160 METER LADDER-LINE* TWIN-LEAD MARCONI ANTENNA
INSTRUCTIONS ARE
ALSO INCLUDED TO SHOW HOW ONE CAN USE IT AS A LONG WIRE ON OTHER BANDS.
TABLE
OF CONTENTS
INTRODUCTION........................................................................................................................................
PART LIST:..................................................................................................................................................
SECTION 1: ASSEMBLY...........................................................................................................................
CUT TO DESIRED FREQUENCY.............................................................................................................
INSTALL SUPPORT ELBOW...................................................................................................................
HANGING THE ANTENNA......................................................................................................................
BARRIER STRIP CONNECTIONS...........................................................................................................
ANTENNA FREQUENCY RESPONSE....................................................................................................
TO USE AS A "LONG WIRE" ANTENNA................................................................................................
SECTION 2: THEORY OF OPERATION..................................................................................................
INTRODUCTION.......................................................................................................................................
GROUND LOSSES....................................................................................................................................
A LITTLE THEORY....................................................................................................................................
TWIN-LEAD MARCONI TO THE RESCUE!...........................................................................................
OTHER ALTERNATIVES........................................................................................................................
CONCLUSION........................................................................................................................................
REFERENCES..........................................................................................................................................
INTRODUCTION
The
Ladder-line Twin-lead Marconi Antenna system is designed for ease of
installation, durability, and optimum performance. The antenna is complete and
only needs two points to which to tie the supporting ends. See Figure 1.
Fig. 1 OVERVIEW OF LADDER-LINE MARCONI ANTENNA SYSTEM
NOTES:
As with any antenna
system one should avoid locating the antenna system near high voltage lines
(including common power service to the home) or other hazardous environments.
Ladder-line wire is rugged,
but can be damaged if abused. Special precaution should be used to avoid
sharp bends and contact with sharp edges.
When determining the cut
lengths for the ladder-line I recommended cutting slightly longer than
suggested in the tables. This will allow sufficient length to accommodate for
influences of ground conditions and effects of nearby objects. For example, if
it is determined that the antenna must be cut by 6" remove only 5"
initially.
It is wise to provide a
simple method of raising and lowering he antenna for the initial tuning
process; many operators use pulleys at each end of the antenna to accomplish
this.
Marconi antennas perform
best when erected as vertically as possible. As with any good end-fed antenna,
a ground radial wire is required with this antenna.
PART LIST:
|
|
QUANTITY |
DESCRIPTION |
|
1 |
130.0 feet (39.6 meters) |
Ladder-line
wire (two #18 conductors ) |
|
2 |
1 |
Two
terminal barrier strip |
|
|
1 |
Clic
Ô clamp (for support
tube) |
|
3 |
1 |
Antenna
insulator (already connected) |
|
4 |
2 |
Friction grips, rubber cord protectors that fit snuggly in
support tube. Can be purchased in building supply stores and electrical
supply stores |
|
5 |
1 |
Support tube,1" electrical plastic conduit, 90 degree
bend |
Fig. 2 ELBOW
SUPPORT CLOSE-UP
SECTION 1: ASSEMBLY
CUT TO DESIRED
FREQUENCY
The
antenna is designed for 1.800 MHz. For operation on other frequencies, see
chart below.
MEASUREMENT
CHART (Remember: Measure twice, cut once!!!)
|
FREQ. |
LADDER-LINE |
RADIAL WIRE |
|
||||||
|
|
MHZ |
FEET |
REMOVE FEET |
METERS |
REMOVE METERS |
FEET |
METERS |
||
|
|
1.800 |
130.00 |
--- |
40.00 |
--- |
130.00 |
39.62 |
||
|
|
1.850 |
126.50 |
3.50 |
38.92 |
1.07 |
126.48 |
38.55 |
||
|
|
1.900 |
123.16 |
6.84 |
37.89 |
2.08 |
123.16 |
37.54 |
||
|
|
1.950 |
120.00 |
10.00 |
36.92 |
3.05 |
120.00 |
36.58 |
||
|
|
2.000 |
117.00 |
13.00 |
36.00 |
3.96 |
117.00 |
35.66 |
||
The
formula for calculating an exact frequency is: 
INSTALL VELOCITY FACTOR "SHORT"
Measure 13 feet (4 meters) from the end that will be farthest from the
transmitter, in other words the end that
will be up in the air when the antenna is installed. Carefully remove the
insulation from the twin-lead at this point, from both conductors. Using about
six inches (15 CM) of bare wire, wrap the wire around the twin-lead so it makes
good contact with both conductors. Solder the connections. If
heat-shrinkable tubing is available, put a piece over this connectionand
shink it. An alternative to heat-shink is to use good quality hot-glue or
coax-seal type putty. Wrap this with electrical tape to keep moisture out. This
short compensates for the velocity factor of
the wire (see the Theory Section).
