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1. By the time of the Graff Zeppelin in 1936, and the Hindenberg in 1937, the 2-wire Zeppelin antenna was reeled electrically from a state-of-the-art radio room in the forward part of the luxurious 20 passenger gondola.
2. For tuning, one wire was extended 0.75 wavelengths (or multiples). Maximum length was 120 meters for use down to the 1,000-2,000 kc range.
3. For matching, the second wire was extended 0.25 wavelength (or multiples) to provided low impedance points anywhere in the high, medium and low frequency bands.
4. The main transmitter's operating range was 17.7 kc to 4.28 kc. CW and Voice.
5. The main receiver ranged from 15 kc to 20,000 kc in ten bands.
Notes:
Spark-gap
transmissions were banned in
1936 in favor of CW transmissions.
In 1960
the unit of frequency CPS (cycles) was
replaced by Hz (Hertz), kc by kHz, mc by mHz.
Related antennas are: J-pole, EDZ (Extended Double Zepp), Half Square, Bobtail Curtain, End-Fed Half-Wave
Figure 1 Here are the two half-wave currents ... end-to-end. |
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Here is the sweep to 28.4 MHz @
1.1 SWR |
and enlargement of 15.4 MHz @
1.6 SWR |
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Graph 1 |
Graph 2 |
Figure 2 |
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Figure 3 |
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Figure 4 |
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Frequency |
SWR |
Wire 2 (X) |
Wire 1 (Acf) |
Stub Length |
Get MHz |
@
SWR = |
28.4 MHz Zepp tune 14.2 MHz Stub tune |
1.33 1.37 |
17.0378 ft. ditto |
8.6253 ft. ditto |
0 ft. 3.6 ft. |
15.0 14.2 |
1.5 2.5 |
After feed moved |
1.37 |
Feed
point on Wire #1 at 30% to 40% from Wire #5 |
3.6 ft. |
14.2 |
1.04 |
Figure 5 |
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For the shortest
antenna, bend the stub down by the separation distance to form a second
parallel matching section. Figure 6
(Wire 5) = sep = 1 ft. (Wire 6) = stub-sep = 6.84 = 5.84
ft.
Wire 2 =16.82 ft. Wire 1 & Wire 4 = 8.102 ft. Stub=6.85
ft. Feed at 10% of Wire 1.
Figure 6 |
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28.4
MHz
2nd Harmonic |
14.2
MHz Stub tuned 1st Harmonic |
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Graph 2
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3D Flyover
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Two lobes at 2 half-waves
elevation, 33 ft |
One lobe at 1 half-wave
elevation, 33 ft. |
Notes
1. The Zepp type
of antenna is unbalanced and notorious for generating common mode
current. It is required to have at the feed point a wide-band choke
(1:1 current balun) suitable for the two frequencies. This is to
prevent the coaxial cable from becoming part of the antenna which
makes tuning difficult if not impossible.
2. The
Zepp+Stub
antenna works
only
where
the
added
stub
length
can
lower
the 1st
harmonic frequency
to
a 2:1 ratio
by
compensating
for the shortness
caused
by end-effect. For example a 6 meter Zepp
dipole can easily be stub tuned longer for 12 meters but not shorter
to 10 meters.
Similarly, is
not possible to stub tune a 10 meter Zepp dipole antenna shorter to
12, 15 or 17 meters.
3. At this writing I have not built a HF amateur radio antenna of this type, only a vertical lab prototype in the VHF range for proof of concept.
4. When connecting a J type of antenna to coaxial cable the shield goes to the short element.
5. An arbitrary spacing of 1 foot has been used to make graphics clear for this articles. To this point there has been no discussion of the spacing between the parallel wires.
Wire Spacing
In the early days open wire feed or “ladder line” spacing was 2 to 5
inches between wires with spacing “rungs” being separated by 5 to
12 inches. We do not know if or what spacing was used by the later
Zeppelins. However wider spacing may have been used because of
the Low (~1 MHz) frequency carryover from spark gap transmitter
use.
