughly* in

balance, but out of phase, so not much radiation occurs from the

feedline.

(If you're familiar with a j-pole antenna, it's the same

principle of operation. And with both the end-fed zepp and the

j-pole, there's controversy over whether they work quite as

they're alleged to. But that's another story, getting into fine

points of feedline balance and what happens at the antenna end of

the open side of the feedline. Also, the resonance of the

flattop on harmonic frequencies isn't exactly an odd or even

multiple of the fundamental frequency, owing to differing impacts

of nearby objects and of end effects. But that, too, can be put

aside in understanding why the W3EDP works.)

Now, if you start pulling the feedline apart, you start to get

more radiation from the two sides of it. If you move the short

side out of the vicinity of the long side, you don't have a

feedline effect at all and you have a W3EDP antenna. But--and

this is crucial to the W3EDP design--the currents at the

transmitter/tuner ends of the two wires are still roughly equal

and roughly out of phase. This means that you can connect them

to a link-coupled tuner without serious imbalance. And this

remains true on harmonic frequencies.

This also explains why, if you try to operate a "standard" W3EDP

on 30 meters, you may find significant hand-capacitance effects

as you try to adjust the tuner.

If you follow through the foregoing analysis, you will see that

there is nothing absolute about the 84 foot and 17 foot

dimensions. In general, what you need is a short side of x feet,

and a long side of (x + y) feet, where y is a half wave at the

lowest frequency. For example, you could set the two sides at 22

and 89 feet, for fundamental operation on 40 meters, and harmonic

operation on 20, 15, and 10 meters. This would likely not be a

good length for 15 meters, however, because the impedance at the

tuner would be quite high. (Think of the input impedance of a

half-wave length of feedline that's terminated in a high

impedance.) Indeed, the "standard" W3EDP is likely to have a

high feedpoint impedance on 10 meters, just as in the case of an

end-fed zepp fed with a half wave feedline. For 10 meters, the

22 and 89 foot dimensions would probably be better in terms of

keeping the impedance at the tuner within a satisfactory range.

An idea for including 30 meters in the W3EDP design is to have a

second short side to switch to. With the standard 84 foot length

for the long side, a short side of 38 feet should give reasonable

balance on 30 meters (the rough conceptual equivalent of a 46

foot flattop and a 38 foot feeder).

There is one *important* point to keep in mind. Unless the short

side is run fairly close to and parallel to lower portion of the

long side, there will be significant radiation from the short

side. This means that it is best *not* to have it on or close to

the ground (in the fashion of the usual counterpoise). If it's

close to ground, it will be radiating into a rather lossy

environment. In fact (without having modeled the different

configurations for comparisons, or having done actual comparative

tests), I'd say there is at least a theoretical advantage in

running the short side in true "feedline" fashion, within several

inches or a foot of the lower portion of the long side and

roughly parallel to it. (Or get the wires close to this

configuration.) The result will be more of the total radiation

occurring at a greater height and thus lower near-field ground

losses.

I've used a W3EDP with one of my z-matches, with no difficulty on

the indicated bands. I'd imagine the W3EDP design should pose no

problem with Roy Gregson's ZM-1 or ZM-2, either, although the two

output links in my design give a little more flexibility. A

standard link-coupled tuner (which L.B. carefully describes on

his Web site) should be fine, too. A single-ended tuner--L

network, T network, etc.--requires a balun (and that may present

its own problems.)

Charlie, w6jjz

clofgren@benson.mckenna.edu

**Next message:**Adam B. Kanis: "(no subject)"**Previous message:**Jim: "KnightLites & Pixie/49er test THANKS !!"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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