INSTALL SUPPORT
ELBOW
Install
the Upper Friction Grip, Support Tube and Lower Friction Grip on the
ladder-line wire. See Fig. 2.
put a small amount of liquid soap
or detergent inside the Grips to help them slide easier.
grasp the friction grips between
the finger and thumb and squeeze into an oval-shaped opening. Slide the grip up
the ladder-line to the point where the elbow will support the antenna.
Force the Upper Friction Grip into the top of the elbow as far as possible. It
will not usually go in all the way. Do the same for the Lower Friction Grip.
The Lower Grip will keep the ladder-line wire from pulling through the elbow
when the unit is installed. The Upper Grip just protects the ladder-line from
coming in contact with the hard surface and relatively sharp inner edges of the
elbow.
Place the clamp on the support bracket so that the rope hole at the top of the
clamp is approximately even with the center of the opening at the top of the
elbow where the ladder-line comes out and squeeze the clamp until it “clicks”
once or twice. See figure 2.
Attach the support rope or wire to the support bracket in one of several ways:
1. Thread a
#6 or #7 braided nylon or hemp rope through the support hole and tie into a
knot. If the knot is too large to pull back through the support hole it will
adequately support the elbow.
2. Install a
¼" x 2" threaded screw eyelet in the clamp hole and tie the cable or
rope to this.
HANGING THE ANTENNA
Fig. 3 END INSULATOR
INSTALLATION
To
hang the antenna, support the other side of the insulator with wire or rope.
See Figure 3. Raise the antenna and check for proper installation. The lower ladder-line
twin-lead should hang freely once installed.
BARRIER STRIP CONNECTIONS
FIG. 4 Barrier
Strip Connection
locate the barrier strip so it is protected from the weather or
covered with a protective material, such as Coax Seal.
support the barrier so it isn't
moving with the wind.
Assure that the ladder-line wire is also supported so it isn't moving at the
barrier strip. If the ladder-line is able to flex back-and-forth it will
eventually fatigue and break the wire at the screw terminal. Standard TV
twin-lead supports, tie-wraps or electrical tape can be used to accomplish this,
depending upon the installation site.
INITIAL
CHECK-OUT (after installation)
Proper
connection to the barrier strip depends on the site ground conditions:
1.
If there are not very many radials on the system, connect both
ladder-line wires to terminal B. Usually when there are not many radials
the ground losses are high and the antenna impedance is often near 50. System efficiency is not at the
maximum obtainable, but adequate performance can be obtained.
2.
If there is an extensive ground system, or the ground conditions at the site
are particularly good, connect the two wires to the barrier strip with one
going to a good ground connection (screw A) and the other to the center
conductor of the coaxial feeder (screw B) as shown in Fig. 4. With very good
ground systems the impedance of the antenna is close to the theoretical
impedance of 10 -15 and the ladder-line transformer action
is necessary.
3.
In some very unusual cases a Unbalanced-to-Unbalanced broadband matching
transformer may be necessary. If the antenna impedance is particularly unusual
the transformer can be used to provide an additional 1:4 step up ratio. This
will very rarely be needed..
A
good ground is absolutely essential on the lower frequency bands. Radials help
provide the ground necessary. They can be on the ground, or buried. A single
ground rod is totally inadequate. See the THEORY OF OPERATION section for a complete
description of the various configurations of the Marconi antenna.
ANTENNA FREQUENCY RESPONSE
The
chart below shows a typical frequency response for the Ladder-line Twin-lead
Marconi antenna. This particular plot is using the antenna with the antenna at an
altitude of 30 feet (10 Meters) and used with the ladder-line transformer
action. The installation was a typical situation with only one radial wire and
a few short ground wires.