In the 1920’s the amateur radio adaptions of the Zepp antenna used 9
inch feeder spacing for 600 Ohms impedance. In 1928 international
amateur radio bands and call signs were established. However, when the
Zepp antenna was used for these harmonic bands, the high impedance at
thousands of Ohms could be difficult to match. In the 1930’s the
Zepp based Collins Radio “Multi-band Antenna System” used 6 inch wire
spacing for 300 Ohms impedance. This is more in line with the
mean harmonic impedance resistance of the harmonics which was generally
under 1200 Ohms.
Coaxial cable and TV flat twin lead cable arrived in 1950’s. Since then
high frequency, HF, mono-band Zepp antennas also use 450 Ohm twin lead
(1 inch spacing) or 300 Ohm (0.3 inch spacing) connected to
coaxial cable through a 9:1 or 4:1 balanced to unbalanced (balun)
transformer.
The arrival of antenna modeling in the 1970’s opened understanding of
how the Zepp works and how to adjust dimensions for 50 Ohm impedance…
and for direct connection to coaxial cable. Development of the
J-Pole version for Very High Frequency, VHF, amateur radio led to
computer optimization of wire spacing using the formula: 22/MHz =
Spacing in Feet. See Table 2 below.
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Tuning
The usual tuning difference between Zepp and J-Pole antennas is the location of the feed point.
For modeling, the Zepp antenna is center-fed at the mid-point of the closed end (figure 7, wire 3). From this point to the end of the parallel section (wires 1 and 4) adds up to the quarter wave matching section. The low mpedance/high current point is in center of the end (wire 3) as seen below. Tuning is by length of wires. Notice that the RF current (color) is symmetrical and highest (red) near the feed point (circle).
Zeppelin |
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Figure7
Center Fed Zeppelin antenna
tuning |
A J-Pole antenna
is
also tuned is by length of wires but with both sides of the matching
section are a quarter-wave in length. (Figure 8) Whatever length
of Wire 4 used for connecting the parallel wires makes the total
RF resonating length too long (low frequency). Tuning
(raising the frequency) is accomplished by finding and connecting coax
to the low impedance/high current point between the parallel wires...
hence matching Zepp tuning. The location of the tuning point
(wire 7) is equal to one connection wire length or more away from the
closed end (Wire 4) of the matching section
J-Pole |
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Figure 8
J-Pole antenna tuning by shorting |
The current
(color) is also high (red) near the feed point (circle on wire 7), but
I notice that the J-pole RF current (red) is not as symmetrical as the
Zepp antenna seen in Figure 1. There is very little current
(light blue) or impedance in the closed end (wires 1,4 and 5).
The extra length of the closed end of a J-pole makes it difficult if
not impossible to do stub tuning for dual band operation.
From my CF
Zeppelin
Stub antenna 4NEC2 model here, this is the “Symbol Conversion file”
definitions and calculated #14 wire dimensions for 14.2 MHz resonance
optimized to 7.2 MHz by Stub tuning.
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Table 3
Effects of Stub Tuning and Feed Point adjustment, 40-20
Meter Zepp antenna |
Below is the current distribution when
using a stub for frequency tuning and making feed point adjustment for
low SWR.
Zepp+Stub One half wave 1st Harmonic |
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7.2 MHz, 1.04 SWR |
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Zepp+Stub Two half waves 2nd Harmonic |
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14.2 MHz 1.07 SWR |
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Figure 9 Currents on a Stub Tuned 40-20 Meter Zepp
Antenna |
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Graph 3 |
The two-band possibilities of a Zeppelin type antenna are:
6 and 12 meters
10 and 20 meters
15 and 30 meters
20 and 40 meters
30 and 60 meters
40 and 80 meters
In summary:
A Zeppelin (Zepp) type antenna resonates as two half waves which is, by definition, a 2nd harmonic.
The 1st harmonic appears at 1.8 to 1.9 ratio lower in frequency.
This lower frequency can be stub-tuned as desired to a 2.0 ratio to create a two-band antenna on any frequency on either band.
On the lower frequency, the antenna is around 15% shorter than a dipole with gain within a dB of a dipole.
A J-Pole type antenna does not ordinarily provide the two-band possibility.