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 ARRL Antenna Handbook,

The American Radio Relay League, Newington, CT

 

Reflections, Transmission Lines and Antennas

Walter Maxwell, W2DU, (Published by ARRL)

 

Radio Handbook

Bill Orr, W6SAI, Howard Sams Co.