TROUBLESHOOTING
Most
difficulties evolve around insufficient ground conditions or bad connections.
Insure that there are no shorts between the terminals on the barrier strip. The
Ladder-Line Marconi antenna depends on good grounds to ensure that it appears
to the transmitter as a 50 ohm load. It is not unusual to see seasonal
variations in this impedance as the ground conductivity changes with moisture
content. When few radials can be installed one should attempt to place one on
the ground beneath the antenna. High SWR conditions usually indicate
insufficient ground, provided all else is as it should be. One can compensate
for poor matching conditions with an antenna tuner, and even under good
conditions a tuner is a wise choice. One can connect the ladder-line wire to a
female coaxial connector rather than the barrier strip, if desired.
TO USE AS A "LONG WIRE" ANTENNA
This
antenna can also be used as a 130' long wire simply by connecting the two
LADDER-LINE conductors together on terminal B and using a tuner to match the
system. Performance is unpredictable but in many cases can prove adequate for
20 Meters or shorter wavelengths.
SECTION 2: THEORY OF OPERATION
INTRODUCTION
Over
the years there have been a host of antennas popularized by amateur radio
operators; the quarter-wave vertical, the ground plane, mobile whips, various
top and center loaded antennas, and even the lowly "Rubber Ducky".
All these antennas have a common thread: their origins go back to the basic
Marconi antenna. A Marconi antenna is simply a quarter-wave vertical radiator
referenced to a true ground plane.
Most
of the variations on the Marconi antenna evolved as solutions to matching
problems. At the lower operating frequencies, particularly on the 160 and 80
meter bands, good ground conditions and adequate height are difficult to
achieve; consequently, compromises must be incorporated to make most antennas
work. The Marconi is no exception.
It
is easy to visualize why impedance matching is a problem. A dipole antenna has
an optimum impedance when it is about a quarter wave above a good reflecting
ground. At 30 MHz (the ten meter band) a dipole antenna a quarter wavelength
above the ground only needs to be 7.8 feet (2.4M) high. Since most dipoles are
usually erected many times higher than this height there is not much of a
problem with matching. However, to get the 160 meter antenna a quarter wave
high would take a tower 130 feet (40M) tall. Some can afford this luxury, but
most cannot. The more typical altitude available to the amateur is between 30
and 50 feet. Mounting a 160 meter dipole at 30 feet would be like mounting the
10 meter antenna at 1.8 feet (.55 M), something very few amateurs would
seriously consider. Yet many operators have 160 meter and 80 meter antennas
that are at less than optimum heights and seem to achieve adequate performance.
What is the secret?
Actually,
there is no secret. The difference is in the composition of the ground itself
and the effect it has on the antenna. At high frequencies the ground reflects well
and works for or against the antenna in a rather predictable fashion; but on
the lower frequencies, the actual ground is several feet below the earth's
surface. This situation creates the problem of how to get a good ground
connection.
160
Meter operators have been plagued with this problem for years. There is really
no good answer. Some have actually used earth moving equipment to bury miles of
copper conductor, this may be a bit of over-kill. Several authors have explored
what is optimum and for the really serious amateur (or commercial AM station) a
radial pattern of wires can be installed to adequately provide the ground
necessary for maximum performance. Between 60 and 120 radials seems to be
the ideal number of radials.
The
more typical installation has a ground rod of 8 feet (2.4 M), which is
practically no ground at all. For 160 meters, more than any other band, a
good ground is very necessary. Understandably not everyone can put in
extensive radial systems. Some reasonable alternatives available are Anchor
fences, buildings with buried metal structures for the foundation and, if one
is really lucky, a pipe into a well or water distribution system. Electric
fence systems that are no longer in operation are also good candidates for a
160 meter ground system. If on-site construction is in progress, just prior to
pouring cement or back-filling the earth one can put in some heavy copper wire.
It may not be a radial pattern, but every little bit helps. Another method
successfully used by more than one amateur is to take a piece of metal rod,
drill a hole through it on one end so it looks like a large needle, thread the
ground radial through it and pass the "needle" through the grass just
along the ground surface of the lawn so it is not visible. It does not have to
be buried, and insulated wire is fine as well.
