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Relics: Radio communications: 11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz

Introduction

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).

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz
11 m Band "Zepp" (EDZ) MkII antenna centred at 27.500 MHz
(click on the image for a larger version)

Description

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.

Wire lengths are calculated as follows

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.

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Construction details
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.

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: EZNEC Wire Dimensions (11 m above ground)
Figure 2: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC Wire Dimensions (11 m above ground)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: EZNEC VSWR
Figure 3: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC VSWR

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: EZNEC side view radiation pattern
Figure 4: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC side view radiation pattern

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: EZNEC top view radiation pattern
Figure 5: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC top view radiation pattern

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: EZNEC antenna model
Figure 6: 11 m Band "Zepp" (EDZ) MkII antenna: EZNEC antenna model

Photographs

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Mast joins (middle section)
11 m Band "Zepp" (EDZ) MkII antenna: Mast joins (middle section)
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Mast joins (top section)
11 m Band "Zepp" (EDZ) MkII antenna: Mast joins (top section)
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Guy wire anchor hooks
11 m Band "Zepp" (EDZ) MkII antenna: Guy wire anchor hooks
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Wood spacers and stand-offs
11 m Band "Zepp" (EDZ) MkII antenna: Wood spacers and stand-offs
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Electrical wire
11 m Band "Zepp" (EDZ) MkII antenna: Electrical wire
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Ladder line
11 m Band "Zepp" (EDZ) MkII antenna: Ladder line
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Wire soldered and sealed with self-amalgamating tape
11 m Band "Zepp" (EDZ) MkII antenna: Wire soldered and sealed with self-amalgamating tape
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Balun mounted with self-amalgamating tape
11 m Band "Zepp" (EDZ) MkII antenna: Balun mounted with self-amalgamating tape
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Check continuity
11 m Band "Zepp" (EDZ) MkII antenna: Check continuity
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Wire wrap guy anchor line and seal with self-amalgamating tape
11 m Band "Zepp" (EDZ) MkII antenna: Wire wrap guy anchor line and seal with self-amalgamating tape
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Tape guy anchor lines in place temporarily
11 m Band "Zepp" (EDZ) MkII antenna: Tape guy anchor lines in place temporarily
(click on the image for a larger version)

 

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: End support mast one
11 m Band "Zepp" (EDZ) MkII antenna: End support mast one
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: End support mast two
11 m Band "Zepp" (EDZ) MkII antenna: End support mast two
(click on the image for a larger version)

11 m Band Extended Double Zeppelin 'Zepp' (EDZ) MkII antenna centred at 27.500 MHz: Installation completed
11 m Band "Zepp" (EDZ) MkII antenna: Installation completed
(click on the image for a larger version)

 

Specifications

Type 1.28 Lambda Extended Double Zeppelin (EDZ) PVC insulated copper wire antenna
Frequency 11 meter Band (centred at 27.500 MHz)
Directivity Bi-directional
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


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