The Extended Double Zeppelin 'Zepp' (EDZ) is a copper wire antenna and is, in my opinion, the best wire antenna I have ever built and used. A similar antenna was used on Zeppelins in the 1920's and 1930's, hence the reference to 'Zeppelin' in the name (although only one wire, which hung from the airship, was connected to the ladder line).
This EDZ design differs to my older one in that an aluminum mast is used as opposed to wood. Using an aluminum mast requires an adjustment be made to the positioning of the ladder line. The ladder line in this case needs to be some distance from the mast to prevent power absorption from the line into the mast. I opted to use wood stand-offs about 20 cm in length (any non-conductive material can be used, e.g. PVC).
The EDZ is a bi-directional antenna with deep nulls off each end of the two wires. It therefore needs to be mounted with the antenna's broadside "pointing" in the desired direction of operation.
"VOACAP HF Propagation Prediction and Ionospheric Communications Analysis" is a very useful online tool to determine bearings of areas of interest: http://www.voacap.com/, specifically the "Point-to-Point Predictions."
It is important to note that True North and Magnetic North are not the same and differ significantly in different parts of the world. Look up the difference for your area and compensate for the difference when "pointing" your antenna using a compass.
Visit "Wikihow" for different methods to find True North: http://www.wikihow.com/Find-True-North-Without-a-Compass.
For this design I used standard PVC insulated electrical copper wire. With respect to RF, the PVC insulation causes the wire to appear longer than it really is, this is caused by a phenomenon called "velocity factor." This has been taken into account and the dimensions I use here have been decreased by 6 % (factor: 0.94) which is the velocity factor of standard PVC insulated electrical copper wire.
Full wavelength: 300 / 27.555 = 10.887 x 0.94 = 10.234 m
Top sections of antenna: 0.64 wavelength: 10.234 x 0.64 = 6.550 m (for each leg)
450 ohm Ladder Line: Approximately 0.2 wavelength: 10.234 x 0.2 = 2.047 m (the length may vary between 1.300 m and 2.047 m depending on surroundings, use trial-and-error to find the ideal feed-point for your antenna by checking the VSWR; you may consider starting at 2.047 meters and shortening as required. Using experimentation and EZNEC calculations 1.660 m appears to be the best compromise. Once the antenna was installed I found that it was a little too long and I was required to cut 10 cm off each end of the two radiator wires. Cutting off or adding anything more than this is not recommended as the length of the two radiating wires affects the radiation pattern. Rather adjust the length of the ladder line.
The EDZ is a balanced antenna and has a high impedance at the feed point. To connect the antenna to 50 ohm coax, which is also un-balanced, it is necessary to transform the high impedance to 50 ohm and to convert the balance to an un-balance respectively. A 450 ohm Ladder Line 0.2 Lambda in length transforms the high impedance of the antenna to 50 ohm and the Current Balun converts the balance to an un-balance; hence the name "bal" for balanced and "un" for un-balanced.
Running this design in EZNEC produces VSWR and Radiation Pattern plots as indicated in figures 3, 4 and 5 respectively. I selected a ground description of poor, rocky, mountainous and PVC insulated copper wire 0.8 mm thick in the simulation. The antenna was placed 11 m above the ground. Figure 6 shows the EZNEC antenna model and current distribution curves. The EZNEC "wire" table is shown in figure 2.
It is important to keep the 450 ohm Ladder Line and 50 ohm coax perpendicular to the antenna for the longest distance possible, in doing so there is less chance for RF being conducted back into the feed line and ultimately into the transmitter. The 450 ohm Ladder Line should also not be twisted so that the two wires touch and not be bent at sharp angles (avoiding angles less than 90 degrees). Refer to figure 1 for the construction details. The EDZ itself should be pulled as straight as possible. Bends in the wire distort the radiation pattern of the antenna which affects gain and directivity respectively. As with most antennas, try to install the antenna as far away as possible from surfaces and objects that reflect RF.
If you have the space why not try building an EDZ, it's a classic antenna and it works well ... enjoy.
Figure 1: 11 m Band "Zepp" (EDZ) MkII antenna: Construction details
You will notice from the photographs that I used 2 mm diameter weedeater (weed wacker) line instead of Dacron as I did not have any at the time.
Figure 2: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC Wire Dimensions (11 m above ground)
Figure 3: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC VSWR
Figure 4: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC side view radiation pattern
Figure 5: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC top view radiation pattern
Figure 6: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC antenna model
|Type||1.28 Lambda Extended Double Zeppelin (EDZ) PVC insulated copper wire antenna|
|Frequency||11 meter Band (centred at 27.500 MHz)|
|Gain||12.69 dBi (at 11 m above ground)|
|Take-off/elevation angle||Approximately 14 degrees (at 11 m above ground)|
|Band width||600 kHz (measured at the 1:1.5 VSWR points)|
|Height above ground||11 m (minimum 7 m)|
|Antenna length||13.150 m (1.28 Lambda) with each leg 6.550 m (0.64 Lambda)|
|450 ohm ladder line length||Approximately 2.047 m (0.2 Lambda) with 5 cm spacing between the two wires|
|Antenna wire||2 to 2.5 mm diameter standard PVC insulated electrical copper wire with insulation approximately 0.8 mm thick|
|Current Balun||7 turns of RG-58U coax close wound side-by-side with a 12 cm diameter air core|
|VSWR||Less than 1:1.1 at 27.500 MHz and 1:1.5 at 27.250 MHz and 27.850 MHz respectively|
Radio Frequency (RF)