GROUND
LOSSES
If
there are no radials, or very few, the ground available may be quite lossy. The
Marconi prefers a perfect ground in all directions so that the radiation pattern
can spread out from the vertical radiator like an opened umbrella. Obviously,
if most of the radials are not there, the pattern will distort, just as the
umbrella loses its form with broken support arms. The energy that does not have
a good return path will still find its way to ground, but now the ground will
present a higher resistance than that of a good radial. This resistance can be
very much like a dummy load and will use up the transmitted energy in the form
of heat. One indication that this may be happening is when an operator answers
CQ's and the station does not respond to the answer and keeps on calling CQ.
That station calling CQ is not ignoring the respondant, the respondant is not
being heard. The responding operator is puzzled because the SWR meter says
everything is great, and yet there is no answer. The fact is that the ground
losses are eating up the transmitted power and very little of it is being
radiated. This can often be the case with a 160 meter dipole antenna.
This
situation can be very deceptive, for the SWR appears to be very good, but a low
SWR to a 50 dummy load looks great too. Too many
amateurs fall into the trap of reading an SWR meter as gospel, SWR readings
have to be interpreted properly. An SWR reading is not a measure of the
antenna's efficiency, it only gives a relative indication of the match
acheived.
Ground
resistance losses are not good. Radiation resistance is desirable, it is the
equivalent resistance that would dissapate power, but instead of dissipating
power, the energy is actually radiated into space, which is the object of the
antenna.
A LITTLE THEORY
|
Figure 1 |
Now
that the ground problem is a little clearer, what does the amateur radio
operator do to make things work properly? All is not as dismal as it may seem.
Even though the grounds that most use are inadequate one can make the antenna
think that things are better than they are. Once this is accomplished the
transmitter will match and the station will successfully communicate.
This
drawing depicts the situation where an antenna is more than one wavelength ( ) long. The feed point is intentionally
not shown. What should be clear here is that the voltage is at a minimum when
the current is at a maximum, and vise-versa. Starting at any zero
crossing (any wave crossing the horizontal line) and going in either direction
one will observe that at each quarter wavelength from this point there is
either a current maximum or a voltage maximum. A quarter wave antenna is open
at the end away from the feed point and is connected to nothing. This will be a
voltage point, so current is minimum, and a quarter wave back from this point
will be a current point. In the case of the Marconi antenna this is the feed
point.
|
|
Here
is a view of a single Wire Marconi. Anyone that has operated two meters has no doubt
seen this antenna, for it is just the 1/4 wave whip that is so popular. This
single wire quarter wave antenna is really half of the complete antenna, for
the other half is an image that is replicated in the ground. In essence the
whip antenna is a half wave antenna where half of it is imaginary in a sense.
Now, recall that a typical dipole antenna is approximately 60 to 90 , with 72 's being the agreed upon theoretical
value. Since the feed point for this antenna only has half the antenna, it only
has half the radiation resistance, or around 30 to 45 , with 36 being the theoretical value. On low
frequencies the radiation resistance is typically much worse. The real value is
usually in the vicinity of 10 to 15 , particularly if the antenna has a
substantial portion in the horizontal configuration. It is readily apparent,
therefore, that a 50 transmitter is not going to match very
well! What can be done?
TWIN-LEAD MARCONI TO THE RESCUE!
In
the above scenario one could use an antenna tuner and compensate for the
impedance mismatch. (It is advisable to always use a tuner, it allows coverage
of the whole band, even when the antenna is only resonant in one portion of the
band. Amateurs have vast spectrum available, why restrict operation to a small
portion of the band?)
Without
a tuner, can this issue be resolved? The Twin-Lead Marconi is in essence a 1:4
transformer matched antenna. If the antenna is 10 - 15, this transformer action will raise the
effective impedance of the antenna to 40 - 60, a very acceptable impedance for a 50 transmitter.
|
Figure 3 |
To
understand how this occurs, one needs to look at how a folded dipole works. A
folded dipole is simply a dipole antenna with a second conductor in parallel with
the dipole. A folded dipole is one-half wavelength long. In Fig. 3 currents c
and d are the typical currents that exist in single wire dipole antenna. The
curve on top shows where the maximum currents exist in this antenna. Notice
that current c is flowing into the + transmission line and the current d is
flowing away from the - transmission line, which means that they are both going
in the same direction.
