An extended double zepp antenna for 15 meters is modeled mounted on two 24 foot poles spaced 60 feet apart. The antenna is cut for 15 meters and is 346 inches each side of center. It is feed in the center with 300 ohm twin lead. Typical feedpoint impedance is 119 ohms resistive and -692 ohms reactive. The EDZ is a fine antenna with several special properties.
Above we see a comparison of the broadside views of of the 15 meter EDZ loaded at 40 meters (grey), 20 meters (yellow) and 15 meters (pink). The 10 meter plot is not included here because it is split on the end as this approaches a long wire on 10 meters. Gain on 15 meters in the favored direction is 11 dbi with a launch angle of 26 degrees.
The EDZ is just about as long as a wire can get in terms of wavelength before the donut broadside pattern typical of a dipole quits sharpening and splits into two lobes. In fact you can see that already the split lobes are appearing at 15 meters. As frequency is increased further, these lobes grow rapidly and the broadside lobe shrinks rapidly. As wavelength is further shortened on a given wire length, it starts acting like a long wire with these split lobes getting closer and closer to the direction of the wire and multiple minor lobes appearing.
Viewed from above we see the 20 meter pattern (grey), the 15 meter pattern (pink) and the 10 meter pattern (yellow). Note how the 10 meter pattern has split and now has maximums that are 42 degrees off axis of the antenna broadside and an unfortunate null directly broadside to the antenna. This changes abruptly the favored angle of radiation of the antenna from dipole to long wire pattern. However, gain on 10 meters is excellent, being close to 12 dbi in the favored directions.
Another major consideration of the EDZ is mounting height. Since the dipole pattern is sharpened, it sends considerable energy towards the ground near the feedpoint. Thus the antenna changes pattern abruptly with height above RF ground. Optimum mounting height for such an antenna is close to .6 wavelength above RF ground. Below that there is some gain loss. Above that a vertical lobe starts to appear that saps energy from the desired low angle radiation.
This means an EDZ for 15 or 10 meters is a practical portable antenna. It can be loaded on the lower bands, can be mounted only a 1/2 wave high which is just 21 feet at 15 meters and 16 feet at 10 meters, and it will perform adequately at lower bands while giving very worthwhile gain on the band it is specifically cut as an EDZ.
Of course the EDZ can be cut for lower frequency bands, but at 40 meters it now wants to be mounted about 70 feet high for best results. At 80 meters that climbs to 130 feet for optimum mounting height. Due to the fact the pattern will split just above the design frequency, the EDZ should be optimized for one of the higher bands you plan on using it on.
As long as you have a decent tuner designed to handle the reactive feedpoint which is highly capacitative and about 120 ohms resistive at the design frequency, you can effectively load an EDZ on many adjacent bands. The 15 meter one models as loading with a reasonable pattern on 40,20,17,15,12,10,6 meters. With the best results being broadside on 17,15 and 12 meters, though it works well on 20 meters.