Assume
for the moment that the top of the folded dipole (a b)is not there and that it is a regular
dipole antenna. Assume also that the current is two amps and that the
transmitter is putting out 200 watts. What is the antenna impedance?
Where
Z is the impedance, P is the power and I is the current.
The
transmitter should be very satisfied with this. If we now return to the drawing
and add in the top part of the folded dipole we find that the current will now
be divided evenly between the two conductors. To understand why this happens,
recall that current on a wire will reverse direction (polarity) each half wave.
[notice that the antenna is a half wave long.] Length c to d has the current
going in one direction for a half-wave; therefore it will go in the other
direction on the top wire. Since the top wire is two right turns at c (or lefts
at d) as the current reverses, so does the direction of the wire. Therefore,
the current on the top wire is in parallel with the bottom wire, both wires are
carrying the same amount of current, and the input current is divided between
the two. The transmission line feeding the system is now seeing a load that is
drawing one amp. If we are feeding the same 200 watts to this system an
interesting phenomenon occurs:
The
load impedance now looks to be four times as great as the single wire dipole!
Thus, if an antenna is 12 because of the previously mentioned
problems, this technique could present a load that looks four times greater in
impedance, 48, and the typical 50 transmitter will readily match with
this.
In
the twin-lead Marconi the situation is similar. Since the antenna is half of
the half wave dipole it also is about half of the impedance, or about 36 with perfect ground. There is still the
mirror image principle so the 1:4 transformer action is still in place. As
previously discussed, the antenna impedance is likely to be much less than 36, and is usually more like 10 to 15. In this case the 1:4 transformer action
presents around 50 to the transmitter.
|
Figure 4 |
VELOCITY FACTOR
If
the velocity factor of the twin-lead is sufficiently high (the speed with which
the wave travels through the wire with respect to traveling in free space) the
transformer is the full length of the Marconi. Usually it is considerably less,
typically .66 to .80. For this reason it is often necessary to make the
transformer section shorter than the full quarter length so that a single wire
extends beyond the transformer section. The single wire length of the antenna
is not as affected by the dielectric material as is the transformer since the
transformer action takes place between the two conductors, whereas the single
wire is independent of the second wire.
OTHER ALTERNATIVES
Another
popular method of feeding a Marconi single wire antenna is to use a broadband
1:4 Unbalanced-to-Unbalanced (Un-Un) transformer. This is very viable method,
but the transformers must be protected from the weather and they unnecessary
expense when twin-lead wire is so readily available. Additionally, the
twin-lead transformer is distributed over much of the length of the antenna, so
it doesn't have the heating problems that concern the Un-Un design.
CONCLUSION
Any
antenna system that is not mounted in a perfect environment will need
individual attention to obtain maximum performance. In the case of the Marconi
the resonant frequency is determined by the length. Once it is resonant it must
be matched. Changing the length will not improve the matching. The
match to the transmitting source is dependent on the ground for the system and
the technique used to adjust the impedance.
It
cannot be over-emphasized that simple test equipment such as a noise generator
and receiver can be invaluable in adjusting such a system. Usually a decent match
can be obtained with a twin-lead Marconi by finding the resonant frequency and
then trying various feeds to determine which gives the best match. If it is a
twin-lead Marconi it is best to check it first by shorting both twin-lead wires
together and observing the performance as a single wire. If this proves
inadequate and insufficient equipment is available to measure the impedance,
then connect it as a true-twin lead antenna and observe if there is an
improvement. If this is unsuccessful a broadband transformer can be used to
match the twin-lead transformer. (This is a 1:16 match, the antenna impedance
would have to be extremely low, but there have been cases of this).
One
advantage the Marconi antenna has over its dipole rivals is the lower angle of
radiation and the potential for better DX performance. Even if the entire
length is not vertical, some amount of the radiation will be in the vertical
plane and can afford great distances under the right conditions.
If
at all possible it is still recommended to use an antenna tuner, even when a
perfectly matched antenna is in use. Tuners, in particular Pi-type
tuners, can assist in harmonic and TVI reduction and allow much greater
bandwidth utilization of any antenna system.
REFERENCES
The
American Radio Relay League, Newington, CT
Reflections, Transmission Lines and Antennas
Walter
Maxwell, W2DU, (Published by ARRL)
Bill
Orr, W6SAI, Howard Sams Co.